View source for Microwave oven

{{short description|Kitchen cooking appliance}}
{{pp-vandalism|small=yes}}
{{Use mdy dates|date=April 2025}}

{{Infobox product
| title = Microwave oven
| logo = 
| logo_caption = alt         =
| image = Panasonic NN-SD69LS 20220410.jpg
| image_size = 250
| alt = 
| caption = A modern microwave oven (2022)
| type = [[Home appliance|Appliance]]
| inventor = [[Percy Spencer]]
| inception = {{start date and age|1947}}
| manufacturer = Various
| available = Globally
| current supplier = 
| last production = 
| models = 
| slogan = 
| website = 
| notes = 
}}

A '''microwave oven''', or simply '''microwave''', is an electric [[oven]] that heats and cooks food by exposing it to [[electromagnetic radiation]] in the [[microwave]] [[frequency]] range.<ref>{{Cite web|url=https://www.britannica.com/technology/microwave-oven|title=Microwave Oven|date=October 26, 2018|website=Encyclopedia Britannica|access-date=January 19, 2019}}</ref> This induces [[Dipole#Molecular dipoles|polar molecules]] in the food to rotate and produce [[thermal energy]] (heat) in a process known as [[dielectric heating]]. Microwave ovens heat food quickly and efficiently because the heating effect is fairly uniform in the outer {{nowrap|25–38 mm}} {{nowrap|(1–1.5 inches)}} of a [[homogeneous]], high-water-content food item.

The development of the [[cavity magnetron]] in the United Kingdom made possible the production of electromagnetic waves of a small enough wavelength ([[microwaves]]) to efficiently heat up water molecules. American [[electrical engineer]] [[Percy Spencer]] is generally credited with developing and patenting the world's first commercial microwave oven, the "Radarange", which was first sold in 1947. He based it on British [[radar]] technology which had been developed before and during [[World War II]].

[[Raytheon]] later licensed its patents for a home-use microwave oven that was introduced by [[Tappan (brand)|Tappan]] in 1955, but it was still too large and expensive for general home use. [[Sharp Corporation]] introduced the first microwave oven with a turntable between 1964 and 1966. The countertop microwave oven was introduced in 1967 by the [[Amana Corporation]]. After microwave ovens became affordable for residential use in the late 1970s, their use spread into commercial and residential [[kitchens]] around the world, and prices fell rapidly during the 1980s. In addition to cooking food, microwave ovens are used for heating in many industrial processes.

Microwave ovens are a common [[kitchen appliance]] and are popular for reheating previously cooked foods and cooking a variety of foods. They rapidly heat foods which can easily burn or turn lumpy if cooked in conventional pans, such as butter, fats, chocolate, or [[porridge]]. Microwave ovens usually do not directly brown or caramelize food, since they rarely attain the necessary temperature to produce [[Maillard reaction]]s. Exceptions occur in cases where the oven is used to heat frying-oil and other oily items (such as bacon), which attain far higher temperatures than that of boiling water.

Microwave ovens have a limited role in professional cooking, because the boiling-range temperatures of a microwave oven do not produce the flavorful chemical reactions that frying, browning, or baking at a higher temperature produces. However, there are hybrid appliances that combine infrared radiation, hot air, and microwaves, such as convection microwave ovens.

== History ==
=== Early developments ===
{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = Cooking with radio waves - Chicago Worlds Fair 1933.jpg
| width1   = 200
| image2   = Cooking with radio waves - Short Wave Craft Nov 1933 cover.jpg
| width2   = 200
| footer    = Demonstration by Westinghouse of cooking sandwiches with a 60 MHz shortwave radio transmitter at the [[1933 Chicago World's Fair]]
}}

The exploitation of high-frequency [[radio wave]]s for heating substances was made possible by the development of [[vacuum tube]] [[radio transmitter]]s around 1920. By 1930 the application of [[short wave]]s to heat human tissue had developed into the medical therapy of [[diathermy]].  At the [[1933 Chicago World's Fair]], [[Westinghouse Electric Corporation|Westinghouse]] demonstrated the cooking of foods between two metal plates attached to a 10&nbsp;kW, 60&nbsp;MHz [[shortwave]] [[transmitter]].<ref>{{cite journal | title = Cooking with Short Waves | journal = Short Wave Craft | volume = 4| issue = 7 | page = 394 | date = November 1933 | url = http://www.americanradiohistory.com/Archive-Short-Wave-Television/30s/SW-TV-1933-11.pdf | access-date = March 23, 2015}}</ref>  The Westinghouse team, led by I. F. Mouromtseff, found that foods like steaks and potatoes could be cooked in minutes.<ref>{{Cite journal |last1=Lovelock |first1=J. E. |last2=Smith |first2=Audrey U. |name-list-style=vanc |year=1956 |title=Studies on Golden Hamsters during Cooling to and Rewarming from Body Temperatures below 0 degrees C. III. Biophysical Aspects and General Discussion |journal=Proceedings of the Royal Society of London. Series B, Biological Sciences |volume=145 |issue=920 |pages=427–442 |bibcode=1956RSPSB.145..427L |doi=10.1098/rspb.1956.0054 |issn=0080-4649 |jstor=83008 |pmid=13359396 |s2cid=6474737}}</ref>

The 1937 United States patent application by Bell Laboratories states:<ref>{{US patent|2147689}} Chaffee, Joseph G., ''Method and apparatus for heating dielectric materials'', filed August 11, 1937; granted February 21, 1939</ref>
{{Blockquote|This invention relates to heating systems for dielectric materials and the object of the invention is to heat such materials uniformly and substantially simultaneously throughout their mass. ... It has been proposed therefore to heat such materials simultaneously throughout their mass by means of the dielectric loss produced in them when they are subjected to a high voltage, high frequency field.}}

However, lower-frequency [[dielectric heating]], as described in the aforementioned patent, is (like [[induction heating]]) an [[electromagnetic]] heating effect, the result of the so-called [[near and far field|near-field]] effects that exist in an electromagnetic cavity that is small compared with the [[wavelength]] of the electromagnetic field. This patent proposed radio frequency heating, at 10 to 20 [[megahertz]] (wavelength 30 to 15 meters, respectively).<ref name=  "patent">{{citation | url = http://pdfpiw.uspto.gov/.piw?PageNum=0&docid=02147689&IDKey=49D95666A76C&HomeUrl=http%3A%2F%2Fpatft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO2%2526Sect2%3DHITOFF%2526u%3D%25252Fnetahtml%25252FPTO%25252Fsearch-adv.htm%2526r%3D1%2526p%3D1%2526f%3DG%2526l%3D50%2526d%3DPALL%2526S1%3D2147689.PN.%2526OS%3Dpn%2F2147689%2526RS%3DPN%2F2147689 | publisher = United States Patent and Trademark Office | title = 2,147,689: Method and Apparatus for Heating Dielectric Materials | first = Joseph G. | last = Chaffee | date = February 21, 1939 | access-date = February 5, 2015 | archive-date = March 19, 2022 | archive-url = https://web.archive.org/web/20220319201814/https://pdfpiw.uspto.gov/.piw?PageNum=0&docid=02147689&IDKey=49D95666A76C&HomeUrl=http%3A%2F%2Fpatft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO2%2526Sect2%3DHITOFF%2526u%3D%25252Fnetahtml%25252FPTO%25252Fsearch-adv.htm%2526r%3D1%2526p%3D1%2526f%3DG%2526l%3D50%2526d%3DPALL%2526S1%3D2147689.PN.%2526OS%3Dpn%2F2147689%2526RS%3DPN%2F2147689 }}</ref> Heating from microwaves that have a wavelength that is small relative to the cavity (as in a modern microwave oven) is due to "far-field" effects that are due to classical [[electromagnetic radiation]] that describes freely propagating light and microwaves suitably far from their source. Nevertheless, the primary heating effect of all types of electromagnetic fields at both radio and microwave frequencies occurs via the dielectric heating effect, as polarized molecules are affected by a rapidly alternating electric field.

=== Cavity magnetron ===
{{Main|Cavity magnetron}}
[[File:Original cavity magnetron, 1940 (9663811280).jpg|thumb|The [[cavity magnetron]] developed by [[John Randall (physicist)|John Randall]] and [[Harry Boot]] in 1940 at the [[University of Birmingham]], England]]

The invention of the [[cavity magnetron]] made possible the production of [[electromagnetic wave]]s of a small enough [[wavelength]] ([[microwave]]s). The cavity magnetron was a crucial component in the development of short wavelength [[radar]] during [[World War II]].<ref>{{cite web | url = http://histru.bournemouth.ac.uk/Oral_History/Talking_About_Technology/radar_research/the_magnetron.html | title = The Magnetron | work = Radar Recollections - A Bournemouth University/CHiDE/HLF project | publisher = Defence Electronics History Society (formerly CHiDE)}}</ref> In 1937&ndash;1940, a multi-cavity magnetron was built by British physicist [[John Randall (physicist)|Sir John Turton Randall, FRSE]] and coworkers, for the British and American military radar installations in World War II.<ref name="Magnetron"/> A higher-powered microwave generator that worked at shorter [[wavelength]]s was needed, and in 1940, at the [[University of Birmingham]] in England, Randall and [[Harry Boot]] produced a working prototype.<ref>{{cite journal|last=Willshaw|first=W. E.|author2=L. Rushforth|author3=A. G. Stainsby|author4=R. Latham|author5=A. W. Balls|author6=A. H. King|title=The High-power Pulsed Magnetron: Development and Design for Radar Applications|journal=Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation|year=1946|volume=93|issue=5|doi=10.1049/ji-3a-1.1946.0188|url=https://ieeexplore.ieee.org/document/5299321|access-date=June 22, 2012|pages=985–1005|archive-url=https://web.archive.org/web/20180505184050/https://ieeexplore.ieee.org/document/5299321/|archive-date=May 5, 2018 |issn=2050-5485|url-access=subscription}}</ref> They invented a [[vacuum tube|valve]] that could produce pulses of microwave radio energy at a wavelength of 10&nbsp;cm, an unprecedented discovery.<ref name="Magnetron">{{cite news|title=Briefcase 'that changed the world'|url=https://news.bbc.co.uk/2/hi/science/nature/6331897.stm|publisher=BBC|date=October 20, 2017}}</ref>

[[Sir Henry Tizard]] traveled to the US in late September 1940 to offer Britain's most valuable technical secrets including the cavity magnetron in exchange for US financial and industrial support (see [[Tizard Mission]]).<ref name="Magnetron"/> An early 6&nbsp;kW version, built in England by the [[General Electric Company]] Research Laboratories, [[Wembley]], London, was given to the [[U.S. government]] in September 1940. The cavity magnetron was later described by American historian James Phinney Baxter III as "[t]he most valuable cargo ever brought to our shores".<ref>{{cite book|first=James Phinney III|last= Baxter|title=Scientists Against Time|location= Boston|publisher=Little, Brown, and Co.|year= 1946|page= 142}}</ref> Contracts were awarded to [[Raytheon]] and other companies for the mass production of the cavity magnetron.

