View source for Ionizing radiation

{{Short description|Harmful high-frequency radiation}}
[[File:Radiation warning symbol.svg|thumb|Ionizing radiation warning symbol]]
'''Ionizing radiation''', also spelled '''ionising radiation''', consists of [[subatomic particle]]s or [[electromagnetic wave]]s that have enough [[energy]] per individual photon or particle to [[ionization|ionize]] atoms or [[molecule]]s by detaching [[electron]]s from them.<ref>{{Cite web|url=https://www.who.int/news-room/fact-sheets/detail/ionizing-radiation-health-effects-and-protective-measures|title=Ionizing radiation, health effects and protective measures|date=29 April 2016|website=World Health Organization|access-date=22 January 2020|archive-date=29 March 2020|archive-url=https://web.archive.org/web/20200329093757/https://www.who.int/news-room/fact-sheets/detail/ionizing-radiation-health-effects-and-protective-measures|url-status=live}}</ref> Some particles can travel up to 99% of the [[speed of light]], and the electromagnetic waves are on the high-energy portion of the [[electromagnetic spectrum]].

[[Gamma ray]]s, [[X-rays]], and the higher energy [[vacuum ultraviolet|ultraviolet]] part of the electromagnetic spectrum are ionizing radiation; whereas the lower energy [[ultraviolet]], visible [[light]], [[infrared]], [[microwave]]s, and [[radio wave]]s are [[non-ionizing radiation]]. Nearly all types of [[laser]] light are non-ionizing radiation. The boundary between ionizing and non-ionizing radiation in the ultraviolet area cannot be sharply defined, as different molecules and atoms ionize at [[Ionization energies of the elements (data page)|different energies]]. The energy of ionizing radiation starts around 10&nbsp;[[electronvolt]]s (eV).<ref name=FCC />

Ionizing subatomic particles include [[alpha particle]]s, [[beta particle]]s, and [[neutron]]s. These particles are created by [[radioactive decay]], and almost all are energetic enough to ionize. There are also secondary [[cosmic particle]]s produced after [[cosmic rays]] interact with Earth's atmosphere, including [[muon]]s, [[meson]]s, and [[positron]]s.<ref name="Woodside">
{{cite book
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 |title       = Environmental, Safety, and Health Engineering
 |publisher   = John Wiley & Sons
 |year        = 1997
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 |isbn        = 978-0471109327
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 |archive-date = 2015-10-19
}}</ref><ref name="Stallcup">
{{cite book
 |last        = Stallcup
 |first       = James G.
 |title       = OSHA: Stallcup's High-voltage Telecommunications Regulations Simplified
 |publisher   = Jones & Bartlett Learning
 |year        = 2006
 |location    = US
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 |url         = https://books.google.com/books?id=CpyeNUgls68C&q=%22ionizing+radiation&pg=SA13-PA3
 |isbn        = 978-0763743475
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20151017165816/https://books.google.com/books?id=CpyeNUgls68C&pg=SA13-PA3&dq=%22ionizing+radiation
 |archive-date = 2015-10-17
}}</ref> [[Cosmic rays]] may also produce [[radioisotope]]s on Earth (for example, [[carbon-14]]), which in turn decay and emit ionizing radiation. Cosmic rays and the decay of radioactive [[isotopes]] are the primary sources of natural ionizing radiation on Earth, contributing to [[background radiation]]. Ionizing radiation is also generated artificially by [[X-ray tube]]s, [[particle accelerators]], and [[nuclear fission]].

Ionizing radiation is not immediately detectable by human senses, so instruments such as [[Geiger counter]]s are used to detect and measure it. However, very high energy particles can produce visible [[Radiobiology|effects on both organic]] and inorganic matter (e.g. water lighting in [[Cherenkov radiation]]) or humans (e.g. [[acute radiation syndrome]]).<ref>{{Cite web |title=Ionizing Radiation - Health Effects {{!}} Occupational Safety and Health Administration |url=https://www.osha.gov/ionizing-radiation/health-effects#:~:text=When%20ionizing%20radiation%20interacts%20with,DNA%20(i.e.,%20mutations). |access-date=2022-06-23 |website=www.osha.gov}}</ref>

Ionizing radiation is used in a wide variety of fields such as [[medicine]], [[nuclear power]], research, and industrial manufacturing, but is a health hazard if proper measures against excessive exposure are not taken. Exposure to ionizing radiation causes cell damage to living [[tissue (anatomy)|tissue]] and [[Organ (biology)|organ damage]]. In high acute doses, it will result in [[radiation burn]]s and [[radiation sickness]], and lower level doses over a protracted time can cause [[Radiation-induced cancer|cancer]].<ref>{{cite journal |last=Ryan |first=Julie |date=5 January 2012 |title=Ionizing Radiation: The Good, the Bad, and the Ugly |journal= The Journal of Investigative Dermatology |volume=132 |issue=3 0 2 |pages=985–993 |pmc=3779131 |pmid=22217743 |doi=10.1038/jid.2011.411}}</ref><ref>Herrera Ortiz AF, Fernández Beaujon LJ, García Villamizar SY, Fonseca López FF. Magnetic resonance versus computed tomography for the detection of retroperitoneal lymph node metastasis due to testicular cancer: A systematic literature review. European Journal of Radiology Open.2021;8:100372. https://doi.org/10.1016/j.ejro.2021.100372 </ref> The [[International Commission on Radiological Protection]] (ICRP) issues guidance on ionizing radiation protection, and the effects of dose uptake on human health.

== Directly ionizing radiation ==
{{main article|Particle radiation}}
[[File:Alfa beta gamma neutron radiation.svg|thumb|Alpha ('''α''') radiation consists of a fast-moving [[helium-4]] ({{sup|4}}He) nucleus and is [[Stopping power (particle radiation)|stopped]] by a sheet of paper. Beta ('''β''') radiation, consisting of [[electron]]s, is halted by an aluminium plate. Gamma ('''γ''') radiation, consisting of energetic [[photon]]s, is eventually absorbed as it penetrates a dense material. Neutron ('''n''') radiation consists of free neutrons that are blocked by light elements, like hydrogen, which slow and/or capture them. Not shown: [[galactic cosmic rays]] that consist of energetic charged nuclei such as [[proton]]s, [[helium]] nuclei, and high-charged nuclei called [[HZE ions]].]]
[[File:Cloud chambers played an important role of particle detectors.jpg|thumb|[[Cloud chamber]]s are used to visualize ionizing radiation. This image show the tracks of particles, which ionize saturated air and leave a trail of water vapour.]]
Ionizing radiation may be grouped as directly or indirectly ionizing.

Any charged particle with mass can ionize [[atom]]s directly by [[fundamental interaction]] through the [[Coulomb force]] if it has enough kinetic energy. Such particles include [[atomic nuclei]], [[electron]]s, [[muon]]s, charged [[pion]]s, [[proton]]s, and energetic charged nuclei stripped of their electrons. When moving at relativistic speeds (near the [[speed of light]], c) these particles have enough kinetic energy to be ionizing, but there is considerable speed variation. For example, a typical alpha particle moves at about 5% of ''c'', but an electron with 33&nbsp;eV (just enough to ionize) moves at about 1% of ''c''.

Two of the first types of directly ionizing radiation to be discovered are [[alpha particles]] which are helium nuclei ejected from the nucleus of an atom during radioactive decay, and energetic electrons, which are called [[beta particle]]s.

