Ionizing (or ionising) radiation is radiation composed of particles that individually can liberate an electron from an atom or molecule, producing ions, which are atoms or molecules with a net electric charge. These tend to be especially chemically reactive, and the reactivity produces the high biological damage caused per unit of energy of ionizing radiation.
The degree and nature of such ionization depends on the energy and type of the individual particles composing the radiation, and less upon the radiation particle number. For example, if a non-ionizing type of radiation does not heat a bulk substance up to ionization temperature, even an intense flood of particles or particle-waves will not cause ionization. In such cases, each particle or particle-wave does not carry enough individual energy to be ionizing (an example is a high-powered radio or microwave beam, which will not ionize if it does not cause high temperatures). Conversely, even very low-intensity radiation will ionize materials at low temperatures and powers, if the individual particles of radiation carry enough energy (e.g., a low-power X-ray beam). In general, particles or photons with energies above about 10 electron volts (eV) are considered ionizing, no matter what their intensity. This particle-energy occurs in electromagnetic waves in the extreme ultraviolet and beyond, to include all of (X-rays and gamma rays).
Free neutrons are able to cause many nuclear reactions in a variety of substances no matter their energy, because in many substances they give rise to high-energy nuclear reactions, and these (or their products) liberate enough energy to cause ionization. For this reason, free neutrons are normally considered effectively ionizing radiation, at any energy (see neutron radiation). Examples of other ionizing particles are alpha particles, beta particles, and cosmic rays. The radiations cause ionization due to the kinetic energy involved in the production of the individual particles, which inevitably exceed 10 eV, and commonly exceed thousands or even millions of eV of energy.
The ability of an electromagnetic wave (photons) to ionize an atom or molecule depends on its frequency, which determines the energy of its associated particle, the photon. Radiation on the high-frequency and short-wavelength end of the electromagnetic spectrum—high-frequency ultraviolet, X-rays, and gamma rays—is ionizing due to the energy of such photons exceeding 10 eV. Lower-energy radiation, such as visible light, infrared, microwaves, and radio waves, are not ionizing. The latter types of low-energy non-ionizing radiation may damage molecules, but the effect is generally indistinguishable from the effects of simple heating. Such heating does not produce free radicals until higher temperatures (for example, flame temperatures or "browning" temperatures, and above) are attained. In contrast, ionizing radiation may produce free radicals, such as reactive oxygen species, even at room temperatures and below. Free radical production is also a primary basis for the particular danger to biological systems of relatively small amounts of ionizing radiation that are far smaller than needed to produce significant heating. Free radicals easily damage DNA, and thus ionizing radiation may also directly damage DNA by ionizing or breaking DNA molecules.
Among radiobiologists, there is a degree of overlap in regard between ionizing radiation, and the lower ultraviolet spectrum that contains a range of molecularly-damaging radiation that is not ionizing, but has somewhat similar biological effects. Although DNA is always susceptible to damage by ionizing radiation, DNA molecules may also be damaged by radiation carrying enough energy to excite certain molecular bonds to form thymine dimers (pyrimidine dimer)s. This energy is less than ionizing, but produces similar types of damage. The ultraviolet spectrum begins at about 3.1 eV (400 nm) at almost exactly the same energy level which can cause sunburn to unprotected skin, as a result of photoreactions in collagen, and (in the UV-B range) also in DNA. Thus, the entire ultraviolet spectrum is damaging to living cells as a result of electronic excitation in molecules which falls short of ionization. Because such damage is similar to ionizing radiation inasmuch as it is larger than is predictable from thermal considerations alone, it has effects similar to ionizing radiation, including the ability to cause skin cancer in relatively small doses.
Ionizing radiation is ubiquitous in the environment, and comes from naturally occurring radioactive materials and cosmic rays. Common artificial sources are artificially produced radioisotopes, X-ray tubes and particle accelerators. Ionizing radiation is invisible and not directly detectable by human senses, so instruments such as Geiger counters are usually required to detect its presence. In some cases it may lead to secondary emission of visible light upon interaction with matter, such as in Cherenkov radiation and radioluminescence. It has many practical uses in medicine, research, construction, and other areas, but presents a health hazard if used improperly. Exposure to ionizing radiation causes damage to living tissue, and can result in mutation, radiation sickness, cancer, and death.