=== Discovery ===
[[File:Wall of microwaves.JPG|thumb|Microwave ovens from the 1980s]]
In 1945, the heating effect of a high-power microwave beam was independently and accidentally discovered by [[Percy Spencer]], an American self-taught engineer from [[Howland, Maine]]. While employed at [[Raytheon]], he noticed that microwaves from an active radar set he was working on started to melt a [[candy bar]] he had in his pocket. The first food deliberately cooked by Spencer was popcorn, and the second was an egg, which exploded in the face of one of the experimenters.<ref>{{cite magazine|magazine=Reader's Digest|date=August 1958|url=http://members.aol.com/spencerlab/history/readdig.htm|title=Percy Spencer and His Itch to Know|author=Don Murray|access-date=January 14, 2025|archive-date=March 6, 2005|archive-url=https://web.archive.org/web/20050306112532/http://members.aol.com/spencerlab/history/readdig.htm|url-status=bot: unknown}}</ref><ref>{{cite web| url=http://www.gallawa.com/microtech/history.html | archive-url=https://web.archive.org/web/20110709081022/http://www.gallawa.com/microtech/history.html | archive-date=July 9, 2011 |title=The History of the Microwave Oven |first=John Carlton|last= Gallawa |year=1998}}</ref><ref>{{YouTube|4h1ESUz2H3E|Radar — Father of the Microwave Oven}}</ref>

To verify his finding, Spencer created a high-density electromagnetic field by feeding microwave power from a magnetron into a metal box from which it had no way to escape. When food was placed in the box with the microwave energy, the temperature of the food rose rapidly. On October 8, 1945, Raytheon filed a United States patent application for Spencer's microwave cooking process, and an oven that heated food using microwave energy from a magnetron was soon placed in a Boston restaurant for testing.<ref>{{US patent|2495429}}, Method of treating foodstuffs; application: October 8, 1945, issued: January 24, 1950, invented by Percy L Spencer. </ref>

=== Commercial availability ===
[[File:NS Savannah microwave oven MD8.jpg|thumb|left|upright|Raytheon RadaRange aboard the [[NS Savannah|NS ''Savannah'']] nuclear-powered cargo ship, installed circa 1961]]
In 1947, Raytheon built the "Radarange", the first commercially available microwave oven.<ref>{{cite web| url=http://www.raytheon.com/ourcompany/history/leadership/ |archive-url=https://web.archive.org/web/20130322044917/http://www.raytheon.com/ourcompany/history/leadership/ |archive-date=March 22, 2013 |title=Technology Leadership |publisher=Raytheon}}</ref> It was almost {{convert|1.8|m|ftin}} tall, weighed {{convert|340|kg|lb}} and cost between $2,000 and $3,000 (${{formatnum:{{Inflation|US|2000|1947|r=-3}}}}  to ${{formatnum:{{Inflation|US|3000|1947|r=-3}}}} in {{Inflation-year|US}} dollars) each. It consumed 3 kilowatts, about three times as much as today's microwave ovens, and was water-cooled. The name was the winning entry in an employee contest.<ref>{{cite book|last= Gallawa|first= J Carlton|chapter= A Brief History of the Microwave Oven|title= The complete microwave oven service handbook: operation, maintenance, troubleshooting, and repair|chapter-url= http://www.smecc.org/microwave_oven.htm|location= Englewood Cliffs, N.J.|publisher= Prentice Hall|date= 1989|access-date= October 11, 2017|isbn= 978-0-13-162017-9|oclc= 18559256|url-access= registration|url= https://archive.org/details/completemicrowav00gall}} Chapter link is hosted at the Southwest Museum of Engineering, Communication and Computation; [[Glendale, Arizona]].</ref> An early Radarange was installed (and remains) in the galley of the nuclear-powered passenger/cargo ship [[NS Savannah|NS ''Savannah'']]. An early commercial model introduced in 1954 consumed 1.6 kilowatts and sold for US$2,000 to US$3,000 (${{formatnum:{{Inflation|US|2000|1954|r=-3}}}} to ${{formatnum:{{Inflation|US|3000|1954|r=-3}}}} in {{Inflation-year|US}} dollars). Raytheon licensed its technology to the [[Tappan (brand)|Tappan]] Stove company of [[Mansfield, Ohio]] in 1952.<ref>{{cite web|url=https://ohiohistory.wordpress.com/2010/11/02/do-you-remember-your-familys-first-microwave/ |title=Do you remember your family's first microwave? |work=Ohio Historical Society |date=November 2, 2010 |archive-url=https://web.archive.org/web/20160422025448/https://ohiohistory.wordpress.com/2010/11/02/do-you-remember-your-familys-first-microwave/ |archive-date=April 22, 2016 }}</ref> Under contract to Whirlpool, Westinghouse, and other major appliance manufacturers looking to add matching microwave ovens to their conventional oven line, Tappan produced several variations of their built-in model from roughly 1955 to 1960. Due to maintenance (some units were water-cooled), in-built requirement, and cost—US$1,295 (${{formatnum:{{Inflation|US|1295|1955|r=-3}}}} in {{Inflation-year|US}} dollars)—sales were limited.<ref>{{cite web | url=https://spectrum.ieee.org/a-brief-history-of-the-microwave-oven | title=A Brief History of the Microwave Oven }}</ref>

Japan's [[Sharp Corporation]] began manufacturing microwave ovens in 1961. Between 1964 and 1966, Sharp introduced the first microwave oven with a turntable, an alternative means to promote more even heating of food.<ref>{{cite web |title=History of Sharp |url=http://www.sharpusa.com/aboutsharp/companyprofile/sharpandtechnologyhistory.aspx |website=[[Sharp Corporation]] |access-date=June 26, 2018 |language=en}}</ref> In 1965, Raytheon, looking to expand their Radarange technology into the home market, acquired [[Amana (appliances)|Amana]] to provide more manufacturing capability. In 1967, they introduced the first popular home model, the countertop Radarange, at a price of US$495 (${{formatnum:{{Inflation|US|495|1967|r=-3}}}} in {{Inflation-year|US}} dollars). Unlike the Sharp models, a motor driven [[mode stirrer]] in the top of the oven cavity rotated allowing the food to remain stationary.

=== Residential use ===
[[File:Fiskarsin mikrouuni 1965 (2).jpg|thumb|Finnish [[Fiskars]] microwave from 1965]]
While uncommon today, combination microwave-ranges were offered by major appliance manufacturers through much of the 1970s as a natural progression of the technology. Both Tappan and General Electric offered units that appeared to be conventional stove top/oven ranges, but included microwave capability in the conventional oven cavity. Such ranges were attractive to consumers since both microwave energy and conventional heating elements could be used simultaneously to speed cooking, and there was no loss of countertop space. The proposition was also attractive to manufacturers as the additional component cost could better be absorbed compared with countertop units where pricing was increasingly market-sensitive.{{Citation needed|date=February 2025}}

By 1972, Litton (Litton Atherton Division, Minneapolis) introduced two new microwave ovens, priced at $349 and $399, to tap into the market estimated at $750 million by 1976, according to Robert I Bruder, president of the division.<ref>[https://www.nytimes.com/1972/07/14/archives/litton-introduces-microwave-ovens.html Litton Introduces Microwave Ovens]. [[New York Times]], July 14, 1972, p. 38.</ref> While prices remained high, new features continued to be added to home models. Amana introduced automatic defrost in 1974 on their RR-4D model, and was the first to offer a microprocessor controlled digital control panel in 1975 with their RR-6 model.
[[File:1971rr4.jpg|thumb|1974 Radarange [[RR-4]].
By the late 1970s, technological advances led to rapidly falling prices. Often called "electronic ovens" in the 1960s, the name "microwave oven" later gained currency, and they are now informally called "microwaves".]]

The late 1970s saw an explosion of low-cost countertop models from many major manufacturers.{{Citation needed|date=February 2025}}

In addition to cooking food, microwave ovens are used for heating in many industrial processes.<ref>{{Cite book |last1=Metaxas |first1=A. C. |title=Industrial microwave heating |last2=Meredith |first2=Roger |date=2008 |publisher=The Institution of Engineering and Technology |isbn=978-0-906048-89-4 |edition=Reprinted |series=IEE power engineering series |location=London}}</ref> Formerly found only in large industrial applications, microwave ovens increasingly became a standard fixture of residential kitchens in [[developed countries]]. By 1986, roughly 25% of households in the U.S. owned a microwave oven, up from only about 1% in 1971;<ref name="CPI_US">{{citation | url = http://www.bls.gov/cpi/cpimwo.htm | title = Hedonic Quality Adjustment Methods For Microwave Ovens In the U.S. CPI | first = Paul R. | last = Liegey | date = October 16, 2001 | access-date = October 5, 2013 | publisher = Bureau of Labor Statistics, United States Department of Labor}}</ref> the U.S. Bureau of Labor Statistics reported that over 90% of American households owned a microwave oven in 1997.<ref name="CPI_US"/><ref>{{citation | last1 = Cox | first1 = W. Michael | last2 = Alm | first2 = Richard | year = 1997 | title = Time Well Spent: The Declining Real Cost of Living in America | publisher = Federal Reserve Bank of Dallas | work = 1997 Annual Report | page = 22 (see Exhibit 8) | url = http://www.dallasfed.org/htm/pubs/pdfs/anreport/arpt97.pdf | access-date = May 8, 2016 | archive-date = October 19, 2004 | archive-url = https://web.archive.org/web/20041019184847/http://www.dallasfed.org/htm/pubs/pdfs/anreport/arpt97.pdf | df = dmy-all }}</ref> In Australia, a 2008 market research study found that 95% of kitchens contained a microwave oven and that 83% of them were used daily.<ref>{{citation | url = http://newsroom.electrolux.com/au/files/2010/01/Westinghouse-How-Australia-Cooks-Report1.pdf | title = The Westinghouse How Australia Cooks Report | date = October 2008 | publisher = Westinghouse | access-date = February 5, 2015 | archive-date = February 5, 2015 | archive-url = https://web.archive.org/web/20150205205537/http://newsroom.electrolux.com/au/files/2010/01/Westinghouse-How-Australia-Cooks-Report1.pdf }}</ref> In Canada, fewer than 5% of households had a microwave oven in 1979, but more than 88% of households owned one by 1998.<ref>{{cite journal | url = http://www.statcan.gc.ca/pub/11-008-x/11-008-x2000003-eng.pdf | title = Income and expenditures | first = Cara | last = Williams | date =Winter 2000 | number = 59 | journal = Canadian Social Trends — Catalogue No. 11-008 | pages = 7–12 | quote = Microwaves have been adopted even more avidly: in 1979, less than 5% of households had one, but by 1998 over 88% did.}}</ref> In France, 40% of households owned a microwave oven in 1994, but that number had increased to 65% by 2004.<ref>{{citation | url = http://www.freedoniagroup.com/brochure/20xx/2015smwe.pdf | title = World Major Household Appliances: World Industry Study with Forecasts to 2009 & 2014 (Study #2015) | date = January 2006 | at = TABLE VI-5: FRANCE COOKING APPLIANCES SUPPLY & DEMAND (million dollars) | publisher = The Freedonia Group | location = Cleveland, Ohio}}</ref>