Natural [[cosmic ray]]s are made up primarily of relativistic protons but also include heavier atomic nuclei like [[helium]] ions and [[HZE ions]]. In the atmosphere such particles are often stopped by air molecules, and this produces short-lived charged pions, which soon decay to muons, a primary type of cosmic ray radiation that reaches the surface of the earth. Pions can also be produced in large amounts in [[particle accelerator]]s.

=== Alpha particles ===
{{Main article|Alpha particle}}
Alpha (α) particles consist of two [[proton]]s and two [[neutron]]s bound together into a particle: a [[helium]]-4 [[atomic nucleus|nucleus]]. Alpha particle emissions are generally produced in the process of [[alpha decay]].

Alpha particles are a strongly ionizing form of radiation, but when emitted by radioactive decay they have low penetration power and can be [[Stopping power (particle radiation)|absorbed]] by a few centimeters of air, or by the top layer of human skin. More powerful alpha particles from [[ternary fission]] are three times as energetic, and penetrate proportionately farther in air. The helium nuclei that form 10–12% of cosmic rays, are also usually of much higher energy than those from radioactive decay and pose shielding problems in space. However, this type of radiation is significantly absorbed by Earth's atmosphere, which is a radiation shield equivalent to about 10 meters of water.<ref>[https://engineering.dartmouth.edu/~d76205x/research/Shielding/docs/Parker_06.pdf One kg of water per cm squared is 10 meters of water] {{webarchive|url=https://web.archive.org/web/20160101094421/http://engineering.dartmouth.edu/~d76205x/research/shielding/docs/Parker_06.pdf |date=2016-01-01 }}</ref>

The alpha particle was named by [[Ernest Rutherford]] after the first letter in the [[Greek alphabet]], [[Alpha|α]], when he ranked the known radioactive emissions in descending order of ionizing effect in 1899. The symbol is α or α{{sup|2+}}. Because they are identical to helium nuclei, they are also called He{{sup|2+}} or {{chem|4|2|He|2+}} indicating helium with a charge of +2&nbsp;[[elementary charge|''e'']] (missing its two electrons). If the ion gains electrons from its environment, the α particle can be written as a normal (electrically neutral) helium atom {{chem|4|2|He}}.

=== Beta particles ===
{{Main article|Beta particles}}
Beta (β) particles are high-energy, high-speed [[electron]]s or [[positron]]s emitted by certain types of [[radioactive]] [[Atomic nucleus|nuclei]], such as [[potassium-40]]. The production of β particles is termed [[beta decay]]. There are two forms of β decay, β{{sup|−}} and β{{sup|+}}, which respectively give rise to the electron and the positron.<ref>{{cite web|title=Beta Decay|url=http://www.lbl.gov/abc/wallchart/chapters/03/2.html|work=Lbl.gov|date=9 August 2000|url-status=dead|archive-url=https://web.archive.org/web/20160303195332/http://www2.lbl.gov/abc/wallchart/chapters/03/2.html|archive-date=3 March 2016|access-date=10 April 2014}}</ref> Beta particles are much less penetrating than gamma radiation, but more penetrating than alpha particles.

High-energy beta particles may produce X-rays known as [[bremsstrahlung]] ("braking radiation") or [[secondary electron]]s ([[delta ray]]) as they pass through matter. Both of these can cause an indirect ionization effect. Bremsstrahlung is of concern when shielding beta emitters, as the interaction of beta particles with some shielding materials produces bremsstrahlung. The effect is greater with material having high atomic numbers, so material with low atomic numbers is used for beta source shielding.

=== Positrons and other types of antimatter ===
{{Main article|Positron|Antimatter}}
The positron or antielectron is the [[antiparticle]] or the [[antimatter]] counterpart of the [[electron]]. When a low-energy positron collides with a low-energy electron, [[annihilation]] occurs, resulting in their conversion into the energy of two or more [[gamma ray]] [[photon]]s (see [[electron–positron annihilation]]). As positrons are positively charged particles they can directly ionize an atom through Coulomb interactions.

Positrons can be generated by [[positron emission]] nuclear decay (through [[weak interaction]]s), or by [[pair production]] from a sufficiently energetic [[photon]]. Positrons are common artificial sources of ionizing radiation used in medical [[PET scan|positron emission tomography]] (PET) scans.

=== Charged nuclei ===
Charged nuclei are characteristic of galactic cosmic rays and solar particle events and except for alpha particles (charged helium nuclei) have no natural sources on Earth. In space, however, very high energy protons, helium nuclei, and HZE ions can be initially stopped by relatively thin layers of shielding, clothes, or skin. However, the resulting interaction will generate secondary radiation and cause cascading biological effects. If just one atom of tissue is displaced by an energetic proton, for example, the collision will cause further interactions in the body. This is called "[[linear energy transfer]]" (LET), which utilizes [[elastic scattering]].

LET can be visualized as a billiard ball hitting another in the manner of the [[conservation of momentum]], sending both away with the energy of the first ball divided between the two unequally. When a charged nucleus strikes a relatively slow-moving nucleus of an object in space, LET occurs and neutrons, alpha particles, low-energy protons, and other nuclei will be released by the collisions and contribute to the total absorbed dose of tissue.<ref name="NASA/TP-1999-209320">[[hdl:2060/19990051001|Contribution of High Charge and Energy (HZE) Ions During Solar-Particle Event of September 29, 1989]] Kim, Myung-Hee Y.; Wilson, John W.; Cucinotta, Francis A.; Simonsen, Lisa C.; Atwell, William; Badavi, Francis F.; Miller, Jack, NASA Johnson Space Center; Langley Research Center, May 1999.</ref>

== Indirectly ionizing radiation ==
Indirectly ionizing radiation is electrically neutral and does not interact strongly with matter, therefore the bulk of the ionization effects are due to secondary ionization.

=== Photon radiation ===
{{see also|Gamma rays|X-rays}}
[[Image:EM-spectrum.svg|right|thumb|upright=1.5|Different types of [[electromagnetic radiation]]]]
[[File:Pb-gamma-xs.svg|upright=1.5|thumb|The total absorption coefficient of lead (atomic number 82) for gamma rays, plotted versus gamma energy, and contributions by the three effects. The photoelectric effect dominates at low energy, but above 5&nbsp;MeV, pair production starts to dominate.]]

Even though photons are electrically neutral, they can ionize [[atom]]s indirectly through the [[photoelectric effect]] and the [[Compton effect]]. Either of those interactions cause the ejection of an electron from an atom at relativistic speeds, turning that electron into a (secondary) beta particle that will ionize other atoms. Since most of the ionized atoms are due to the [[secondary beta]] particles, photons are indirectly ionizing radiation.<ref>{{cite web|last=European Centre of Technological Safety|title=Interaction of Radiation with Matter|url=http://www.tesec-int.org/TechHaz-site%2008/Radiation-interaction.pdf|work=Radiation Hazard|access-date=5 November 2012|url-status=dead|archive-url=https://web.archive.org/web/20130512170010/http://www.tesec-int.org/TechHaz-site%2008/Radiation-interaction.pdf|archive-date=12 May 2013}}</ref>

Radiated photons are called [[gamma ray]]s if they are produced by a [[nuclear reaction]], [[subatomic particle]] decay, or [[radioactive decay]] within the nucleus. They are called [[x-ray]]s if produced outside the nucleus. The generic term "photon" is used to describe both.<ref>
{{cite book
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}}</ref>

X-rays normally have a lower energy than gamma rays, and an older convention was to define the boundary as a wavelength of 10<sup>−11</sup> m (or a photon energy of 100&nbsp;keV).<ref>
{{cite book
 |last = Charles Hodgman, Ed.
 |title = CRC Handbook of Chemistry and Physics, 44th Ed.
 |publisher = Chemical Rubber Co.
 |year = 1961
 |location = USA
 |page = 2850
}}</ref> That threshold was driven by historic limitations of older X-ray tubes and low awareness of [[isomeric transition]]s. Modern technologies and discoveries have shown an overlap between X-ray and gamma energies. In many fields they are functionally identical, differing for terrestrial studies only in origin of the radiation. In astronomy, however, where radiation origin often cannot be reliably determined, the old energy division has been preserved, with X-rays defined as being between about 120&nbsp;eV and 120&nbsp;keV, and gamma rays as being of any energy above 100 to 120&nbsp;keV, regardless of source. Most astronomical "[[gamma-ray astronomy|gamma-rays]]" are known ''not'' to originate from radioactivity but, rather, result from processes like those that produce astronomical X-rays, except driven by much more energetic electrons.