US Federal Communications Commission material defines ionizing radiation as that with a photon energy of greater than 10 eV (equivalent to a far ultraviolet wavelength of 124 nanometers). Roughly, this corresponds to both the first ionization energy of oxygen, and the ionization energy of hydrogen, both about 14 eV  In some Environmental Protection Agency references, the ionization of a typical water molecule at an energy of 33 eV is referenced  as the appropriate biological threshold for ionizing radiation: this value represents the so-called W-value, the colloquial name for the ICRU's mean energy expended in a gas per ion pair formed, which combines ionization energy plus the energy lost to other processes such as excitation. At 38 nanometers wavelength for electromagnetic radiation, 33 eV is close to the energy at the conventional 10 nm wavelength transition between extreme ultraviolet, and X-ray radiation, which occurs at about 125 eV. Thus, X-ray radiation is always ionizing, but only extreme-ultraviolet radiation can be considered ionizing under all definitions.
As noted, the biological effect of ionizing radiation on cells somewhat resembles that of a broader spectrum of molecularly-damaging radiation, which overlaps ionizing radiation and extends beyond, to somewhat lower energies. Although 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 bonds to form thymine dimers. This energy may be less than ionizing, but near to it. A good example is ultraviolet spectrum energy which begins at about 3.1 eV (400 nm) at almost exactly the same energy level which can cause sunburn to unprotected skin, as a result of photoreactions in collagen and (in the UV-B range) also damage in DNA (for example, pyrimidine dimers). Thus, the lower ultraviolet electromagnetic spectrum is damaging to biological tissues as a result of electronic excitation in molecules which falls short of ionization, but produce similar non-thermal effects.
Various types of ionizing radiation may be produced by radioactive decay, nuclear fission and nuclear fusion, and by particle accelerators and naturally occurring cosmic rays. Muons and many types of mesons (in particular charged pions) are also ionizing.
In order for a particle to be ionizing, it must both have a high enough energy and interact with the atoms of a target.
Photons interact electromagnetically with charged particles, so photons of sufficiently high energy also are ionizing. The energy at which this begins to happen with photons (light) lies in the high-frequency end of the ultraviolet (UV) region of the electromagnetic spectrum. As noted above, most UV is not ionizing, but all UV may cause molecular damage in a somewhat similar way, and thus all UV (like ionizing radiation) is more biologically harmful than expected from its heating effect and simple energy deposition.
Charged particles such as electrons, positrons, muons, protons, alpha particles, and heavy atomic nuclei from accelerators or cosmic rays also interact electromagnetically with electrons of an atom or molecule, and all may cause ionization. Muons contribute to background radiation due to cosmic rays, but by themelves are thought to be of little hazard-importance, due to their relatively low dose. Pions (another very short-lived sometimes-charged particle) may be produced in large amounts in the largest particle accelerators. Pions are not a theoretical biological hazard except near such operating accelerator machines, which are then subject to heavy security.
Neutrons, on the other hand, having zero electrical charge, do not interact electromagnetically with electrons, and so they cannot directly cause ionization by this mechanism. However, fast neutrons will interact with the protons in hydrogen (in the manner of a billiard ball hitting another, head on, sending it away with all of the first ball's energy of motion), and this mechanism produces proton radiation (fast protons). These protons are ionizing because they are of high energy, are charged, and interact with the electrons in matter.
A neutron can also interact with other atomic nuclei, depending on the nucleus and the neutron's velocity; these reactions happen with fast neutrons and slow neutrons, depending on the situation. Neutron interactions in this manner often produce radioactive nuclei, which produce ionizing radiation when they decay. In fissile materials, secondary neutrons may produce nuclear chain reactions, sometimes causing a larger amount of ionization.