Adoption has been slower in [[Developing country|less-developed countries]], as households with disposable income concentrate on more important household appliances like [[refrigerators]] and ovens. In [[India]], for example, only about 5% of households owned a microwave oven in 2013, well behind refrigerators at 31% ownership.<ref>{{cite web | title = Household penetration rate of home appliances in India in 2013 | url = http://www.statista.com/statistics/370635/household-penetration-home-appliances-india/ | access-date = February 5, 2015 | publisher = Statistica}}</ref> However, microwave ovens are gaining popularity. In Russia, for example, the number of households with a microwave oven grew from almost 24% in 2002 to almost 40% in 2008.<ref name=USDA-microwave/> Almost twice as many households in South Africa owned microwave ovens in 2008 (38.7%) as in 2002 (19.8%).<ref name=USDA-microwave/> Microwave oven ownership in Vietnam in 2008 was at 16% of households, versus 30% ownership of refrigerators; this rate was up significantly from 6.7% microwave oven ownership in 2002, with 14% ownership for refrigerators that year.<ref name=USDA-microwave>{{citation | url = http://www.ers.usda.gov/media/612721/householdamenities.xls | archive-url = https://web.archive.org/web/20130626225554/http://www.ers.usda.gov/media/612721/householdamenities.xls | archive-date = June 26, 2013 | format = XLS | title = Ownership of household amenities among selected countries | publisher = Economic Research Service, United States Department of Agriculture | year = 2009 | access-date = February 5, 2015 }}</ref>

Consumer household microwave ovens usually come with a cooking power of between 600 and 1200 watts. Microwave cooking power, also referred to as output wattage, is lower than its input wattage, which is the manufacturer's listed [[power rating]].{{Citation needed|date=February 2025}}

The size of household microwave ovens can vary, but usually have an internal volume of around {{convert|20|L|cuin cuft|sp=us|}}, and external dimensions of approximately {{convert|45|-|60|cm|ftin|abbr=on|}} wide, {{convert|35|-|40|cm|ftin|abbr=on|}} deep and {{convert|25|-|35|cm|ftin|abbr=on|}} tall.<ref>{{Cite web|last=Francis|first=Andrew|date=April 12, 2017|title=Microwave Sizes Comparison|url=https://www.reviewho.com/microwave-sizes-and-dimensions-guide/|access-date=March 10, 2021|website=reviewho.com|language=en}}</ref> Countertop microwaves vary in weight 23 – 45 lbs.<ref>{{Cite web |date=November 17, 2020 |title=How Much Do Microwaves Weigh? Averages from 54 Models |url=https://applianceanalysts.com/microwave-weights/ |access-date=December 17, 2024 |website=ApplianceAnalysts |language=en}}</ref>

Simply radiating microwaves into a chamber intrinsically results in very uneven energy distribution, over- and under-heating various parts of the food. Only the cheapest units lack mechanisms that mitigate this problem, requiring the user to more frequently stop the cooking to stir or move the food. For reasonably even heating, a microwave oven must have either multiple magnetrons, common in commercial units, a microwave stirrer, which is a fan-like mechanism in which microwave-reflective blades spin to vary the radiation pattern (usually at the top of the chamber, hidden above a microwave-transparent plastic cover), or a turntable, which effectively reduces the capacity of the oven to the space above the turntable, wasting at least nearly a quarter of the volume of a square chamber. Food is always placed above the bottom of the microwave-reflecting chamber, allowing microwave energy to reach the bottom of the food; in a ''flatbed'' unit the floor of the visible chamber is of a microwave-transparent material above the microwave-reflecting bottom, and in better units some of the microwave radiation enters through the transparent floor. In a non-flatbed unit there is either a glass tray shaped to fit the chamber, keeping the food some distance from the reflective bottom, or, in a unit with a turntable, the turntable itself is microwave-transparent and sits above the reflective bottom.<ref name=":1">{{Cite book|last=Williams|first=Alison|url=https://escholarship.org/content/qt3s29h7wd/qt3s29h7wd.pdf|title=Surveys of Microwave Ovens in U.S. Homes|publisher=Lawrence Berkeley National Laboratory|date=December 5, 2012|pages=6, 18 and so on}}</ref><ref>{{Cite journal|last=ANNIS|first=PATTY J.|date=August 1, 1980|title=Design and Use of Domestic Microwave Ovens|journal=Journal of Food Protection|volume=43|issue=8|pages=629–632|doi=10.4315/0362-028X-43.8.629|pmid=30822984|issn=0362-028X|doi-access=free}}</ref><ref name=":2">{{Cite journal|date=July 1, 2016|title=Thawing in a microwave cavity: Comprehensive understanding of inverter and cycled heating|url=https://www.sciencedirect.com/science/article/abs/pii/S0260877416300395|journal=Journal of Food Engineering|language=en|volume=180|pages=87–100|doi=10.1016/j.jfoodeng.2016.02.007|issn=0260-8774|last1=Chen|first1=Fangyuan|last2=Warning|first2=Alexander D.|last3=Datta|first3=Ashim K.|last4=Chen|first4=Xing|url-access=subscription}}</ref>

By position and type, [[US DOE]] classifies them as (1) [[countertop]] or (2) [[Kitchen stove|over the range]] and built-in (wall oven for a [[Cabinetry|cabinet]] or a [[drawer]] model).<ref name=":1" />

A conventional microwave oven has only one output power level, with intermediate power settings achieved using [[duty cycle|duty-cycle modulation]], switching the magnetron on and off every few seconds, with more time on for higher settings.{{Citation needed|date=February 2025}} An ''inverter''-type unit, however, can effectively sustain lower power output instead of switching full power on and off, heating food more evenly. Apart from offering superior cooking ability, these microwaves are generally more energy-efficient.<ref name=":3">{{Cite journal|last1=Kako|first1=H.|last2=Nakagawa|first2=T.|last3=Narita|first3=R.|date=August 1991|title=Development of compact inverter power supply for microwave oven|journal=IEEE Transactions on Consumer Electronics|volume=37|issue=3|pages=611–616|doi=10.1109/30.85575|s2cid=108870083 |issn=1558-4127}}</ref><ref name=":2" /><ref name=":4">{{Cite journal|last1=Lee|first1=Min-Ki|last2=Koh|first2=Kang-Hoon|last3=Lee|first3=Hyun--Woo|date=2004|title=A Study on Constant Power Control of Half Bridge Inverter for Microwave Oven|url=https://www.koreascience.or.kr/article/JAKO200411922336424.page|journal=KIEE International Transaction on Electrical Machinery and Energy Conversion Systems|volume=4B|issue=2|pages=73–79|issn=1598-2602}}</ref>

{{As of|2020}}, the majority of countertop microwave ovens (regardless of brand) sold in the United States were manufactured by the [[Midea Group]].<ref>{{cite news |last1=McCabe |first1=Liam |last2=Sullivan |first2=Michael |title=The Best Microwave |url=https://thewirecutter.com/reviews/best-microwave/ |access-date=May 21, 2020 |work=Wirecutter |publisher=The New York Times |date=May 20, 2020}}</ref>

=== Categories ===
[[File:Symbol Microwave oven.svg|thumb|50x50px|Microwave-safe symbol]]
Domestic microwave ovens are typically marked with the microwave-safe symbol, next to the device's approximate IEC 60705 output power rating, in watts (typically either: 600W, 700W, 800W, 900W, 1000W), and a voluntary Heating Category (A-E).<ref>{{Cite web |title=Microwave Fact Sheets |url=https://www.microwaveassociation.org.uk/factsheets/index.php |access-date=March 12, 2023 |website=www.microwaveassociation.org.uk}}</ref>

== Principles ==
{{further|Dielectric heating}}
[[File:Weibolu.jpg|thumb|250px|A microwave oven, c. 2005]]
[[File:Microwaveoventransient.webm|thumb|250px|Simulation of the electric field inside a microwave oven for the first 8 ns of operation]]

A microwave oven heats food by passing [[microwave radiation]] through it. Microwaves are a form of [[non-ionizing]] [[electromagnetic radiation]] with a [[frequency]] in the so-called [[Microwave|microwave region]] (300&nbsp;MHz to 300&nbsp;GHz). Microwave ovens use frequencies in one of the [[ISM band|ISM (industrial, scientific, medical) bands]], which are otherwise used for communication amongst devices that do not need a license to operate, so they do not interfere with other vital radio services.

It is a common misconception that microwave ovens heat food by operating at a special resonance of water molecules in the food.<ref>{{cite book|last=Schmitt|first=Ron|year=2002|title=Electromagnetics Explained|isbn=978-0-7506-7403-4 |url=https://books.google.com/books?id=MLzPNpJQz9UC|page=343|publisher=Newnes |quote=Referring to the Figure 15.2, you can see that there is no resonance of water at this frequency. The first resonant peak occurs above 1THz, and the highest loss occurs well into the infrared. There is no special significance of 2.45 GHz, except that it is allocated by the FCC as being allowable for microwave oven usage.}}</ref>{{Verify source|date=July 2025|reason=the quote doesn't verify this, but maybe something else in the source does}} Instead, microwave ovens heat by causing molecules to spin under the influence of a constantly changing electric field ([[dielectric heating]]), usually in the microwave frequencies range, and a higher wattage power of the microwave oven results in faster cooking times. Typically, consumer ovens work around a nominal 2.45&nbsp;[[gigahertz]] (GHz) – a [[wavelength]] of {{convert|12.2|cm|sigfig=3}} in the 2.4&nbsp;GHz to 2.5&nbsp;GHz ISM band – while large industrial / commercial ovens often use 915&nbsp;[[megahertz]] (MHz) – {{convert|32.8|cm|sigfig=3}}.<ref>{{cite web |title=For heat, tune to 915 or 2450&nbsp;Megacycles |year=2007 |publisher=[[Litton Industries]] |orig-date=1965 |via=Southwest Museum of Engineering, Communications, and Computation |url=http://www.smecc.org/litton_-_for_heat,_tune_to_915_or_2450_megacycles.htm |access-date=December 12, 2006}}</ref> Among other differences, the longer wavelength of a commercial microwave oven allows the initial heating effects to begin deeper within the food or liquid, and therefore become evenly spread within its bulk sooner, as well as raising the temperature deep within the food more quickly.<ref name=depth>{{cite web|title=Cooking with Microwave Ovens|website=Food Safety and Inspection Service|publisher=[[U.S. Department of Agriculture]]|url=https://www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/food-safety-basics/cooking-microwave-ovens#3|access-date=June 22, 2022|archive-date=May 2, 2025|archive-url=https://web.archive.org/web/20250502014426/https://www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/food-safety-basics/cooking-microwave-ovens#3}}</ref>