Photoelectric absorption is the dominant mechanism in organic materials for photon energies below 100&nbsp;keV, typical of classical X-ray tube originated [[X-ray]]s. At energies beyond 100&nbsp;keV, photons ionize matter increasingly through the [[Compton effect]], and then indirectly through [[pair production]] at energies beyond 5&nbsp;MeV. The accompanying interaction diagram shows two Compton scatterings happening sequentially. In every scattering event, the gamma ray transfers energy to an electron, and it continues on its path in a different direction and with reduced energy.

=== Definition boundary for lower-energy photons ===
{{see also|Ultraviolet}}
The lowest ionization energy of any element is 3.89&nbsp;eV, for [[caesium]]. However, US Federal Communications Commission material defines ionizing radiation as that with a [[photon energy]] greater than 10&nbsp;eV (equivalent to a far [[ultraviolet]] wavelength of 124 [[nanometer]]s).<ref name=FCC>{{cite web |url=http://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet56/oet56e4.pdf |access-date=2011-12-07 |url-status=live |archive-url=https://web.archive.org/web/20111020131849/http://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet56/oet56e4.pdf |archive-date=2011-10-20 |author1=Robert F. Cleveland, Jr. |author2=Jerry L. Ulcek |title=Questions and Answers about Biological Effects and Potential Hazards of Radiofrequency Electromagnetic Fields |publisher=OET (Office of Engineering and Technology) Federal Communications Commission |location=Washington, D.C. |edition=4th |date=August 1999}}</ref> Roughly, this corresponds to both the first [[ionization energy]] of oxygen, and the ionization energy of hydrogen, both about 14&nbsp;eV.<ref>{{cite web |author=Jim Clark |year=2000 |url=http://www.chemguide.co.uk/atoms/properties/ies.html |title=Ionisation Energy |access-date=2011-12-07 |url-status=live |archive-url=https://web.archive.org/web/20111126131813/http://www.chemguide.co.uk/atoms/properties/ies.html |archive-date=2011-11-26}}</ref> In some [[United States Environmental Protection Agency|Environmental Protection Agency]] references, the ionization of a typical water molecule at an energy of 33&nbsp;eV is referenced<ref name=EPA>{{cite web |url=http://epa.gov/radiation/understand/ionize_nonionize.html |title=Ionizing & Non-Ionizing Radiation |website=Radiation Protection |publisher=EPA |access-date=2015-01-09 |url-status=dead |archive-url=https://web.archive.org/web/20150212174819/http://www.epa.gov/radiation/understand/ionize_nonionize.html |archive-date=2015-02-12|date=2014-07-16 }}</ref> as the appropriate biological threshold for ionizing radiation: this value represents the so-called ''W-value'', the colloquial name for the [[International Commission on Radiation Units and Measurements|ICRU]]'s [[Stopping power (particle radiation)#Stopping power, Bragg curve and range|mean energy expended in a gas per ion pair formed]],<ref>{{cite journal | journal=Journal of the ICRU | volume=11 | issue=1 | title=Fundamental Quantities and Units for Ionizing Radiation (ICRU Report 85) | year=2011 | url=http://www.icru.org/index.php?option=com_content&task=view&id=202 | url-status=live | archive-url=https://web.archive.org/web/20120420015714/http://www.icru.org/index.php?option=com_content&task=view&id=202 |archive-date=2012-04-20 }}</ref> which combines ionization energy plus the energy lost to other processes such as [[excited state|excitation]].<ref>{{cite book |chapter-url=http://www.science.mcmaster.ca/medphys/images/files/courses/4R06/note3.pdf |chapter=Gas Filled Detectors |url-status=dead |archive-url=https://web.archive.org/web/20120617125708/http://www.science.mcmaster.ca/medphys/images/files/courses/4R06/note3.pdf |archive-date=2012-06-17 |author=Hao Peng |publisher=MacMaster University, Department of Medical Physics and Radiation Sciences |title=Lecture notes for MED PHYS 4R06/6R03 – Radiation & Radioisotope Methodology}}</ref> At 38 nanometers wavelength for [[electromagnetic radiation]], 33&nbsp;eV is close to the energy at the conventional 10&nbsp;nm wavelength transition between extreme ultraviolet and X-ray radiation, which occurs at about 125&nbsp;eV. Thus, X-ray radiation is always ionizing, but only extreme-ultraviolet radiation can be considered ionizing under all definitions.

[[File:Strahlenarten en.svg|upright=1.5|thumb|Radiation interaction: gamma rays are represented by wavy lines, charged particles and neutrons by straight lines. The small circles show where ionization occurs.]]

=== Neutrons ===
{{main article|Neutron|neutron radiation}}
{{More citations needed section|date=May 2024}}
[[Neutron]]s have a neutral electrical charge often misunderstood as zero electrical charge and thus often do not ''directly'' cause ionization in a single step or interaction with matter. However, fast neutrons will interact with the protons in hydrogen via [[linear energy transfer]], energy that a particle transfers to the material it is moving through. This mechanism scatters the nuclei of the materials in the target area, causing direct ionization of the hydrogen atoms. When neutrons strike the hydrogen nuclei, proton radiation (fast protons) results. These protons are themselves ionizing because they are of high energy, are charged, and interact with electrons.

Neutrons that strike other nuclei besides hydrogen, transfer less energy to the other particle if linear energy transfer does occur. But, for many nuclei struck by neutrons, [[inelastic scattering]] occurs. Whether elastic or inelastic scatter occurs is dependent on the speed of the neutron, whether [[Neutron temperature|fast]] or [[Neutron temperature|thermal]] or somewhere in between. It is also dependent on the nuclei it strikes and its [[neutron cross section]].

In inelastic scattering, neutrons are readily absorbed in a type of [[nuclear reaction]] called [[neutron capture]] and attributes to the [[neutron activation]] of the nucleus. Neutron interactions with most types of matter in this manner usually produce [[radioactive]] nuclei. [[Oxygen-16]], for example, undergoes neutron activation, rapidly decays by a proton emission forming [[nitrogen-16]], which decays to oxygen-16. The short-lived nitrogen-16 decay emits a powerful beta ray. This process can be written as:
<blockquote>{{sup|16}}O (n,p) {{sup|16}}N (fast neutron capture possible with >11&nbsp;MeV neutron)
{{sup|16}}N → {{sup|16}}O + β{{sup|−}} (Decay t{{sub|1/2}} = 7.13&nbsp;s)
This high-energy β{{sup|−}} further interacts rapidly with other nuclei, emitting high-energy γ via [[Bremsstrahlung]]</blockquote>

While not a favorable reaction, the {{sup|16}}O (n,p) {{sup|16}}N reaction is a major source of X-rays emitted from the cooling water of a [[pressurized water reactor]] and contributes enormously to the radiation generated by a water-cooled [[nuclear reactor]] while operating.