An ionization event normally produces a positive atomic ion and an electron. High-energy beta particles may produce bremsstrahlung as they pass through matter, or secondary electrons (δ-electrons); both can ionize in turn. Energetic Beta-particles, like those emitted by 32P, are quickly decelerated by surrounding matter. The energy lost to deceleration is emitted in the form of X-rays called "bremsstrahlung," which translates to "braking radiation". Bremsstrahlung is of concern when shielding beta emitters. The intensity of bremsstrahlung increases with the increase in energy of the electrons and the atomic number of the absorbing medium.
Unlike alpha or beta particles (see particle radiation), gamma rays do not ionize all along their path, but rather interact with matter in one of three ways: the photoelectric effect, the Compton effect, and pair production. By way of example, the figure shows Compton effect: two Compton scatterings that happen 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.
In the same figure, the 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.
See the Biological effects section below for detail.
Non-ionizing radiation up to the energy of visible light is thought to be essentially harmless below the levels that cause heating. Ultraviolet radiation, although non-ionizing except at its very highest frequencies, can be biologically hazardous in all of its forms, due to its ability to electronically excite molecules and cause biological damage by breaking or re-arranging chemical bonds (for example, in DNA). Ionizing radiation is always far more dangerous per energy unit of direct exposure than non-ionizing radidation, although the degree of danger for some types of radiation remains a subject of debate.
The negatively-charged electrons and positively charged ions created by ionizing radiation may cause damage in living tissue. If the dose is sufficient, the effect may be seen almost immediately, in the form of radiation poisoning. See criticality accident for a number of cases of accidental radiation poisoning and their outcomes.
Lower doses may cause cancer or other long-term problems. The effect of the very low doses encountered in normal circumstances (from both natural and artificial sources, like cosmic rays, medical X-rays and nuclear power plants) is a subject of current debate. A 2005 report released by the U.S. National Research Council (the BEIR VII report, summarized in ) indicated that the overall cancer risk associated with background sources of radiation was relatively low. Some even propose that low-level doses of ionizing radiation are beneficial, by stimulating the immune system and self-repair mechanisms of cells. This hypothesis is called radiation hormesis.
Radioactive materials usually release alpha particles (which are the nuclei of helium), beta particles (which are quickly moving electrons or positrons), gamma rays, or neutrons. Alpha and beta particles can often be stopped by a piece of paper or a sheet of aluminium, respectively. They cause most damage when they are emitted inside the human body. Gamma rays are less ionizing than either alpha or beta particles, but protection against gammas requires thicker shielding. The damage they produce is similar to that caused by X-rays, and include burns and also cancer, through mutations. Neutron radiation also causes mutations through interactions with the hydrogen atoms (including hydrocarbons) in the body. Neutrons can be extremely dangerous because of their ability to penetrate through most materials.
Humans and animals are also exposed to ionizing radiation internally: As radioactive isotopes are present in the environment, they can be taken into the body. For example, radioactive iodine is treated as normal iodine by the body and used by the thyroid; its accumulation there can lead to thyroid cancer. Some radioactive elements also bioaccumulate.
The units used to measure ionizing radiation are rather complex. The ionizing effects of radiation are measured by units of exposure:
However, the amount of damage done to matter (especially living tissue) by ionizing radiation is more closely related to the amount of energy deposited rather than the charge. This is called the absorbed dose.
Equal doses of different types or energies of radiation cause different amounts of damage to living tissue. For example, 1 Gy of alpha radiation causes about 20 times as much damage as 1 Gy of X-rays. Therefore, the equivalent dose was defined to give an approximate measure of the biological effect of radiation. It is calculated by multiplying the absorbed dose by a weighting factor WR, which is different for each type of radiation (see above table). This weighting factor is also called the Q (quality factor), or RBE (relative biological effectiveness of the radiation).
For comparison, the average 'background' dose of natural radiation received by a person per year is assumed by BRET to be 6.6 μSv (660 mrem), but the average in the US is around 3.6 mSv (360 mrem), and in a small area in India is as high as 30mSv (3000 mrem).  The lethal full-body dose of radiation for a human is around 4–5 Sv (400–500 rem).