A microwave oven takes advantage of the electric [[dipole]] structure of [[water]] [[molecule]]s, fats, and many other substances in the food, using a process known as [[dielectric heating]]. These molecules have a partial positive charge at one end and a partial negative charge at the other. In an alternating electric field, they will constantly spin around as they continually try to align themselves with the electric field. This can happen over a wide range of frequencies.<ref name="Soltysiak">{{cite conference |last1=Soltysiak |first1=Michal |last2=Celuch |first2=Malgorzata |last3=Erle |first3=Ulrich |title=Measured and simulated frequency spectra of the household microwave oven |conference=2011 IEEE MTT-S International Microwave Symposium |date=June 2011 |pages=1–4 |doi=10.1109/MWSYM.2011.5972844 |isbn=978-1-61284-754-2 |s2cid=41526758}}</ref><ref name=microwave>{{cite web |author=Bloomfield, Louis |title=Question 1456 |website=How Everything Works |url=http://www.howeverythingworks.org/page1.php?QNum=1456 |access-date=February 9, 2012 |archive-url=https://web.archive.org/web/20131017005928/http://www.howeverythingworks.org/page1.php?QNum=1456 |archive-date=October 17, 2013}}</ref><ref>{{cite web |first=Christopher S. |last=Baird |date=October 15, 2014 |title=Why are the microwaves in a microwave oven tuned to water |website=Science Questions with Surprising Answers |publisher=[[West Texas A&M University]] |place=Canyon, TX |url=http://wtamu.edu/~cbaird/sq/2014/10/15/why-are-the-microwaves-in-a-microwave-oven-tuned-to-water/}}</ref> The electric field's energy is absorbed by the dipole molecules as rotational energy.  Then they hit non-dipole molecules, making them move faster as well. This energy is shared deeper into the substance as molecular rotation and translational movement occurs, signifying an increase in the [[Temperature#Kinetic theory approach|temperature]] of the food. Once the electrical field's energy is initially absorbed, heat will gradually spread through the object similarly to any other heat transfer by contact with a hotter body.<ref>{{cite book |last=Zitzewitz |first=Paul W. |date=February 2011 |title=The Handy Physics Answer Book |publisher=Visible Ink Press |isbn=978-1-57859-357-6 |language=en |url=https://books.google.com/books?id=qGTkgFZBJZQC&q=microwave+oven+principle&pg=PA153 |via=Google Books}}</ref>

=== Defrosting ===
Microwave heating is more efficient on liquid water than on frozen water, where the movement of molecules is more restricted.  Defrosting is done at a low power setting, allowing time for conduction to carry heat to still frozen parts of food. Dielectric heating of liquid water is also temperature-dependent: At 0&nbsp;°C, [[dielectric loss]] is greatest at a field frequency of about 10&nbsp;GHz, and for higher water temperatures at higher field frequencies.<ref name=Martin>{{cite web |last=Chaplin |first=Martin |title=Water and microwaves |url=http://www.lsbu.ac.uk/water/microwave.html |series=Water Structure and Science |publisher=[[London South Bank University]] |access-date=December 4, 2012 |date=May 28, 2012}}</ref>

=== Fats and sugar ===
[[Sugar]]s and [[triglyceride]]s (fats and oils) absorb microwaves due to the dipole moments of their [[hydroxyl groups]] or [[ester|ester groups]]. Due to their [[molecular dipole moment]], however, they are heated less efficiently.{{efn|Here "efficient" means that more energy is deposited and temperature rises faster, not necessarily that the temperature rises to a higher maximum. The maximum temperature is also a function of the material's [[specific heat capacity]], which for most substances is lower than water. For a practical example, milk heats slightly faster than water in a microwave oven, but only because milk solids have less heat capacity than the water they replace.{{Citation needed|date=January 2009}}}}

Although fats and sugar typically absorb energy less efficiently than water, paradoxically their temperatures rise faster and higher than water when cooking: Fats and oils require less energy delivered per gram of material to raise their temperature by 1&nbsp;°C than does water (they have lower [[specific heat capacity]]) and they begin cooling off by "boiling" only after reaching a higher temperature than water (the temperature they require to [[vaporize]] is higher), so inside microwave ovens they normally reach higher temperatures – sometimes ''much'' higher.<ref name=Martin/> This can induce temperatures in oil or fatty foods like bacon far above the boiling point of water, and high enough to induce some browning reactions, much in the manner of conventional [[grilling|broiling (UK: grilling)]], braising, or deep fat frying.{{citation needed|date=July 2020}}

The effect is most often noticed by consumers from unexpected damage to plastic containers when microwaving foods high in sugar, starch, or fat generates higher temperatures.{{Citation needed|date=February 2025}} Foods high in water content and with little oil rarely exceed the boiling temperature of water and do not damage plastic.{{Citation needed|date=February 2025}}

=== Cookware ===
[[Cookware]] must be transparent to microwaves. Conductive cookware, such as metal pots, reflects microwaves, and prevents the microwaves from reaching the food.{{Citation needed|date=February 2025}} Cookware made of materials with high [[Relative permittivity|electrical permittivity]] will absorb microwaves, resulting in the cookware heating rather than the food. Cookware made of [[melamine resin]] is a common type of cookware that will heat in a microwave oven, reducing the effectiveness of the microwave oven and creating a hazard from burns or shattered cookware.{{Citation needed|date=February 2025}}

=== Thermal runaway ===
Microwave heating can cause localized [[thermal runaway]]s in some materials with low thermal conductivity which also have dielectric constants that increase with temperature. An example is glass, which can exhibit thermal runaway in a microwave oven to the point of melting if preheated. Additionally, microwaves can melt certain types of rocks, producing small quantities of molten rock. Some ceramics can also be melted, and may even become clear upon cooling. Thermal runaway is more typical of electrically conductive liquids such as salty water.<ref>{{cite conference |last1=Jerby |first1=Eli |last2=Meir |first2=Yehuda |last3=Faran |first3=Mubarak |title=Basalt melting by localized-microwave thermal-runaway instability |doi=10.13140/2.1.4346.1126 |url=https://www.eng.tau.ac.il/~jerby/Jerby_Basalt_Ampere-2013_Proc_PDF.pdf |conference=14th International Conference on Microwave and High Frequency Heating, AMPERE-2013 |location=Nottingham, UK |date=September 2013}}</ref>

=== Penetration ===
Another misconception is that microwave ovens cook food "from the inside out", meaning from the center of the entire mass of food outwards.{{Citation needed|date=February 2025}} This idea arises from heating behavior seen if an absorbent layer of water lies beneath a less absorbent drier layer at the surface of a food; in this case, the deposition of heat energy inside a food can exceed that on its surface.{{Citation needed|date=February 2025}} This can also occur if the inner layer has a lower heat capacity than the outer layer causing it to reach a higher temperature, or even if the inner layer is more thermally conductive than the outer layer making it feel hotter despite having a lower temperature. In most cases, however, with uniformly structured or reasonably homogeneous food item, microwaves are absorbed in the outer layers of the item at a similar level to that of the inner layers.{{Citation needed|date=February 2025}}

Depending on water content, the depth of initial heat deposition may be several centimetres or more with microwave ovens, in contrast with [[grilling|broiling / grilling]] (infrared) or convection heating methods which thinly deposit heat at the food surface. Penetration depth of microwaves depends on [[food composition]] and the frequency, with lower microwave frequencies (longer wavelengths) penetrating deeper.<ref name="depth"/>

=== Energy consumption ===
In use, microwave ovens can be as low as 50% efficient at converting electricity into microwaves,<ref>{{cite web |last1=Wirfs-Brock |first1=Jordan |last2=Jacobson |first2=Rebecca |date=February 23, 2016 |title=A watched pot: What is the most energy-efficient way to boil water? |website=Inside Energy |url=http://insideenergy.org/2016/02/23/boiling-water-ieq/}}</ref> but energy-efficient models can exceed 64% efficiency.<ref>{{cite web |title=Energy Label- Criteria |website=www.energylabel.org.tw |publisher=Bureau of Energy, Ministry of Economic Affairs, Taiwan |url=https://www.energylabel.org.tw/englishlabel/application_en/efficiency/upt.aspx?Cid=48 |access-date=March 7, 2022 |language=zh-Hant-TW}}</ref> Stovetop cooking is 40–90% efficient, depending on the type of appliance used.<ref>{{cite web |title=Level-up your cooking game with an energy-efficient stovetop |date=August 24, 2020 |url=https://leap-va.org/energy-news/level-up-your-cooking-game-with-an-energy-efficient-stovetop/ |access-date=December 23, 2022 |archive-date=December 23, 2022 |archive-url=https://web.archive.org/web/20221223161204/https://leap-va.org/energy-news/level-up-your-cooking-game-with-an-energy-efficient-stovetop/ }}</ref>

Because they are used fairly infrequently, the average residential microwave oven consumes only 72&nbsp;kWh per year.<ref>{{cite journal |last1=Gallego-Schmid |first1=Alejandro |last2=Mendoza |first2=Joan Manuel F. |last3=Azapagic |first3=Adisa |title=Environmental assessment of microwaves and the effect of European energy efficiency and waste management legislation |journal=Science of the Total Environment |date=March 2018 |volume=618 |pages=487–499 |doi=10.1016/j.scitotenv.2017.11.064 |pmid=29145100 |bibcode=2018ScTEn.618..487G |url=https://www.research.manchester.ac.uk/portal/en/publications/environmental-assessment-of-microwaves-and-the-effect-of-european-energy-efficiency-and-waste-management-legislation(ac7e65c2-c53e-4d0f-9bc8-b03ceb65541f).html }}</ref> Globally, microwave ovens used an estimated 77&nbsp;TWh per year in 2018, or 0.3% of global electricity generation.<ref>{{cite journal |last1=Detz |first1=Remko J. |last2=van der Zwaan |first2=Bob |title=Surfing the microwave oven learning curve |journal=Journal of Cleaner Production |date=October 20, 2020 |volume=271 |article-number=122278 |doi=10.1016/j.jclepro.2020.122278 |s2cid=225872878 |doi-access=free |bibcode=2020JCPro.27122278D }}</ref>

A 2000 study by [[Lawrence Berkeley National Laboratory]] found that the average microwave drew almost 3 watts of [[standby power]] when not being used,<ref>{{cite book |last1=Ross |first1=J.P. |last2=Meier |first2=Alan |title=Energy Efficiency in Household Appliances and Lighting |chapter=Whole-House Measurements of Standby Power Consumption |year=2001 |pages=278–285 |doi=10.1007/978-3-642-56531-1_33 |osti=793739 |isbn=978-3-540-41482-7 |chapter-url=https://www.osti.gov/biblio/793739 }}</ref> which would total approximately 26&nbsp;kWh per year. New efficiency standards imposed in 2016 by the [[United States Department of Energy]] require less than 1&nbsp;watt, or approximately 9&nbsp;kWh per year, of standby power for most types of microwave ovens.<ref>{{cite web |last=de&nbsp;Laski |first=Andrew |date=June 3, 2013 |title=New standards cut "vampire" energy waste |website=Appliance Standards Awareness Project (ASAP) |url=https://appliance-standards.org/blog/new-standards-cut-vampire-energy-waste |access-date=October 4, 2021}}</ref>

== Components ==
[[File:Magnetron2.jpg|thumb|A magnetron with section removed (magnet is not shown)]]
[[File:-125wiki.jpg|thumb|Inner space of a microwave oven and its control panel]]
A microwave oven generally consists of:
* a high-voltage DC power source, either:
** a large high voltage [[transformer]] with a [[Voltage doubler#Villard circuit|voltage doubler]] (a high-voltage [[capacitor]] and a [[diode]])
** an electronic [[power converter]] usually based around an inverter.
* a [[cavity magnetron]], which converts the high-voltage DC electric energy to microwave radiation
* a magnetron control circuit (usually with a [[microcontroller]])
* a short [[Waveguide (electromagnetism)|waveguide]] (to couple microwave power from the magnetron into the cooking chamber)
* a turntable and/or [[mode stirrer|metal wave guide stirring fan]]
* a control panel