For the best shielding of neutrons, hydrocarbons that have an abundance of [[hydrogen]] are used.

In [[fissile]] materials, secondary neutrons may produce [[nuclear chain reaction]]s, causing a larger amount of ionization from the [[daughter product]]s of fission.

Outside the nucleus, free neutrons are unstable and have a mean lifetime of 14 minutes, 42 seconds. Free neutrons decay by emission of an electron and an electron antineutrino to become a proton, a process known as [[beta decay]]:<ref>{{cite journal |author=W.-M. Yao |display-authors=etal |url=http://pdg.lbl.gov/2007/tables/bxxx.pdf |title=Particle Data Group Summary Data Table on Baryons |journal=J. Phys. G |volume=33 |issue=1 |url-status=dead |archive-url=https://web.archive.org/web/20110910125729/http://pdg.lbl.gov/2007/tables/bxxx.pdf |archive-date=2011-09-10 |year=2007 |access-date=2012-08-16}}</ref>

In the adjacent diagram, a neutron collides with a proton of the target material, and then becomes a fast recoil proton that ionizes in turn. At the end of its path, the neutron is captured by a nucleus in an (n,γ)-reaction that leads to the emission of a [[neutron capture]] photon. Such photons always have enough energy to qualify as ionizing radiation.

== Physical effects ==
{{main article|Radiation damage}}
[[File:Cyclotron with glowing beam.jpg|thumb|right|Ionized air glows blue around a beam of particulate ionizing radiation from a [[cyclotron]]]]

=== Nuclear effects ===
Neutron radiation, alpha radiation, and extremely energetic gamma (> ~20&nbsp;MeV) can cause [[nuclear transmutation]] and [[induced radioactivity]]. The relevant mechanisms are [[neutron activation]], [[alpha absorption]], and [[photodisintegration]]. A large enough number of transmutations can change macroscopic properties and cause targets to become radioactive themselves, even after the original source is removed.

=== Chemical effects ===
{{main article|Radiation chemistry}}
Ionization of molecules can lead to [[radiolysis]] (breaking chemical bonds), and formation of highly reactive [[free radicals]]. These free radicals may then react chemically with neighbouring materials even after the original radiation has stopped (e.g. [[ozone cracking]] of polymers by ozone formed by ionization of air). Ionizing radiation can also accelerate existing chemical reactions such as polymerization and corrosion, by contributing to the activation energy required for the reaction. Optical materials deteriorate under the effect of ionizing radiation.

High-intensity ionizing radiation in air can produce a visible [[ionized air glow]] of telltale bluish-purple color. The glow can be observed, e.g., during [[criticality accident]]s, around [[mushroom cloud]]s shortly after a [[nuclear explosion]], or the inside of a damaged nuclear reactor like during the [[Chernobyl disaster]].

Monatomic fluids, e.g. molten [[sodium]], have no chemical bonds to break and no crystal lattice to disturb, so they are immune to the chemical effects of ionizing radiation. Simple diatomic compounds with very negative [[enthalpy of formation]], such as [[hydrogen fluoride]] will reform rapidly and spontaneously after ionization.

=== Electrical effects ===
The ionization of materials temporarily increases their conductivity, potentially permitting damaging current levels. This is a particular hazard in [[semiconductor]] microelectronics used in electronic equipment; subsequent currents introduce operation errors or even permanently damage the devices. Devices intended for high-radiation environments such as the nuclear industry or outer space, may be made [[radiation hardening|''radiation hard'']] to resist such effects through design, material selection, and fabrication methods.

Ionizing radiation can cause an increase in the density of interface traps by reactivating passivated [[dangling bonds]] at interfaces between two materials, such as the Si/SiO<sub>2</sub> interface in [[CMOS]] devices.<ref>
{{cite journal
| last1 = Khoshnoud
| first1 = A.
| last2 = Yavandhassani
| first2 = J.
| title = Modeling of total ionizing dose (TID) effects on the nonuniform distribution of Si/SiO<sub>2</sub> interface trap energy states in MOS devices
| journal = Scientific Reports
| volume = 15
| pages = 17082
| year = 2025
| doi = 10.1038/s41598-025-01325-3
| url = https://www.nature.com/articles/s41598-025-01325-3
| pmc = 12084595
}}</ref> These traps can capture charge carriers, resulting in parasitic effects including [[Electron mobility|mobility]] degradation, increased [[Noise_(electronics)|noise]], and [[threshold voltage]] shifts.

Proton radiation found in space can also cause [[single-event upset]]s in digital circuits. The electrical effects of ionizing radiation are exploited in gas-filled radiation detectors, e.g. the [[Geiger counter]] or the [[ion chamber]].

== Health effects ==
{{main article|Radiobiology}}

Most adverse health effects of exposure to ionizing radiation may be grouped in two general categories:
* deterministic effects (harmful tissue reactions) due in large part to killing or malfunction of cells following high doses from [[radiation burn]]s.
* stochastic effects, i.e., [[Radiation-induced cancer|cancer]] and heritable effects involving either cancer development in exposed individuals owing to [[mutation]] of somatic cells or heritable disease in their offspring owing to mutation of reproductive (germ) cells.{{sfn | ICRP | 2007 | loc= paragraph 55}}

The most common impact is stochastic [[radiation-induced cancer]] with a latent period of years or decades after exposure. For example, ionizing radiation is one cause of [[chronic myelogenous leukemia]],<ref>{{Cite book |title=Understanding pathophysiology |publisher=Elsevier |last1=Huether |first1=Sue E. |last2=McCance |first2=Kathryn L. |isbn=9780323354097 |edition=6th |location=St. Louis, Missouri |pages=530 |oclc=740632205|date=2016-01-22 }}</ref><ref name="LLSCML">{{cite web |title=Chronic myeloid leukemia (CML) |url=https://www.lls.org/leukemia/chronic-myeloid-leukemia |publisher=[[Leukemia & Lymphoma Society]] |access-date=22 September 2019 |date=2015-02-26 |archive-date=2019-09-22 |archive-url=https://web.archive.org/web/20190922220402/https://www.lls.org/leukemia/chronic-myeloid-leukemia |url-status=live }}</ref><ref name="MedlinePlusCML">{{cite web |title=Chronic myelogenous leukemia (CML) Chronic myelogenous leukemia (CML) |url=https://medlineplus.gov/ency/article/000570.htm |website=[[Medline Plus]] Medical Encyclopedia |publisher=[[U.S. National Library of Medicine]] |access-date=22 September 2019 |archive-date=29 September 2019 |archive-url=https://web.archive.org/web/20190929155346/https://medlineplus.gov/ency/article/000570.htm |url-status=live }}</ref> although most people with CML have not been exposed to radiation.<ref name="LLSCML"/><ref name="MedlinePlusCML"/> The mechanism by which this occurs is well understood, but quantitative models predicting the level of risk remain controversial.{{Citation needed|date=March 2021}}

The most widely accepted model, the [[linear no-threshold model]] (LNT), holds that the incidence of cancers due to ionizing radiation increases linearly with [[effective radiation dose]] at a rate of 5.5% per [[sievert]].{{sfn | ICRP | 2007}} If this is correct, then natural [[background radiation]] is the most hazardous source of radiation to general public health, followed by medical imaging as a close second. Other stochastic effects of ionizing radiation are [[teratogenesis]], [[Radiation-induced cognitive decline|cognitive decline]], and [[heart disease]].{{Citation needed|date=March 2021}}