Ionizing radiation has many uses, including killing cancerous cells and power generation. However, although ionizing radiation has many applications, overuse can be hazardous to human health. For example, at one time, assistants in shoe shops used X-rays to check a child's shoe size, but this practice was halted when it was discovered that ionizing radiation is dangerous.
Nuclear reactors produce large quantities of ionizing radiation as a byproduct of fission during operation. In addition, they produce highly radioactive nuclear waste, which will emit ionizing radiation for thousands of years for some of the fission byproducts. The safe disposal of this waste in a way that protects future generations from radiation exposure is currently imperfect and remains a highly controversial issue.
Radiation emissions from high level nuclear waste decrease extremely slowly, which requires long term containment and storage for thousands of years before it is considered safe. During normal conditions, radioactive emissions from nuclear power plants are generally lower than coal-burning plants; though several high profile nuclear accidents have released dangerous levels of radioactivity.
See main article: Industrial radiography. Since some ionizing radiation (principly gamma) can penetrate matter, they are used for a variety of measuring methods.
X-rays and gamma rays are used to make images of the inside of solid products, as a means of nondestructive testing and inspection. The piece to be radiographed is placed between the source and a photographic film in a cassette. After a certain exposure time, the film is developed and it shows internal defects of the material if there are any.
Two ionisation chambers are placed next to each other. Both contain a small source of 241Am that gives rise to a small constant current. One is closed and serves for comparison, the other is open to ambient air; it has a gridded electrode. When smoke enters the open chamber, the current is disrupted as the smoke particles attach to the charged ions and restore them to a neutral electrical state. This reduces the current in the open chamber. When the current drops below a certain threshold, the alarm is triggered.
The largest use of ionizing radiation in medicine is in medical radiography to make images of the inside of the human body using x-rays. This is the largest artificial source of radiation exposure for humans. Radiation is also used to treat diseases in radiation therapy. Tracer methods (mentioned above) are used in nuclear medicine to diagnose diseases, and widely used in biological research.
In biology and agriculture, radiation is used to induce mutations to produce new or improved species. Another use in insect control is the sterile insect technique, where male insects are sterilized by radiation and released, so they have no offspring, to reduce the population.
In industrial and food applications, radiation is used for sterilization of tools and equipment. An advantage is that the object may be sealed in plastic before sterilization. An emerging use in food production is the sterilization of food using food irradiation.
Detractors of food irradiation have concerns about the health hazards of induced radioactivity. Also, a report for the American Council on Science and Health entitled "Irradiated Foods" states: "The types of radiation sources approved for the treatment of foods have specific energy levels well below that which would cause any element in food to become radioactive. Food undergoing irradiation does not become any more radioactive than luggage passing through an airport X-ray scanner or teeth that have been X-rayed." 
Natural and artificial radiation sources are similar in their effects on matter.
The average exposure for Americans is about 360 mrem (3.6 mSv) per year, 81 percent of which comes from natural sources of radiation. The remaining 19 percent results from exposure to human-made radiation sources such as medical X-rays, most of which is deposited in people having had CT scans. However, in some areas, the average background dose can be over 1,000 mrem (10 mSv) per year. 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.
The background rate for radiation varies considerably with location, being as low as 1.5 mSv/a (1.5 mSv per year) in some areas and over 100 mSv/a in others. People in some parts of Ramsar, a city in northern Iran, receive an annual absorbed dose from background radiation that is up to 260 mSv/a. Despite having lived for many generations in these high-background areas, inhabitants of Ramsar show no significant cytogenetic differences compared to people in normal background areas. This has led to the suggestion that high but steady levels of radiation are easier for humans to sustain than sudden radiation bursts.
See also: Cosmic ray. The Earth, and all living things on it, are constantly bombarded by radiation from outside our solar system. This cosmic radiation consists of positively-charged ions from protons to iron nuclei. The energy of this radiation can far exceed that which humans can create even in the largest particle accelerators (see ultra-high-energy cosmic ray). This radiation interacts in the atmosphere to create secondary radiation that rains down, including x-rays, muons, protons, alpha particles, pions, electrons, and neutrons.