In most ovens, the magnetron is driven by a linear transformer which can only feasibly be switched completely on or off. (One variant of the GE Spacemaker had two taps on the transformer primary, for high and low power modes.) Usually choice of power level does not affect intensity of the microwave radiation; instead, the magnetron is cycled on and off every few seconds, thus altering the large scale [[duty cycle]]. Newer models use ''inverter'' power supplies that use [[pulse-width modulation]] to provide effectively continuous heating at reduced power settings, so that foods are heated more evenly at a given power level and can be heated more quickly without being damaged by uneven heating.<ref>{{cite web|url=http://www.thetelegram.com/opinion/columnists/a-notable-advance-in-microwave-technology-135137/|title=A notable advance in microwave technology|work=[[The Telegram]]|date=September 22, 2013|access-date=May 10, 2018|archive-url=https://web.archive.org/web/20180502233757if_/http://www.thetelegram.com/opinion/columnists/a-notable-advance-in-microwave-technology-135137/|archive-date=May 2, 2018}}</ref><ref name=":3" /><ref name=":2" /><ref name=":4" />

The microwave frequencies used in microwave ovens are chosen based on regulatory and cost constraints. The first is that they should be in one of the [[ISM band|industrial, scientific, and medical (ISM) frequency band]]s set aside for unlicensed purposes. For household purposes, 2.45&nbsp;GHz has the advantage over 915&nbsp;MHz in that 915&nbsp;MHz is only an ISM band in some countries ([[ITU Region]] 2) while 2.45&nbsp;GHz is available worldwide.<ref>{{cite web |title=Low-Power RF Devices and the ISM Bands |url=https://www.allaboutcircuits.com/textbook/radio-frequency-analysis-design/electromagnetic-spectrum/low-power-rf-devices-and-the-ism-bands/ |website=All About Circuits}}</ref> Three additional ISM bands exist in the microwave frequencies, but are not used for microwave cooking. Two of them are centered on 5.8&nbsp;GHz and 24.125&nbsp;GHz, but are not used for microwave cooking because of the very high cost of power generation at these frequencies.{{citation needed|date=May 2019}} The third, centered on 433.92&nbsp;MHz, is a narrow band that would require expensive equipment to generate sufficient power without creating interference outside the band, and is only available in some countries.{{citation needed|date=May 2019}}

The cooking chamber is similar to a [[Faraday cage]] to prevent the waves from coming out of the oven. Even though there is no continuous metal-to-metal contact around the rim of the door, [[Waveguide flange#Choke connection|choke connections]] on the door edges act like metal-to-metal contact, at the frequency of the microwaves, to prevent leakage. The oven door usually has a window for easy viewing, with a layer of conductive mesh some distance from the outer panel to maintain the shielding. Because the size of the perforations in the mesh is much less than the microwaves' wavelength (12.2&nbsp;cm for the usual 2.45&nbsp;GHz), microwave radiation cannot pass through the door, while [[visible light]] (with its much shorter wavelength) can.<ref>{{Cite web|last=Staff|first=Straight Dope|date=November 4, 2003|title=What keeps microwave radiation from leaking out the oven door?|url=https://www.straightdope.com/21343269/what-keeps-microwave-radiation-from-leaking-out-the-oven-door|access-date=March 1, 2021|website=The Straight Dope|language=en}}</ref>

=== Control panel ===
Modern microwave ovens use either an analog dial-type [[timer]] or a digital [[Control panel (engineering)|control panel]] for operation. Control panels feature an [[LED]], [[LCD]] or vacuum fluorescent display, buttons for entering the cook time and a power level selection feature.  A defrost option is typically offered, as either a power level or a separate function.  Some models include pre-programmed settings for different food types, typically taking weight as input.  In the 1990s, brands such as Panasonic and GE began offering models with a scrolling-text display showing cooking instructions.

Power settings are commonly implemented not by actually varying the power output, but by switching the emission of microwave energy off and on at intervals. The highest setting thus represents continuous power. Defrost might represent power for two seconds followed by no power for five seconds. To indicate cooking has completed, an audible warning such as a bell or a beeper is usually present, and/or "End" usually appears on the display of a digital microwave.

Microwave control panels are often considered awkward to use and are frequently employed as examples for user interface design.<ref>{{cite web|url=https://www.theguardian.com/technology/2015/jul/13/microwave-oven-user-interface-digital-complicated |title=User Interfaces: Why are Microwave Ovens All So Difficult to Use?|work=[[The Guardian]]|date= July 13, 2015|access-date= January 4, 2019}}</ref>

== Variants and accessories ==
A variant of the conventional microwave oven is the convection microwave oven. A convection microwave oven is a combination of a standard microwave oven and a [[convection oven]]. It allows food to be cooked quickly, yet come out browned or crisped, as from a convection oven. Convection microwave ovens are more expensive than conventional microwave ovens. Some convection microwave ovens—those with exposed heating elements—can produce smoke and burning odors as food spatter from earlier microwave-only use is burned off the heating elements. Some ovens use high speed air; these are known as impingement ovens and are designed to cook food quickly in restaurants, but cost more and consume more power.

In 2000, some manufacturers began offering high power [[quartz]] [[halogen bulb]]s to their convection microwave oven models,<ref>{{cite news|last= Fabricant |first= Florence |title= Son of Microwave: Fast and Crisp |url= https://www.nytimes.com/2000/09/27/dining/test-kitchen-son-of-microwave-fast-and-crisp.html |date=September 27, 2000|access-date=January 6, 2015|work=[[The New York Times]]}}</ref> marketing them under names such as "Speedcook", "[[Advantium]]", "Lightwave" and "Optimawave" to emphasize their ability to cook food rapidly and with good browning. The bulbs heat the food's surface with [[infrared]] (IR) radiation, browning surfaces as in a conventional oven. The food browns while also being heated by the microwave radiation and heated through conduction through contact with heated air. The IR energy which is delivered to the outer surface of food by the lamps is sufficient to initiate browning [[caramelization]] in foods primarily made up of carbohydrates and [[Maillard reaction]]s in foods primarily made up of protein. These reactions in food produce a texture and taste similar to that typically expected of conventional oven cooking rather than the bland boiled and steamed taste that microwave-only cooking tends to create.

In order to aid [[browning (food process)|browning]], sometimes an accessory browning tray is used, usually composed of glass or [[porcelain]]. It makes food crisp by [[oxidizing]] the top layer until it turns [[Browning (chemical process)|brown]].<ref>{{Cite web |last=Fredericks |first=Jonae |date=May 3, 2017 |title=How to Use a Browning Dish for Microwave Cooking |url=https://www.leaf.tv/articles/how-to-use-a-browning-dish-for-microwave-cooking/ |access-date=January 1, 2025 |website=LEAFtv |language=en}}</ref> Ordinary plastic [[cookware]] is unsuitable for this purpose because it could melt.

[[Frozen dinner]]s, pies, and [[microwave popcorn]] bags often contain a [[susceptor]] made from thin [[Metallised film|aluminium film]] in the packaging or included on a small paper tray. The metal film absorbs microwave energy efficiently and consequently becomes extremely hot and radiates in the infrared, concentrating the heating of oil for popcorn or even browning surfaces of frozen foods. Heating packages or trays containing susceptors are designed for a single use and are then discarded as waste.

== Heating characteristics ==
[[File:Microwave tunnel closeup.jpg|thumb|In addition to their use in heating food, microwave ovens are widely used for heating in industrial processes, such as this microwave tunnel oven for softening plastic rods prior to extrusion.]]

Microwave ovens have a limited role in professional cooking,<ref name="herve">{{cite book|first=Hervé |last=This|title= Révélations gastronomiques|language=fr|publisher= Éditions Belin|isbn=978-2-7011-1756-0|year=1995}}</ref> because the boiling-range temperatures of a microwave oven do not produce the flavorful chemical reactions that frying, browning, or baking at a higher temperature produces. However, such high-heat sources can be added to microwave ovens in the form of a convection microwave oven.<ref>{{Cite journal|last1=Datta|first1=A. K.|last2=Rakesh|first2=V.|date=2013|title=Principles of Microwave Combination Heating|journal=Comprehensive Reviews in Food Science and Food Safety|language=en|volume=12|issue=1|pages=24–39|doi=10.1111/j.1541-4337.2012.00211.x|issn=1541-4337|doi-access=}}</ref>

Microwave ovens produce heat directly within the food, but despite the common misconception that microwaved food cooks from the inside out, 2.45&nbsp;GHz microwaves can only penetrate approximately {{cvt|1|cm|sp=us|sigfig=1}} into most foods. The inside portions of thicker foods are mainly heated by heat conducted from the outer 1&nbsp;cm.<ref name="Püschner">{{cite web |title=Microwave penetration depths |website=pueschner.com |publisher=Püschner GMBH + CO KG MicrowavePowerSystems |url=http://www.pueschner.com/en/microwave-technology/penetration-depths |access-date=June 1, 2018 |lang=en}}</ref><ref name="FDA">{{cite web  |title=Microwave oven radiation |date=December 12, 2017 |department=Radiation-emitting products / Resources for you |website=fda.gov |series=Center for Devices and Radiological Health |publisher=U.S. [[Food and Drug Administration]] |url=https://www.fda.gov/Radiation-EmittingProducts/ResourcesforYouRadiationEmittingProducts/ucm252762.htm |archive-url=https://web.archive.org/web/20110501123509/http://www.fda.gov/Radiation-EmittingProducts/ResourcesforYouRadiationEmittingProducts/ucm252762.htm |archive-date=May 1, 2011 |access-date=June 1, 2018 |lang=en }}</ref>

Uneven heating in microwaved food can be partly due to the uneven distribution of microwave energy inside the oven, and partly due to the different rates of energy absorption in different parts of the food. The first problem is reduced by a stirrer, a type of fan that [[Reflection (physics)|reflects]] microwave energy to different parts of the oven as it rotates, or by a turntable or carousel that turns the food; turntables, however, may still leave spots, such as the center of the oven, which receive uneven energy distribution.