Though [[DNA]] is always susceptible to damage by ionizing radiation, the DNA molecule may also be damaged by radiation with enough energy to excite certain [[molecular bond]]s to form [[pyrimidine dimer]]s. This energy may be less than ionizing, but near to it. A good example is ultraviolet spectrum energy which begins at about 3.1&nbsp;eV (400&nbsp;nm) at close to the same energy level which can cause [[sunburn]] to unprotected skin, as a result of [[photochemistry|photoreactions]] in [[collagen]] and (in the [[UV-B]] range) also damage in DNA (for example, pyrimidine dimers). Thus, the mid and lower ultraviolet electromagnetic spectrum is damaging to biological tissues as a result of electronic excitation in molecules which falls short of ionization, but produces similar non-thermal effects. To some extent, visible light and also ultraviolet A (UVA) which is closest to visible energies, have been proven to result in formation of [[reactive oxygen species]] in skin, which cause indirect damage since these are electronically excited molecules which can inflict reactive damage, although they do not cause sunburn (erythema).<ref>{{cite journal |vauthors = Liebel F, Kaur S, Ruvolo E, Kollias N, Southall MD |title=Irradiation of skin with visible light induces reactive oxygen species and matrix-degrading enzymes |journal=J. Invest. Dermatol. |volume=132 |issue=7 |pages=1901–1907 |year=2012 |pmid=22318388 |doi=10.1038/jid.2011.476|doi-access=free }}</ref> Like ionization-damage, all these effects in skin are beyond those produced by simple thermal effects.{{Citation needed|date=March 2021}}

== Measurement of radiation ==
[[File:Radioactivity and radiation.png|thumb|upright=1.2|Relationship between radioactivity and detected ionizing radiation. Key factors are strength of the radioactive source, trans&shy;mission effects and instrument sensitivity]]
{{radiation_units.svg|1=none{{!}}upright=2}}
The table below shows radiation and dose quantities in SI and non-SI units.{{Clear|right}}
{| class="wikitable plainrowheaders"
|+Methods of radiation measurement
|-
!scope="col"| Quantity
!scope="col"| [[Particle detector|Detector]]
!scope="col"| [[CGS units]]
!scope="col"| [[SI units]]
!scope="col"| Other units
|-
!scope="row"| Disintegration rate
| || [[Curie (unit)|curie]] || [[becquerel]] ||
|-
!scope="row"| Particle [[flux]]
| [[Geiger counter]], [[proportional counter]], [[scintillator]] || {{sfrac|counts|cm{{sup|2}} · second}} || {{sfrac|counts|metre{{sup|2}} · second}} || [[counts per minute]], particles per cm{{sup|2}} per second
|-
!scope="row"| Energy [[fluence]]
| [[thermoluminescent dosimeter]], [[film badge dosimeter]] || {{sfrac|MeV|cm{{sup|2}}}} || {{sfrac|joule|metre{{sup|2}}}} ||
|-
!scope="row"| Beam energy
| [[proportional counter]] || [[electronvolt]] || [[joule]] ||
|-
!scope="row"| [[Linear energy transfer]]
| derived quantity || {{sfrac|MeV|cm}} || {{sfrac|[[Joule]]|metre}} || {{sfrac|keV|μm}}
|-
!scope="row"| [[Kerma (physics)|Kerma]]
| [[ionization chamber]], [[semiconductor detector]], [[quartz fiber dosimeter]], [[Kearny fallout meter]] || {{sfrac|[[Statcoulomb|esu]]|cm{{sup|3}}}} || [[gray (unit)|gray]] ({{sfrac|joule|kg}}) || [[roentgen (unit)|roentgen]]
|-
!scope="row"| [[Absorbed dose]]
| [[calorimeter]] || [[rad (unit)|rad]] || [[gray (unit)|gray]] || [[Röntgen equivalent physical|rep]]
|-
!scope="row"| [[Equivalent dose]]
| derived quantity || [[rem (unit)|rem]] || [[sievert]] ({{sfrac|joule|kg}} × [[Sievert#Calculating protection dose quantities|''W{{sub|R}}'']])||
|-
!scope="row"| [[Effective dose (radiation)|Effective dose]]
| derived quantity || [[rem (unit)|rem]] || [[sievert]] ({{sfrac|joule|kg}} × [[Sievert#Calculating protection dose quantities|''W{{sub|R}}'' × ''W{{sub|T}}'']])|| [[Background radiation equivalent time|BRET]]
|-
!scope="row"| [[Committed dose]]
| derived quantity || [[rem (unit)|rem]] || [[sievert]] || [[banana equivalent dose]]
|}

== Uses of radiation ==
{{main article|Nuclear technology|Radiobiology}}
Ionizing radiation has many industrial, military, and medical uses. Its usefulness must be balanced with its hazards, a compromise that has shifted over time. For example, at one time, assistants in shoe shops in the US [[Shoe-fitting fluoroscope|used X-rays to check a child's shoe size]], but this practice was halted when the risks of ionizing radiation were better understood.<ref>{{cite journal |title=The Shoe-fitting Fluoroscope as a Radiation Hazard |journal=California Medicine |first1=Leon |last1=Lewis |first2=Paul E |last2=Caplan |volume=72 |issue=1 |pmid=15408494 |date=January 1, 1950 |pmc=1520288 |pages=26–30 [27]}}</ref>

Neutron radiation is essential to the working of [[nuclear reactor]]s and [[nuclear weapon]]s. The penetrating power of x-ray, gamma, beta, and positron radiation is used for [[medical imaging]], [[nondestructive testing]], and a variety of industrial gauges. [[Radioactive tracers]] are used in medical and industrial applications, as well as biological and [[radiation chemistry]]. Alpha radiation is used in [[static eliminator]]s and [[smoke detectors]]. The sterilizing effects of ionizing radiation are useful for cleaning medical instruments, [[food irradiation]], and the [[sterile insect technique]]. Measurements of [[carbon-14]], is used for [[radiocarbon dating]].

== Sources of radiation ==
Ionizing radiation is generated through nuclear reactions, nuclear decay, by very high temperature, or via acceleration of charged particles in electromagnetic fields. Natural sources include the Sun, lightning and supernova explosions. Artificial sources include nuclear reactors, particle accelerators, and [[x-ray tube]]s.

The [[United Nations Scientific Committee on the Effects of Atomic Radiation]] (UNSCEAR) itemized types of human exposures.