The 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.
Most materials on Earth contain some radioactive atoms, 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 radionuclides of concern for terrestrial radiation are isotopes of potassium, uranium, and thorium. Each of these sources has been decreasing in activity since the birth of the Earth.
All Earthly materials that are the building-blocks of life contain a radioactive component. As humans, plants, and animals 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. Other radionuclides, like carbon-14, have such a long half-life that they can be used to date the remains of long-dead organisms (such as wood that is thousands of years old). These internal radiation sources contribute to an individual's total radiation dose from natural background radiation.
Radon-222 is a gas produced by the 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. Since radon is a gas, it can accumulate in homes. Accumulation is dependent upon home location as well as building methods. Among non-smokers, Radon is the number one cause of lung cancer and, overall, the second leading cause.
Above the background level of radiation exposure, the U.S. Nuclear Regulatory Commission (NRC) requires that its licensees limit human-made radiation exposure for individual members of the public to 100 mrem (1 mSv) per year, and limit occupational radiation exposure to adults working with radioactive material to 5,000 mrem (50 mSv) per year.Occupationally exposed individuals are exposed according to the sources with which they work.The radiation exposure of these individuals is carefully monitored with the use of pocket-pen-sized instruments called dosimeters.
Examples of industries where occupational exposure is a concern include:
Medical procedures, such as diagnostic X-rays, 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 radionuclides used are I-131, Tc-99, Co-60, Ir-192, and Cs-137. These are rarely released into the environment. The public also is exposed to radiation from consumer products, such as tobacco (polonium-210), building materials, combustible fuels (gas, coal, etc.), ophthalmic glass, televisions, luminous watches and dials (tritium), airport X-ray systems, smoke detectors (americium), road construction materials, electron tubes, fluorescent lamp starters, and lantern mantles (thorium). A typical dose for radiation therapy might be 7 Gy spread daily (on weekdays) over two months.
Of lesser magnitude, members of the public are exposed to radiation from the nuclear fuel cycle, which includes the entire sequence from mining and milling of uranium to the disposal of the spent fuel, as well as the coal power cycle due to the release and emission of radioactive contaminants that were trapped in the coal. The effects of such exposure have not been reliably measured due to the extremely low doses involved. Estimates of exposure are low enough that proponents of nuclear power liken them to the mutagenic power of wearing trousers for two extra minutes per year (because heat causes mutation). Opponents use a cancer per dose model to assert that such activities cause several hundred cases of cancer per year, an application of the controversial Linear no-threshold model (LNT).
In a nuclear war, gamma rays from fallout of nuclear weapons would probably cause the largest number of casualties. Immediately downwind of targets, doses would exceed 300 Gy per hour. As a reference, 4.5 Gy (around 15,000 times the average annual background rate) is fatal to half of a normal population, without medical treatment.
The biological effects of radiation are thought of in terms of their effects on living cells. For low levels of radiation, the biological effects are so small they may not be detected in epidemiological studies. The body repairs many types of radiation and chemical damage. Biological effects of radiation on living cells may result in a variety of outcomes, including:
Other observations at the tissue level are more complicated. These include:
DNA double-strand breaks (DSBs) are generally accepted to be the most biologically significant lesion by which ionizing radiation causes cancer and hereditary disease. In vitro, each cell's DNA suffers 35 DSBs per gray. Most of the induced DSBs are repaired within 24h after exposure, however, 25% of the repaired strands are repaired incorrectly and about 20% of fibroblast cells that were exposed to 200mGy died within 4 days after exposure. 
Acute radiation exposure is an exposure to ionizing radiation that occurs during a short period of time. There are routine brief exposures, and the boundary at which it becomes significant is difficult to identify. Extreme examples include
The effects of acute events are more easily studied than those of chronic exposure.
Exposure to ionizing radiation over an extended period of time is called chronic exposure. The term chronic (Greek cronos = time) refers to the duration, not the magnitude or seriousness. The natural background radiation is chronic exposure, but a normal level is difficult to determine due to variations. Geographic location and occupation often affect chronic exposure.