: The location of dead spots and hot spots in a microwave oven can be mapped out by placing a damp piece of [[thermal paper]] in the oven: When the water-saturated paper is subjected to the microwave radiation it becomes hot enough to cause the dye to be darkened which can provide a visual representation of the microwaves. If multiple layers of paper are constructed in the oven with a sufficient distance between them a three-dimensional map can be created. Many store receipts are printed on thermal paper which allows this to be easily done at home.<ref>{{cite web | first = Maarten | last = Rutgers | year = 1999 | title = Finding the hot spots in your microwave with fax paper | department = Physics inside a microwave oven |website = maartenrutgers.org | url = http://maartenrutgers.org/fun/microwave/microwave.html#fax | archive-url = https://web.archive.org/web/20030720183703/http://maartenrutgers.org/fun/microwave/microwave.html | archive-date = July 20, 2003}}</ref>

The second problem is due to food composition and geometry, and must be addressed by the cook, by arranging the food so that it absorbs energy evenly, and periodically testing and [[RF shielding|shielding]] any parts of the food that overheat. In some materials with low [[thermal conductivity]], where [[dielectric constant]] increases with temperature, microwave heating can cause localized [[thermal runaway]]. Under certain conditions, glass can exhibit thermal runaway in a microwave oven to the point of melting.<ref>{{YouTube |title=Video of microwave effects |id=cskB5c0mJ58#t=98s }}</ref>

Due to this phenomenon, microwave ovens set at too-high power levels may even start to cook the edges of frozen food while the inside of the food remains frozen. Another case of uneven heating can be observed in baked goods containing berries. In these items, the berries absorb more energy than the drier surrounding bread and cannot dissipate the heat due to the low thermal conductivity of the bread. Often this results in overheating the berries relative to the rest of the food. "Defrost" oven settings either use low power levels or repeatedly turn the power off and on – intended to allow time for heat to be conducted within frozen foods from areas that absorb heat more readily to those which heat more slowly. In turntable-equipped ovens, more even heating can take place by placing food off-center on the turntable tray instead of exactly in the center, as this results in more even heating of the food throughout.<ref>{{cite thesis |degree=Masters |last=Pitchai |first=K. |year=2011 |title=Electromagnetic and Heat Transfer Modeling of Microwave Heating in Domestic Ovens |publisher=[[University of Nebraska–Lincoln]] |place=[[Lincoln, Nebraska]] |url=https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1038&context=foodscidiss |access-date=August 28, 2020 }}</ref>

There are microwave ovens on the market that allow full-power defrosting.  They do this by exploiting the properties of the electromagnetic radiation [[Longitudinal-section mode|LSM mode]]s. LSM full-power defrosting may actually achieve more even results than slow defrosting.<ref>{{cite book |first=P. |last=Risman |year=2009 |section=Advanced topics in microwave heating uniformity |pages=76–77 |editor1-first=M.W. |editor1-last=Lorence |editor2-first=P.S. |editor2-last=Pesheck |title=Development of Packaging and Products for Use in Microwave Ovens |publisher=Elsevier |isbn=978-1-84569-657-3 }}</ref>

Microwave heating can be deliberately uneven by design. Some microwavable packages (notably pies) may include materials that contain [[ceramic]] or aluminium flakes, which are designed to absorb microwaves and heat up, which aids in baking or crust preparation by depositing more energy shallowly in these areas. The technical term for such a microwave-absorbing patch is a ''[[susceptor]]''. Such ceramic patches affixed to cardboard are positioned next to the food, and are typically smokey blue or gray in colour, usually making them easily identifiable; the cardboard sleeves included with [[Hot Pockets]], which have a silver surface on the inside, are a good example of such packaging. Microwavable cardboard packaging may also contain overhead ceramic patches which function in the same way.<ref>{{cite journal | last1 = Labuza | first1 = T. | last2 = Meister | first2 = J. | year = 1992 | title = An alternate method for measuring the heating potential of microwave susceptor films | journal = Journal of Microwave Power and Electromagnetic Energy | volume = 27 | issue = 4 | pages = 205–208 | doi =  10.1080/08327823.1992.11688192 | bibcode = 1992JMPEE..27..205L | url = http://www.jmpee.org/JMPEE_PDFs/27-4_bl/JMPEE-Vol27-Pg205-Labuza.pdf | access-date = September 23, 2011 | archive-url = https://web.archive.org/web/20111104020217/http://www.jmpee.org/JMPEE_PDFs/27-4_bl/JMPEE-Vol27-Pg205-Labuza.pdf | archive-date = November 4, 2011 | df = dmy-all }}</ref>

=== Effects on food and nutrients ===

Any form of cooking diminishes overall nutrient content in food, particularly [[water-soluble]] [[vitamin]]s common in vegetables, but the key variables are how much water is used in the cooking, how long the food is cooked, and at what temperature.<ref name=harvard-med>{{cite web |title=Microwave cooking and nutrition |date=February 6, 2019 |series=Family Health Guide |publisher=[[Harvard Medical School]] |website=health.harvard.edu |url=http://www.health.harvard.edu/fhg/updates/Microwave-cooking-and-nutrition.shtml |access-date=April 13, 2021 |archive-url=https://web.archive.org/web/20110717050842/http://www.health.harvard.edu/fhg/updates/Microwave-cooking-and-nutrition.shtml |archive-date=July 17, 2011 }}</ref><ref name=NYTimes>{{cite news |first=Anahad |last=O'Connor |date=October 17, 2006 |newspaper=[[The New York Times]] |title=The claim: Microwave ovens kill nutrients in food |url=https://www.nytimes.com/2006/10/17/health/17real.html|access-date=April 13, 2021}}</ref> Nutrients are primarily lost by leaching into cooking water, which tends to make microwave cooking effective, given the shorter cooking times it requires and that the water heated is in the food.<ref name="harvard">{{cite web|title=Microwave cooking and nutrition|url=http://www.health.harvard.edu/fhg/updates/Microwave-cooking-and-nutrition.shtml|work=Family Health Guide|publisher=Harvard Medical School|date=February 6, 2019|access-date=April 13, 2021|archive-url=https://web.archive.org/web/20110717050842/http://www.health.harvard.edu/fhg/updates/Microwave-cooking-and-nutrition.shtml|archive-date=July 17, 2011}}</ref> Like other heating methods, microwaving converts [[vitamin B12|vitamin B{{sub|12}}]] from an active to inactive form; the amount of conversion depends on the temperature reached, as well as the cooking time. Boiled food reaches a maximum of {{convert|100|Celsius}} (the boiling point of water), whereas microwaved food can get internally hotter than this, leading to faster breakdown of vitamin B{{sub|12}}. The higher rate of loss is partially offset by the shorter cooking times required.<ref name=pmid10554220>{{cite journal |first1=Fumio |last1=Watanabe |first2=Katsuo |last2=Abe |first3=Tomoyuki |last3=Fujita |first4=Mashahiro |last4=Goto |first5=Miki |last5=Hiemori |first6=Yoshihisa |last6=Nakano |date=January 1998|title=Effects of microwave heating on the loss of vitamin B(12) in foods |journal=Journal of Agricultural and Food Chemistry |volume=46 |issue=1 |pages=206–210 |pmid=10554220 |doi=10.1021/jf970670x |bibcode=1998JAFC...46..206W |s2cid=23096987 }}</ref>

Spinach retains nearly all its [[folate]] when cooked in a microwave oven; when boiled, it loses about 77%, leaching nutrients into the cooking water.<ref name=harvard/> Bacon cooked by microwave oven has significantly lower levels of [[nitrosamine]]s than conventionally cooked bacon.<ref name=NYTimes/> Steamed vegetables tend to maintain more nutrients when microwaved than when cooked on a stovetop.<ref name="NYTimes"/> Microwave [[blanching (cooking)|blanching]] is 3–4&nbsp;times more effective than boiled-water blanching for retaining of the water-soluble vitamins, folate, [[thiamin]] and [[riboflavin]], with the exception of {{nobr|[[vitamin C]],}}  of which 29% is lost (compared with a 16% loss with boiled-water blanching).<ref>{{cite web |first1=M.A. |last1=Osinboyejo |first2=L.T. |last2= Walker |first3=S. |last3=Ogutu |first4=M. |last4=Verghese |title=Effects of microwave blanching vs. boiling water blanching on retention of selected water-soluble vitamins in turnips, foods, and greens using HPLC |series=National Center for Home Food Preservation |publisher=[[University of Georgia]] |url=http://www.uga.edu/nchfp/papers/2003/03iftturnipgreensposter.html |date=July 15, 2003 |access-date=July 23, 2011 }}</ref>

=== Safety benefits and features ===
All microwave ovens use a timer to switch off the oven at the end of the cooking time.

Microwave ovens heat food without getting hot themselves. Taking a pot off a stove, unless it is an [[induction cooktop]], leaves a potentially dangerous heating element or [[trivet]] that remains hot for some time. Likewise, when taking a [[casserole]] out of a conventional oven, one's arms are exposed to the very hot walls of the oven.  A microwave oven does not pose this problem.

Food and cookware taken out of a microwave oven are rarely much hotter than {{convert|100|C}}. Cookware used in a microwave oven is often much cooler than the food because the cookware is transparent to microwaves; the microwaves heat the food directly and the cookware is indirectly heated by the food. Food and cookware from a conventional oven, on the other hand, are the same temperature as the rest of the oven; a typical cooking temperature is {{convert|180|C}}.  That means that conventional stoves and ovens can cause more serious burns.

The lower temperature of cooking (the boiling point of water) is a significant safety benefit compared with baking in the oven or frying, because it eliminates the formation of tars and [[Char (chemistry)|char]], which are [[carcinogenic]].<ref>{{cite web |title=The five worst foods to grill |publisher=Physicians Committee for Responsible Medicine |year=2005 |url=http://www.pcrm.org/health/reports/worst_grill.html |archive-url=https://web.archive.org/web/20101230043041/http://www.pcrm.org/health/reports/worst_grill.html |archive-date=December 30, 2010 }}</ref> Microwave radiation also penetrates deeper than direct heat, so that the food is heated by its own internal water content. In contrast, direct heat can burn the surface while the inside is still cold. Pre-heating the food in a microwave oven before putting it into the grill or pan reduces the time needed to heat up the food and reduces the formation of carcinogenic char. Unlike frying and baking, microwaving does not produce [[acrylamide]] in potatoes,<ref>{{cite web | title = Acrylamide: Information on diet, food storage, and food preparation | date = May 22, 2008 | publisher = U.S. [[Food and Drug Administration]] | website = fda.gov | url = https://www.fda.gov/Food/FoodborneIllnessContaminants/ChemicalContaminants/ucm151000.htm | archive-url = https://web.archive.org/web/20130710071745/http://www.fda.gov/Food/FoodborneIllnessContaminants/ChemicalContaminants/ucm151000.htm | archive-date = July 10, 2013 | quote =  Boiling potatoes and microwaving whole potatoes with skin on, to make "microwaved baked potatoes", does not produce acrylamide.⁽¹⁾ (Footnote&nbsp;(1): Based on FDA studies.)}}</ref> however unlike deep-frying at high-temperatures, it is of only limited effectiveness in reducing glycoalkaloid (i.e., [[solanine]]) levels.<ref>{{cite report | first1 = Raymond | last1 = Tice | first2 = Brigette | last2 = Brevard | date = February 1999  | title = 3-Picoline [108-99-6]: Review of toxicological literature | publisher = Integrated Laboratory Systems | place = Research Triangle Park, NC | url = http://ntp.niehs.nih.gov/ntp/htdocs/Chem_Background/ExSumPdf/Picoline_508.pdf | archive-url = https://web.archive.org/web/20150205230417/http://ntp.niehs.nih.gov/ntp/htdocs/chem_background/exsumpdf/picoline_508.pdf | archive-date = February 5, 2015 }}</ref> Acrylamide has been found in other microwaved products like popcorn.