{| class="wikitable"
|+ Type of radiation exposures
|-
| bgcolor="#dfffdf" colspan="3" | '''Public exposure'''
|-
| rowspan=10 | Natural Sources || rowspan=2 |Normal occurrences || [[Cosmic radiation]]
|-
| [[Terrestrial background radiation|Terrestrial radiation]]
|-
| rowspan=8 | Enhanced sources || [[Metal mining]] and [[smelting]]
|-
| [[Phosphate]] industry
|-
| [[Coal mining]] and power production from coal
|-
| [[Oil well|Oil]] and gas drilling
|-
| [[Rare earth mineral|Rare earth]] and [[titanium dioxide]] industries
|-
| [[Zirconium]] and [[ceramic materials|ceramics]] industries
|-
| Application of [[radium]] and [[thorium]]
|-
| Other exposure situations
|-
| rowspan=5 | Human-made sources || rowspan=3 |Peaceful purposes || [[Nuclear power]] production
|-
| Transport of nuclear and radioactive material
|-
| Application other than nuclear power
|-
| rowspan=2 |Military purposes || [[Nuclear tests]]
|-
| Residues in the environment. [[Nuclear fallout]]
|-
| colspan=3| Historical situations
|-
| colspan=3| Exposure from accidents
|-
| bgcolor="#dfffdf" colspan="3" | '''Occupational radiation exposure'''
|-
| rowspan=4 | Natural Sources || || Cosmic ray exposures of [[aircrew]] and [[Astronaut|space crew]]
|-
| || Exposures in extractive and processing industries
|-
| || Gas and oil extraction industries
|-
| || Radon exposure in workplaces other than mines
|-
| rowspan=5 | Human-made sources || rowspan=4 |Peaceful purposes || Nuclear power industries
|-
| [[Radiology|Medical uses of radiation]]
|-
| Industrial uses of radiation
|-
| Miscellaneous uses
|-
| Military purposes || Other exposed workers
|-
| colspan=3| <small>Source [http://www.unscear.org/docs/reports/2008/09-86753_Report_2008_Annex_B.pdf UNSCEAR 2008 Annex B] retrieved 2011-7-4</small>
|}

The [[International Commission on Radiological Protection]] manages the International System of Radiological Protection, which sets recommended limits for dose uptake.

=== Background radiation ===
{{main article|Background radiation}}
Background radiation comes from both natural and human-made sources.

The global average exposure of humans to ionizing radiation is about 3&nbsp;mSv (0.3&nbsp;rem) per year, 80% of which comes from nature. The remaining 20% results from exposure to human-made radiation sources, mainly [[medical imaging]]. Average human-made exposure is much higher in developed countries, mostly due to [[CT scans]] and [[nuclear medicine]].

Natural [[background radiation]] comes from five primary sources: cosmic radiation, solar radiation, external terrestrial sources, radiation in the human body, and [[radon]].

The background rate for natural radiation varies considerably with location, being as low as 1.5&nbsp;mSv/a (1.5&nbsp;mSv per year) in some areas and over 100&nbsp;mSv/a in others. The highest level of purely natural radiation recorded on the Earth's surface is 90&nbsp;μGy/h (0.8&nbsp;Gy/a) on a Brazilian black beach composed of [[monazite]].<ref>{{cite book|last=[[United Nations Scientific Committee on the Effects of Atomic Radiation]]|title=Sources and Effects of Ionizing Radiation|year=2000|publisher=United Nations|chapter-url=http://www.unscear.org/unscear/publications/2000_1.html|access-date=11 November 2012|page=121|volume=1|chapter=Annex B|url-status=dead|archive-url=https://web.archive.org/web/20120804174956/http://www.unscear.org/unscear/publications/2000_1.html|archive-date=4 August 2012}}</ref> The highest background radiation in an inhabited area is found in [[Ramsar, Mazandaran|Ramsar]], mainly due to naturally radioactive limestone used as a building material. Some 2000 of the most exposed residents receive an average [[radiation dose]] of 10&nbsp;[[mGy]] per year, (1&nbsp;[[rad (unit)|rad]]/yr) ten times more than the ICRP recommended limit for exposure to the public from artificial sources.<ref name="Mortazavi2005">{{cite journal|last=Mortazavi|first=S.M.J.|author2=P.A. Karamb|title=Apparent lack of radiation susceptibility among residents of the high background radiation area in Ramsar, Iran: can we relax our standards?|journal=Radioactivity in the Environment|year=2005|volume=7|pages=1141–1147|doi=10.1016/S1569-4860(04)07140-2|issn=1569-4860|isbn=9780080441375}}</ref> Record levels were found in a house where the [[effective radiation dose]] due to external radiation was 135&nbsp;mSv/a, (13.5&nbsp;rem/yr) and the [[committed dose]] from [[radon]] was 640&nbsp;mSv/a (64.0&nbsp;rem/yr).<ref name="Sohrabi2005">{{cite journal|last=Sohrabi|first=Mehdi|author2=Babapouran, Mozhgan|journal=International Congress Series |title=New public dose assessment from internal and external exposures in low- and elevated-level natural radiation areas of Ramsar, Iran|year=2005|volume=1276|pages=169–174 |doi=10.1016/j.ics.2004.11.102}}</ref> This unique case is over 200 times higher than the world average background radiation. Despite the high levels of background radiation that the residents of Ramsar receive there is no compelling evidence that they experience a greater health risk. The ICRP recommendations are conservative limits and may represent an over representation of the actual health risk. Generally radiation safety organization recommend the most conservative limits assuming it is best to err on the side of caution. This level of caution is appropriate but should not be used to create fear about background radiation danger. Radiation danger from background radiation may be a serious threat but is more likely a small overall risk compared to all other factors in the environment.

==== Cosmic radiation ====
{{see also|Cosmic ray}}
The Earth, and all living things on it, are constantly bombarded by radiation from outside the [[Solar System]]. This cosmic radiation consists of relativistic particles: positively charged nuclei (ions) from {{val|1|ul=Da}} [[proton]]s (about 85% of it) to ~{{val|56|u=Da}} [[iron]] [[atomic nucleus|nuclei]] and even beyond. (The high-atomic number particles are called [[HZE ions]].) The energy of this radiation can far exceed that which humans can create, even in the largest [[particle accelerator]]s (see [[ultra-high-energy cosmic ray]]). This radiation interacts in the atmosphere to create secondary radiation that rains down, including [[x-rays]], [[muon]]s, [[proton]]s, [[antiproton]]s, [[alpha particle]]s, [[pion]]s, [[electron]]s, [[positron]]s, and [[neutron]]s.

The [[Dose (biochemistry)|dose]] from cosmic radiation is largely from muons, neutrons, and electrons, with a dose rate that varies in different parts of the world and based largely on the geomagnetic field, altitude, and solar cycle. The cosmic-radiation dose rate on airplanes is so high that, according to the United Nations UNSCEAR 2000 Report (see links at bottom), airline flight crew workers receive more dose on average than any other worker, including those in nuclear power plants. Airline crews receive more cosmic rays if they routinely work flight routes that take them close to the North or South pole at high altitudes, where this type of radiation is maximal.

Cosmic rays also include high-energy gamma rays, which are far beyond the energies produced by solar or human sources.

==== External terrestrial sources ====
Most materials on Earth contain some radioactive [[atom]]s, even if in small quantities. Most of the dose received from these sources is from gamma-ray emitters in building materials, or rocks and soil when outside. The major [[radionuclide]]s of concern for '''terrestrial radiation''' are isotopes of [[potassium]], [[uranium]], and [[thorium]]. Each of these sources has been decreasing in activity since the formation of the Earth.

==== Internal radiation sources ====
{{see also|Committed dose}}
All earthly materials that are the building blocks of life contain a radioactive component. As organisms consume food, air, and water, an inventory of radioisotopes builds up within the organism (see [[banana equivalent dose]]). Some radionuclides, like [[potassium-40]], emit a high-energy gamma ray that can be measured by sensitive electronic radiation measurement systems. These internal radiation sources contribute to an individual's total radiation dose from [[natural background radiation]].

==== Radon ====
{{see also|Health effects of radon}}
An important source of natural radiation is [[radon]] gas, which seeps continuously from bedrock but can, because of its high density, accumulate in poorly ventilated houses.