=== Use in cleaning kitchen sponges ===
Studies have investigated the use of the microwave oven to clean non-metallic [[kitchen sponge|domestic sponges]] which have been thoroughly wetted. A 2006 study found that microwaving wet sponges for 2&nbsp;minutes (at 1000-watt power) removed 99% of [[coliform]]s, ''[[E. coli]]'', and [[MS2 phage]]s. ''[[Bacillus cereus]]'' spores were killed at 4&nbsp;minutes of microwaving.<ref>{{cite journal | last1 = Taché | first1 = J. | last2 = Carpentier | first2 = B.  | date = January 2014 | title=Hygiene in the home kitchen: Changes in behaviour and impact of key microbiological hazard control measures | journal=Food Control | volume=35 | issue = 1 | pages=392–400 | doi = 10.1016/j.foodcont.2013.07.026 }}</ref>

A 2017 study was less affirmative: About 60% of the germs were killed but the remaining ones quickly re-colonized the sponge.<ref>{{cite journal | last1 = Egert | first1 = Markus | last2 = Schnell | first2 = Sylvia | last3 = Lueders | first3 = Tillmann | last4 = Kaiser | first4 = Dominik | last5 = Cardinale | first5 = Massimiliano | date = July 19, 2017 | df=dmy-all  | title=Microbiome analysis and confocal microscopy of used kitchen sponges reveal massive colonization by ''Acinetobacter'', ''Moraxella'', and ''Chryseobacterium'' species | journal=Scientific Reports | volume = 7 | issue = 1 | page = 5791 | doi = 10.1038/s41598-017-06055-9 | pmid = 28725026 | pmc = 5517580 | bibcode = 2017NatSR...7.5791C }}</ref>

== Issues ==
=== High temperatures ===
==== Closed containers ====
Closed containers, such as [[egg (food)|egg]]s, can explode when heated in a microwave oven due to the increased pressure from [[steam]]. Intact fresh egg yolks outside the shell also explode as a result of superheating. Insulating plastic foams of all types generally contain closed air pockets, and are generally not recommended for use in a microwave oven, as the air pockets explode and the foam (which can be toxic if consumed) may melt. Not all plastics are microwave-safe, and some plastics absorb microwaves to the point that they may become dangerously hot.<ref>{{Cite web |date=January 17, 2024 |title=Microwave safe plastics - How safe are they? {{!}} CAG |url=https://www.cag.org.in/blogs/microwave-safe-plastics-how-safe-are-they |access-date=May 29, 2024 |website=www.cag.org.in |language=en}}</ref>

==== Fires ====
[[File:Burnt popcorn from a microwave.jpg|thumb|Charred popcorn burnt by leaving the microwave oven on too long]]
Products that are heated for too long  can catch fire. Though this is inherent to any form of cooking, the rapid cooking and unattended nature of the use of microwave ovens results in additional hazard.

==== Superheating ====
In rare cases, water and other [[wiktionary:homogeneous|homogeneous]] liquids can [[superheating|superheat]]<ref>{{cite web |author=Mike P. |author2=Alcir Grohmann |author3=Darin Wagner |author4=Richard E. Barrans Jr |author5=Vince Calder |year=2001–2002 |title=Superheated Water |url=http://www.newton.dep.anl.gov/askasci/chem00/chem00636.htm |archive-url=https://web.archive.org/web/20090322201613/http://newton.dep.anl.gov/askasci/chem00/chem00636.htm |archive-date=March 22, 2009 |access-date=March 28, 2009 |work=[[NEWTON Ask-A-Scientist]] |publisher=[[Argonne National Laboratory]]}} (from the U.S. Dept. of Energy "Ask A Scientist" series's "Chemistry Archive" 2001315)</ref><ref>{{cite web |title=Superheating and microwave ovens |url=http://www.phys.unsw.edu.au/~jw/superheating.html |access-date=October 25, 2010 |work=School of Physics |publisher=[[University of New South Wales]]}}</ref> when heated in a microwave oven in a container with a smooth surface. That is, the liquid reaches a temperature slightly above its normal boiling point without bubbles of vapour forming inside the liquid. The boiling process can start [[steam explosion|explosively]] when the liquid is disturbed, such as when the user takes hold of the container to remove it from the oven or while adding solid ingredients such as powdered creamer or sugar. This can result in spontaneous boiling ([[nucleation]]) which may be violent enough to eject the boiling liquid from the container and cause severe [[scalding]].<ref>{{cite web |last=Beaty |first=William J. |title=High Voltage in your Kitchen: Unwise Microwave Oven Experiments |url=http://amasci.com/weird/microwave/voltage3.html#dirt |access-date=January 21, 2006 |publisher=Amasci.com}}</ref>

=== Metal objects ===

Contrary to popular assumptions, metal objects can be safely used in a microwave oven, but with some restrictions.<ref>{{Cite web|url=https://www.goodhousekeeping.com/food-recipes/cooking/tips/a19797/microwave-metal/|title = Yes, You Can Microwave Metal|date = January 31, 2014}}</ref><ref>{{cite web |url=https://www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/food-safety-basics/cooking-microwave-ovens#11 |title=Cooking with Microwave Ovens: What containers and wraps are safe to use in the microwave oven? |website=[[Food Safety and Inspection Service]] |access-date=October 17, 2021 |archive-date=May 2, 2025 |archive-url=https://web.archive.org/web/20250502014426/https://www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/food-safety-basics/cooking-microwave-ovens#11 }}</ref> Any metal or conductive object placed into the microwave oven acts as an [[antenna (radio)|antenna]] to some degree, resulting in an electric [[alternating current|current]]. This causes the object to act as a [[ohmic heating|heating]] element. This effect varies with the object's shape and composition, and is sometimes utilized for cooking.

Any object containing pointed metal can create an [[electric arc]] (sparks) when microwaved. This includes [[cutlery]], crumpled [[aluminium foil]] (though some foil used in microwave ovens is safe, see below), twist-ties containing metal wire, the metal wire carry-handles in [[oyster pail]]s, or almost any metal formed into a poorly conductive foil or thin wire, or into a pointed shape.<ref>{{cite web|url=http://www.conagrafoods.com/utilities/mwbasics.jsp?cookietest=true|title=Microwave cooking|at=sec. "Q: What is a microwave-safe plate or container?"|website=ConagraFoods.com|archive-url=https://web.archive.org/web/20120330102812/http://www.conagrafoods.com/utilities/mwbasics.jsp?cookietest=true|archive-date=March 30, 2012|access-date=October 25, 2009}}</ref> Forks are a good example: the [[Tine (structural)|tine]]s of the fork respond to the electric field by producing high concentrations of electric charge at the tips. This has the effect of exceeding the [[dielectric breakdown]] of air, about 3 [[volt|megavolt]]s per meter. The air forms a conductive [[plasma (physics)|plasma]], which is visible as a spark. The plasma and the tines may then form a conductive loop, which may be a more effective antenna, resulting in a longer lived spark. When dielectric breakdown occurs in air, some [[ozone]] and [[nitrogen oxide]]s are formed, both of which are unhealthy in large quantities.

[[File:Microwave metal shelf.JPG|thumb|A microwave oven with a metal shelf]]

Microwaving an individual smooth metal object without pointed ends, for example, a spoon or shallow metal pan, usually does not produce sparking. Thick metal wire racks can be part of the interior design in microwave ovens (see illustration). In a similar way, the interior wall plates with perforating holes which allow light and air into the oven, and allow interior-viewing through the oven door, are all made of conductive metal formed in a safe shape.

[[File:RainbowDVD.jpg|thumb|A microwaved [[DVD-R]] disc showing the effects of electrical discharge through its metal film]]
The effect of microwaving thin metal films can be seen clearly on a [[Compact Disc]] or [[DVD]] (particularly the factory pressed type). The microwaves induce electric currents in the metal film, which heats up, melting the plastic in the disc and leaving a visible pattern of concentric and radial scars. Similarly, [[porcelain]] with thin metal films can also be destroyed or damaged by microwaving. Aluminium foil is thick enough to be used in microwave ovens as a shield against heating parts of food items, if the foil is not badly warped. When wrinkled, aluminium foil is generally unsafe in microwaves, as manipulation of the foil causes sharp bends and gaps that invite sparking. The [[USDA]] recommends that aluminium foil used as a partial food shield in microwave oven cooking cover no more than one quarter of a food object, and be carefully smoothed to eliminate sparking hazards.<ref>{{cite web|title=Microwave Ovens and Food Safety |url=http://www.fsis.usda.gov/PDF/Microwave_Ovens_and_Food_Safety.pdf |work=[[Food Safety and Inspection Service]] |publisher=[[United States Department of Agriculture]] |access-date=August 10, 2011 |date=October 2011 |archive-url=https://web.archive.org/web/20110108064919/http://www.fsis.usda.gov/PDF/Microwave_Ovens_and_Food_Safety.pdf |archive-date=January 8, 2011 }}</ref>

Another hazard is the resonance of the magnetron tube itself. If the microwave oven is run without an object to absorb the radiation, a [[standing wave]] forms. The energy is reflected back and forth between the tube and the cooking chamber. This may cause the tube to overload and burn out. High reflected power may also cause magnetron arcing, possibly resulting in primary power fuse failure, though such a causal relationship is not easily established. Thus, [[dehydrated food]], or food wrapped in metal which does not arc, is problematic for overload reasons, without necessarily being a fire hazard.

Certain foods such as grapes, if properly arranged, can produce an [[electric arc]].<ref>{{cite web|last=Popa|first=Adrian|title=Re: Why do grapes spark in the microwave?|url=http://madsci.org/posts/archives/dec97/882909591.Ph.r.html|archive-url=https://web.archive.org/web/20010305093727/http://www.madsci.org/posts/archives/dec97/882909591.Ph.r.html|archive-date=March 5, 2001|publisher=[[MadSci Network]]|access-date=February 23, 2006|date=December 23, 1997}}</ref> Prolonged arcing from food carries similar risks to arcing from other sources as noted above.

Some other objects that may conduct sparks are plastic/holographic print [[Thermos flasks]] and other heat-retaining containers (such as [[Starbucks]] novelty cups) or cups with metal lining. If any bit of the metal is exposed, all the outer shell can burst off the object or melt.{{citation needed|date=March 2013}}

The high electrical fields generated inside a microwave oven often can be illustrated by placing a [[radiometer]] or neon glow-bulb inside the cooking chamber, creating glowing plasma inside the low-pressure bulb of the device.

=== Direct microwave exposure ===
{{Further|Microwave burn|Microwave#Effects on health}}
Direct microwave exposure is not generally possible, as microwaves emitted by the source in a microwave oven are confined in the oven by the material out of which the oven is constructed. Furthermore, ovens are equipped with redundant safety interlocks, which remove power from the magnetron if the door is opened. This safety mechanism is required by United States federal regulations.<ref>[http://www.gpo.gov/fdsys/pkg/CFR-2012-title21-vol8/pdf/CFR-2012-title21-vol8-sec1030-10.pdf 21 C.F.R. 1030.10] Retrieved August 12, 2014.</ref> Tests have shown confinement of the microwaves in commercially available ovens to be so nearly universal as to make routine testing unnecessary.<ref>{{cite web|url=http://www.arpansa.gov.au/RadiationProtection/Factsheets/is_Microwave.cfm#6|title=Radiation Emissions from Microwave ovens: How safe are Microwave Ovens?|publisher=[[ARPANSA]]|access-date=March 5, 2009|archive-url=https://web.archive.org/web/20090306112844/http://www.arpansa.gov.au/RadiationProtection/Factsheets/is_Microwave.cfm#6|archive-date=March 6, 2009}}</ref> According to the [[United States Food and Drug Administration]]'s Center for Devices and Radiological Health, a U.S. Federal Standard limits the amount of microwaves that can leak from an oven throughout its lifetime to 5 milliwatts of microwave radiation per square centimeter at approximately {{nowrap|5 cm}} (2&nbsp;in) from the surface of the oven.<ref>{{cite web|title=Microwave Oven Radiation: Microwave Oven Safety Standard|url=https://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/HomeBusinessandEntertainment/ucm142616.htm#4|archive-url=https://web.archive.org/web/20110529213915/http://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/HomeBusinessandEntertainment/ucm142616.htm#4|archive-date=May 29, 2011|publisher=U.S. [[Food and Drug Administration]]|access-date=February 16, 2009|date=January 13, 2010}}</ref> This is far below the exposure level currently considered to be harmful to human health.<ref>{{cite web|title=Advanced Measurements of Microwave Oven Leakage|url=http://www.arpansa.gov.au/pubs/emr/microwave.pdf|year=2004|publisher=[[ARPANSA]]|access-date=January 8, 2011|archive-url=https://web.archive.org/web/20110124131856/http://www.arpansa.gov.au/pubs/emr/microwave.pdf|archive-date=January 24, 2011}}</ref>

The radiation produced by a microwave oven is non-ionizing. It therefore does not have the cancer risks associated with [[ionizing radiation]] such as [[X-ray]]s and [[radioactive decay|high-energy particles]]. Long-term [[rodent studies]] to assess cancer risk have so far failed to identify any carcinogenicity from {{nowrap|2.45 GHz}} microwave radiation even with chronic exposure levels (i.e. large fraction of life span) far larger than humans are likely to encounter from any leaking ovens.<ref>{{cite journal | doi= 10.2307/3579874 | last1= Frei | first1= MR | last2= Jauchem | first2= JR | last3= Dusch | first3= SJ | last4= Merritt | first4= JH | last5= Berger | first5= RE | last6= Stedham | first6= MA | title= Chronic, low-level (1.0 W/kg) exposure of mice prone to mammary cancer to 2450 MHz microwaves | journal= Radiation Research | volume= 150 | issue= 5 | pages= 568–76 | year= 1998 | pmid= 9806599 | jstor= 3579874 | bibcode= 1998RadR..150..568F }}</ref><ref>{{cite journal | last1= Frei | first1= MR | last2= Berger | first2= RE | last3= Dusch | first3= SJ | last4= Guel | first4= V | last5= Jauchem | first5= JR | last6= Merritt | first6= JH | last7= Stedham | first7= MA | title= Chronic exposure of cancer-prone mice to low-level 2450 MHz radiofrequency radiation | journal= Bioelectromagnetics | volume= 19 | issue= 1 | pages= 20–31 | year= 1998 | pmid= 9453703 | doi= 10.1002/(SICI)1521-186X(1998)19:1<20::AID-BEM2>3.0.CO;2-6 }}</ref> However, with the oven door open, the radiation may cause damage by heating. Microwave ovens are sold with a protective [[interlock]] so that it cannot be run when the door is open or improperly latched.

Microwaves generated in microwave ovens cease to exist once the electrical power is turned off. They do not remain in the food when the power is turned off, any more than light from an electric lamp remains in the walls and furnishings of a room when the lamp is turned off. They do not make the food or the oven radioactive. In contrast with conventional cooking, the nutritional content of some foods may be altered differently, but generally in a positive way by preserving more [[micronutrient]]s – [[#Effects on food and nutrients|see above]]. There is no indication of detrimental health issues associated with microwaved food.<ref>{{Cite web |url=http://www.arpansa.gov.au/radiationprotection/factsheets/is_microwave.cfm#safe |title=ARPANSA - Microwave Ovens and Health<!-- Bot generated title --> |access-date=March 26, 2015 |archive-url=https://web.archive.org/web/20090306112844/http://www.arpansa.gov.au/RadiationProtection/Factsheets/is_Microwave.cfm#safe |archive-date=March 6, 2009  }}</ref>

There are, however, a few cases where people have been exposed to direct microwave radiation, either from appliance malfunction or deliberate action.<ref name="childinjury">{{cite book|last=Frost|first=Joe L.|title=Children and Injuries|publisher=Lawyers & Judges Publishing|page=593|isbn= 978-0-913875-96-4|url=https://books.google.com/books?id=pHXdUbiRiA8C&q=microwave+injury&pg=PA87|access-date=January 29, 2011|date=September 30, 2001}}</ref><ref name="handbelechaz">{{cite book |last1=Geddesm |first1=Leslie Alexander |last2=Roeder |first2=Rebecca A. |isbn=978-0-913875-44-5 |title=Handbook of electrical hazards and accidents |pages=369ff |url=https://books.google.com/books?id=Pb4lUnSsMa0C&q=microwave+injury&pg=PA370 |publisher=Lawyers & Judges Publishing |year=2006}}</ref> This exposure generally results in physical burns to the body, as human tissue, particularly the outer fat and muscle layers, has a similar composition to some foods that are typically cooked in microwave ovens and so experiences similar dielectric heating effects when exposed to microwave electromagnetic radiation.

=== Chemical exposure ===
[[File:Symbol Microwave oven.svg|thumb|50x50px|Microwave-safe symbol]]
The use of unmarked plastics for microwave cooking raises the issue of [[plasticizer]]s leaching into the food.<ref name="harvhealth">{{cite web|title=Microwaving food in plastic: Dangerous or not?|date=December 2019|url=https://www.health.harvard.edu/staying-healthy/microwaving-food-in-plastic-dangerous-or-not|publisher=Harvard Health Publishing (Harvard U.)}}</ref>

The plasticizers which received the most attention are [[bisphenol A]] (BPA) and [[phthalates]],<ref name="harvhealth" /><ref>{{Cite journal|title=Phthalates and attributable mortality: A population-based longitudinal cohort study and cost analysis|first1=Leonardo|last1=Trasande|first2=Buyun|last2=Liu|first3=Wei|last3=Bao|date=January 1, 2022|journal=Environmental Pollution|volume=292|issue=Pt A |article-number=118021|doi=10.1016/j.envpol.2021.118021|pmid=34654571 |pmc=8616787 |bibcode=2022EPoll.29218021T }}</ref> although it is unclear whether other plastic components present a toxicity risk.  Other issues include melting and flammability.  An alleged issue of release of [[dioxins]] into food has been dismissed<ref name="harvhealth" /> as an intentional [[red herring]] distraction from actual safety issues.

Some current plastic containers and food [[plastic wrap|wraps]] are specifically designed to resist radiation from microwaves.  Products may use the term "microwave safe",  may carry a microwave symbol (three lines of waves, one above the other) or simply provide instructions for proper microwave oven use. Any of these is an indication that a product is suitable for microwaving when used in accordance with the directions provided.<ref>{{cite web | url=http://www.plasticsinfo.org/s_plasticsinfo/sec_level2_faq.asp?CID=703&DID=2837 | title=FAQs: Using Plastics in the Microwave | publisher=American Chemistry Council | access-date=May 12, 2010 | archive-url=https://web.archive.org/web/20100926171011/http://plasticsinfo.org/s_plasticsinfo/sec_level2_faq.asp?CID=703&DID=2837 | archive-date=September 26, 2010 }}</ref>

Plastic containers can release [[microplastics]] into food when heated in microwave ovens.<ref>{{Cite web |date=July 21, 2023 |title=Nebraska study finds billions of nanoplastics released when microwaving containers |url=https://news.unl.edu/newsrooms/today/article/nebraska-study-finds-billions-of-nanoplastics-released-when-microwaving/ |access-date=September 4, 2023 |website=[[University of Nebraska–Lincoln]]|language=en}}</ref>

=== Uneven heating ===
Microwave ovens are frequently used for reheating [[leftovers|leftover food]], and bacterial contamination may not be repressed if the microwave oven is used improperly. If [[Cooking#Food safety|safe temperature]] is not reached, this can result in [[foodborne illness]], as with other reheating methods. While microwave ovens can destroy bacteria as well as conventional ovens can, they cook rapidly and may not cook as evenly, similar to frying or grilling, leading to a risk of some food regions failing to reach recommended temperatures. Therefore, a standing period after cooking to allow temperatures in the food to equalize is recommended, as well as the use of a food thermometer to verify internal temperatures.<ref name="FSIS">{{cite web |title=Microwave Ovens and Food Safety |url=https://www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/food-safety-basics/cooking-microwave-ovens |website=Food Safety and Inspection Service |publisher=United States Department of Agriculture |access-date=June 1, 2018 |date=August 8, 2013}}</ref>

=== Interference ===
Microwave ovens, although shielded for safety purposes, still emit low levels of microwave radiation. This is not harmful to humans, but can sometimes cause interference to [[Wi-Fi]] and [[Bluetooth]] and other devices that communicate on the [[2.4 GHz radio use|2.45&nbsp;GHz wavebands]], particularly at close range.<ref>
{{cite news
 |first=Karl S. |last=Kruszelnicki
 |date=September 25, 2012
|title= WiFi frozen? Blame the microwave oven
 |series=[[ABC News and Current Affairs]]
 |publisher=[[Australian Broadcasting Corporation]]
 |url=http://www.abc.net.au/science/articles/2012/09/25/3595484.htm
 |access-date=January 19, 2019
}}
</ref>
Conventional transformer ovens do not operate continuously over the mains cycle, but can cause significant slowdowns for many metres around the oven, whereas inverter-based ovens can stop nearby networking entirely while operating.<ref>
{{cite web
 |title=Part&nbsp;1
 |department=Interference from non-WiFi sources
 |series=WiSE Article
 |website=cwnp.com
 |url=https://www.cwnp.com/the-wise-article-series-interference-from-non-wifi-sources-part-1/
}}
</ref>

== See also ==
{{portal|Cooking}}
* [[Countertop]]
* [[Electromagnetic reverberation chamber]]
* [[Induction cooker]]
* [[List of cooking appliances]]
* [[List of home appliances]]
* [[Microwave chemistry]]
* [[Peryton (astronomy)]]
* [[Robert V. Decareau]]
* [[Thelma Pressman]]
* [[Wall oven]]

== Notes ==
{{notelist}}

== References ==
{{reflist}}

== External links ==
{{Commons category|Microwave ovens}}
* {{US patent|2495429}}: Percy Spencer's original patent
* [http://www.newton.dep.anl.gov/askasci/chem00/chem00636.htm Ask a Scientist Chemistry Archives] {{Webarchive|url=https://web.archive.org/web/20150226223720/http://newton.dep.anl.gov/askasci/chem00/chem00636.htm |date=February 26, 2015 }}, Argonne National Laboratory
* [http://www.smecc.org/microwave_oven.htm Further Reading On The History Of Microwaves and Microwave Ovens]
* [https://web.archive.org/web/20051102093130/http://www.inventionandtechnology.com/xml/2005/4/it_2005_4_feat_4.xml Microwave oven history] from ''[[American Heritage (magazine)|American Heritage]]'' magazine
* [http://www.animations.physics.unsw.edu.au/jw/superheating.htm Superheating and Microwave Ovens, University of New South Wales (includes video)]
* [https://web.archive.org/web/20120425091608/http://emlab.uiuc.edu/ece350/suppnotes/moven.pdf "The Microwave Oven"]: Short explanation of microwave oven in terms of [[Microwave cavity|microwave cavities]] and [[waveguides]], intended for use in a class in [[electrical engineering]]
* [https://www.youtube.com/watch?v=Lux6js2TtOw How Things Work: Microwave Ovens],  David Ruzic, University of Illinois
{{Home appliances}}
{{Authority control}}

[[Category:Ovens]]
[[Category:Microwave technology|Owen]]
[[Category:American inventions]]
[[Category:Radiation effects]]
[[Category:Products introduced in 1947]]
[[Category:20th-century inventions]]
[[Category:Home appliances]]