[[Radon-222]] is a gas produced by the α-decay of [[radium]]-226. Both are a part of the natural [[uranium]] decay chain. Uranium is found in soil throughout the world in varying concentrations. Radon is the largest cause of lung cancer among non-smokers and the second-leading cause overall.<ref>{{cite web |url=http://www.epa.gov/radon/healthrisks.html |title=Health Risks |website=Radon |publisher=EPA |access-date=2012-03-05 |url-status=dead |archive-url=https://web.archive.org/web/20081020231845/http://www.epa.gov/radon/healthrisks.html |archive-date=2008-10-20}}</ref>

== Radiation exposure ==
[[File:G radiation-level scale 01.png|thumb|right| Radiation level in a range of situations, from normal activities up to the Chernobyl reactor accident. Each step up the scale indicates a tenfold increase in radiation level.]]
[[File:Exposure chart-XKCD.svg|thumb|Various doses of radiation in sieverts, ranging from trivial to lethal.]]
[[File:Radiological exposure from daily life.png|thumb|Visual comparison of radiological exposure from daily life activities.]]
{{main article|Radiation protection}}
There are three standard ways to limit exposure:
# '''Time''': For people exposed to radiation in addition to natural background radiation, limiting or minimizing the exposure time will reduce the dose from the source of radiation.
# '''Distance''': Radiation intensity decreases sharply with distance, according to an [[inverse-square law]] (in an absolute vacuum).<ref name=Camphausen>Camphausen KA, Lawrence RC. [http://www.cancernetwork.com/cancer-management-11/chapter02/article/10165/1399960 "Principles of Radiation Therapy"] {{webarchive|url=https://web.archive.org/web/20090515031929/http://www.cancernetwork.com/cancer-management-11/chapter02/article/10165/1399960 |date=2009-05-15 }} in Pazdur R, Wagman LD, Camphausen KA, Hoskins WJ (Eds) [http://www.cancernetwork.com/cancer-management-11/ Cancer Management: A Multidisciplinary Approach] {{webarchive|url=https://web.archive.org/web/20131004224102/http://www.cancernetwork.com/cancer-management-11/ |date=2013-10-04 }}. 11 ed. 2008.</ref>
# '''Shielding''': Air or skin can be sufficient to substantially attenuate alpha radiation, while sheet metal or plastic is often sufficient to stop beta radiation. Barriers of [[lead]], [[concrete]], or water are often used to give effective protection from more penetrating forms of ionizing radiation such as gamma rays and [[neutrons]]. Some radioactive materials are stored or handled underwater or by [[remote control]] in rooms constructed of thick concrete or lined with lead. There are special [[plastic]] shields that stop beta particles, and air will stop most alpha particles. The effectiveness of a material in shielding radiation is determined by its [[half-value layer|half-value thicknesses]], the thickness of material that reduces the radiation by half. This value is a function of the material itself and of the type and energy of ionizing radiation. Some generally accepted thicknesses of attenuating material are 5&nbsp;mm of aluminum for most beta particles, and 3&nbsp;inches of lead for gamma radiation.

These can all be applied to natural and human-made sources. For human-made sources the use of '''Containment''' is a major tool in reducing dose uptake and is effectively a combination of shielding and isolation from the open environment. Radioactive materials are confined in the smallest possible space and kept out of the environment such as in a [[hot cell]] (for radiation) or [[glove box]] (for contamination). [[Radioactive isotope]]s for medical use, for example, are dispensed in closed handling facilities, usually gloveboxes, while [[nuclear reactor]]s operate within closed systems with multiple barriers that keep the radioactive materials contained. Work rooms, hot cells and gloveboxes have slightly reduced air pressures to prevent escape of airborne material to the open environment.

In nuclear conflicts or civil nuclear releases [[civil defense]] measures can help reduce exposure of populations by reducing ingestion of isotopes and occupational exposure. One is the issue of [[potassium iodide]] (KI) tablets, which blocks the uptake of [[iodine-131|radioactive iodine]] (one of the major radioisotope products of [[nuclear fission]]) into the human [[thyroid]] gland.

=== Occupational exposure ===
Occupationally exposed individuals are controlled within the regulatory framework of the country they work in, and in accordance with any local nuclear licence constraints. These are usually based on the recommendations of the [[International Commission on Radiological Protection]].
The ICRP recommends limiting artificial irradiation. For occupational exposure, the limit is 50 mSv in a single year with a maximum of 100 mSv in a consecutive five-year period.{{sfn | ICRP | 2007 }}

The radiation exposure of these individuals is carefully monitored with the use of [[dosimeter]]s and other radiological protection instruments which will measure radioactive particulate concentrations, area gamma dose readings and [[radioactive contamination]]. A legal record of dose is kept.

Examples of activities where occupational exposure is a concern include:
* Airline crew (the most exposed population)
* [[Industrial radiography]]
* Medical [[radiology]] and [[nuclear medicine]]<ref>{{cite journal|doi=10.1088/0952-4746/16/4/004|vauthors = Pattison JE, Bachmann DJ, Beddoe AH|title=Gamma Dosimetry at Surfaces of Cylindrical Containers|journal=Journal of Radiological Protection|volume= 16|issue=4|pages=249–261|year=1996|bibcode = 1996JRP....16..249P | s2cid=71757795 }}</ref><ref>{{cite journal|doi=10.1097/00004032-199911000-00006|author=Pattison, J.E.|title= Finger Doses Received during Samarium-153 Injections|journal=Health Physics|volume=77|issue=5|pages=530–5|year=1999|pmid=10524506}}</ref>
* Uranium mining
* [[Nuclear power plant]] and [[nuclear fuel reprocessing plant]] workers
* Research laboratories (government, university and private)

Some human-made radiation sources affect the body through direct radiation, known as [[effective dose (radiation)]] while others take the form of [[radioactive contamination]] and [[irradiate]] the body from within. The latter is known as [[committed dose]].

=== Public exposure ===
Medical procedures, such as diagnostic [[X-ray]]s, [[nuclear medicine]], and [[radiation therapy]] are by far the most significant source of human-made radiation exposure to the general public. Some of the major [[radionuclide]]s used are [[I-131]], [[Tc-99m]], [[Co-60]], [[iridium|Ir-192]], and [[caesium|Cs-137]]. The public is also exposed to radiation from consumer products, such as [[tobacco]] ([[polonium]]-210), combustible fuels (gas, [[coal]], etc.), [[television]]s, luminous [[watch]]es and dials ([[tritium]]), airport [[X-ray]] systems, [[smoke detector]]s ([[americium]]), electron tubes, and gas lantern mantles ([[thorium]]).

Of lesser magnitude, members of the public are exposed to radiation from the [[nuclear fuel]] cycle, which includes the entire sequence from processing [[uranium]] to the disposal of the spent fuel. The effects of such exposure have not been reliably measured due to the extremely low doses involved. Opponents use a cancer per dose model to assert that such activities cause several hundred cases of cancer per year, an application of the widely accepted [[Linear no-threshold model]] (LNT).

The International Commission on Radiological Protection recommends limiting artificial irradiation to the public to an average of 1&nbsp;mSv (0.001&nbsp;Sv) of effective dose per year, not including medical and occupational exposures.{{sfn | ICRP | 2007 }}

In a [[nuclear war]], gamma rays from both the initial weapon explosion and [[fallout]] would be sources of radiation exposure.

==== Spaceflight ====
Massive particles are a concern for astronauts outside the [[Earth's magnetic field]] who would receive solar particles from [[solar proton event]]s (SPE) and [[galactic cosmic rays]] from cosmic sources. These high-energy charged nuclei are blocked by Earth's magnetic field but pose a [[Health threat from cosmic rays|major health concern]] for astronauts traveling to the Moon and to any distant location beyond Earth orbit. Highly charged HZE ions in particular are known to be extremely damaging, though protons make up the vast majority of galactic cosmic rays. Evidence indicates past SPE radiation levels that would have been lethal for unprotected astronauts.<ref>{{cite web|url=https://www.newscientist.com/article/dn7142|title=Superflares could kill unprotected astronauts|publisher=New Scientist|date=21 March 2005|url-status=live|archive-url=https://web.archive.org/web/20150327023245/http://www.newscientist.com/article/dn7142|archive-date=27 March 2015}}</ref>

==== Air travel ====
Air travel exposes people on aircraft to increased radiation from space as compared to sea level, including [[cosmic rays]] and from [[solar flare]] events.<ref>{{cite web |title=Effective Dose Rate |url=http://sol.spacenvironment.net/raps_ops/current_files/globeView.html |website=NAIRAS (Nowcast of Atmospheric Ionizing Radiation System) |url-status=live |archive-url=https://web.archive.org/web/20160305061814/http://sol.spacenvironment.net/raps_ops/current_files/globeView.html |archive-date=2016-03-05 }}</ref><ref name=davis>{{cite book |url=https://books.google.com/books?id=_6hymYAgC6MC&pg=PA229 |author1=Jeffrey R. Davis |author2=Robert Johnson |author3=Jan Stepanek |title=Fundamentals of Aerospace Medicine |year=2008 |pages=221–230 |publisher=Lippincott Williams & Wilkins |via=Google Books |isbn=9780781774666 |access-date=2015-06-27 |archive-date=2020-08-03 |archive-url=https://web.archive.org/web/20200803180145/https://books.google.com/books?id=_6hymYAgC6MC&pg=PA229 |url-status=live }}</ref> Software programs such as [[Epcard]], CARI, SIEVERT, PCAIRE are attempts to simulate exposure by aircrews and passengers.<ref name=davis/> An example of a measured dose (not simulated dose) is 6&nbsp;μSv per hour from London Heathrow to Tokyo Narita on a polar route.<ref name=davis/> However, dosages can vary, such as during periods of high solar activity.<ref name=davis/> The United States FAA requires airlines to provide flight crew with information about cosmic radiation, and an [[International Commission on Radiological Protection]] recommendation for the general public is no more than 1&nbsp;mSv per year.<ref name=davis/> Also, many airlines do not allow [[pregnancy|pregnant]] flightcrew members, to comply with a European Directive.<ref name=davis/> The FAA has a recommended limit of 1&nbsp;mSv total for a pregnancy, and no more than 0.5&nbsp;mSv per month.<ref name=davis/> Information originally based on ''Fundamentals of Aerospace Medicine'' published in 2008.<ref name=davis/>

=== Radiation hazard warning signs ===
Hazardous levels of ionizing radiation are signified by the trefoil sign on a yellow background. These are usually posted at the boundary of a radiation controlled area or in any place where radiation levels are significantly above background due to human intervention.

The red ionizing radiation warning symbol (ISO 21482) was launched in 2007, and is intended for [[IAEA]] Category 1, 2 and 3 sources defined as dangerous sources capable of death or serious injury, including food irradiators, teletherapy machines for cancer treatment and industrial radiography units. The symbol is to be placed on the device housing the source, as a warning not to dismantle the device or to get any closer. It will not be visible under normal use, only if someone attempts to disassemble the device. The symbol will not be located on building access doors, transportation packages or containers.<ref name=radiationsymbol>{{cite web |title=New Symbol Launched to Warn Public About Radiation Dangers |date=February 15, 2007 |url=http://www.iaea.org/newscenter/news/2007/radiationsymbol.html |publisher=International Atomic Energy Agency |url-status=live |archive-url=https://web.archive.org/web/20070217075115/http://www.iaea.org/NewsCenter/News/2007/radiationsymbol.html |archive-date=2007-02-17 }}</ref>
<gallery class="center" widths="250px" heights="200px">
File:Radioactive.svg|Ionizing radiation [[hazard symbol]]
File:Logo iso radiation.svg|2007 ISO [[radioactivity]] danger symbol intended for [[IAEA]] Category 1, 2 and 3 sources defined as dangerous sources capable of death or serious injury.<ref name=radiationsymbol/>
</gallery>

== See also ==
{{Portal|Energy|Nuclear technology}}
{{cols|colwidth=15em}}
* [[European Committee on&nbsp;Radiation Risk]]
* [[International Commission on&nbsp;Radiological Protection]] – manages the International System of Radiological Protection
* [[Ionometer]]
* [[Irradiated mail]]
* [[National Council on Radiation Protection and Measurements]] – US national organisation
* [[Nuclear safety]]
* [[Nuclear semiotics]]
* [[Radiant energy]]
* [[Exposure (radiation)]]
* [[Radiation hormesis]]
* [[Radiation physics]]
* [[Radiation protection]]
* [[Radiation Protection Convention, 1960]]
* [[Radiation protection of patients]]
* [[Sievert]]
* [[Treatment of infections after accidental or hostile exposure to ionizing radiation]]
{{colend}}

== References ==
{{reflist|30em}}

== Literature ==
* {{cite book |author=ICRP |title=The 2007 Recommendations of the International Commission on Radiological Protection (Annals of the ICRP) |year=2007 |volume=37:2–4 |series=ICRP publication 103 |url=http://www.icrp.org/publication.asp?id=ICRP%20Publication%20103 |access-date=17 May 2012 |isbn=978-0-7020-3048-2 |url-status=dead |archive-url=https://web.archive.org/web/20121116084754/http://www.icrp.org/publication.asp?id=ICRP+Publication+103 |archive-date=16 November 2012 }}

== External links ==
* [http://www.nrc.gov The Nuclear Regulatory Commission] regulates most commercial radiation sources and non-medical exposures in the US:
* [https://web.archive.org/web/20180618152308/https://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@na+@rel+ionizing+radiation NLM Hazardous Substances Databank – Ionizing Radiation]
* [[United Nations Scientific Committee on the Effects of Atomic Radiation]] 2000 Report [http://www.unscear.org/unscear/en/publications/2000_1.html Volume 1: Sources], [http://www.unscear.org/unscear/en/publications/2000_2.html Volume 2: Effects]
* [http://www.npl.co.uk/educate-explore/posters/ionising-radiation/ Beginners Guide to Ionising Radiation Measurement]
* {{Cite web|url=http://www.xrayrisk.com|title=XrayRisk.com : Radiation Risk Calculator. Calculate Radiation Dose and Cancer Risk|author=Mike Hanley}} (from CT scans and xrays).
* [http://www.x-raysafety.com/rad-safety.htm Free Radiation Safety Course] {{Webarchive|url=https://web.archive.org/web/20180416160511/http://www.x-raysafety.com/rad-safety.htm |date=2018-04-16 }}
* [http://www.radiationanswers.org Health Physics Society Public Education Website]
* [https://web.archive.org/web/20040623102755/http://www.ornl.gov/sci/env_rpt/aser95/appa.htm Oak Ridge Reservation] Basic Radiation Facts

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{{Carcinogen}}
{{Nuclear technology}}
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{{Ionising radiation related quantities}}
{{Authority control}}

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[[Category:Ionizing radiation| ]]
[[Category:Carcinogens]]
[[Category:Mutagens]]
[[Category:Radioactivity]]
[[Category:Radiobiology]]
[[Category:Radiation health effects]]
[[Category:Radiation protection]]