The associations between ionizing radiation exposure and the development of cancer are based mostly on populations exposed to relatively high levels of ionizing radiation, such as Japanese atomic bomb survivors, and recipients of selected diagnostic or therapeutic medical procedures.
Cancers associated with high-dose exposure include leukemia, thyroid, breast, bladder, colon, liver, lung, esophagus, ovarian, multiple myeloma, and stomach cancers. United States Department of Health and Human Services literature also suggests a possible association between ionizing radiation exposure and prostate, nasal cavity/sinuses, pharyngeal and laryngeal, and pancreatic cancer.
The period of time between radiation exposure and the detection of cancer is known as the latent period. Those cancers that may develop as a result of radiation exposure are indistinguishable from those that occur naturally or as a result of exposure to other carcinogens. Furthermore, National Cancer Institute literature indicates that chemical and physical hazards and lifestyle factors, such as smoking, alcohol consumption, and diet, significantly contribute to many of these same diseases.
Although radiation may cause cancer at high doses and high dose rates, public health data regarding lower levels of exposure, below about 1,000 mrem (10 mSv), are harder to interpret. To assess the health impacts of lower radiation doses, researchers rely on models of the process by which radiation causes cancer; several models that predict differing levels of risk have emerged.
Studies of occupational workers exposed to chronic low levels of radiation, above normal background, have provided mixed evidence regarding cancer and transgenerational effects. Cancer results, although uncertain, are consistent with estimates of risk based on atomic bomb survivors and suggest that these workers do face a small increase in the probability of developing leukemia and other cancers. One of the most recent and extensive studies of workers was published by Cardis, et al. in 2005 .
The linear dose-response model suggests that any increase in dose, no matter how small, results in an incremental increase in risk. The linear no-threshold model (LNT) hypothesis is accepted by the Nuclear Regulatory Commission (NRC) and the EPA and its validity has been reaffirmed by a National Academy of Sciences Committee (see the BEIR VII report, summarized in ). Under this model, about 1% of a population would develop cancer in their lifetime as a result of ionizing radiation from background levels of natural and man-made sources.
Ionizing radiation damages tissue by causing ionization, which disrupts molecules directly and also produces highly reactive free radicals, which attack nearby cells. The net effect is that biological molecules suffer local disruption; this may exceed the body's capacity to repair the damage and may also cause mutations in cells currently undergoing replication.
Approximately 134 plant workers and fire fighters engaged at the Chernobyl power plant received high radiation doses (70,000 to 1,340,000 mrem or 700 to 13,400 mSv) and suffered from acute radiation sickness. Of these, 28 died from their radiation injuries.
Longer-term effects of the Chernobyl disaster have also been studied. There is a clear link (see the UNSCEAR 2000 Report, Volume 2: Effects) between the Chernobyl accident and the unusually large number, approximately 1,800, of thyroid cancers reported in contaminated areas, mostly in children. These were fatal in some cases. Other health effects of the Chernobyl accident are subject to current debate.
In March 2011, an earthquake and tsunami caused damage that led to explosions and partial meltdowns at the Fukushima I Nuclear Power Plant in Japan. Radiation levels at the stricken Fukushima I power plant have varied spiking up to 10,000 mSv/h (millisievert per hour), which is a level that can cause fatal radiation poisoning from less than one hour of exposure. Significant release in emissions of radioactive particles took place following hydrogen explosions at three reactors, as technicians tried to pump in seawater to keep the uranium fuel rods cool, and bled radioactive gas from the reactors in order to make room for the seawater. Concerns about the possibility of a large scale radiation leak resulted in 20 km exclusion zone being set up around the power plant and people within the 20–30 km zone being advised to stay indoors. Later, the UK, France and some other countries told their nationals to consider leaving Tokyo, in response to fears of spreading nuclear contamination. New Scientist has reported that emissions of radioactive iodine and cesium from the crippled Fukushima I nuclear plant have approached levels evident after the Chernobyl disaster in 1986. On March 24, 2011, Japanese officials announced that "radioactive iodine-131 exceeding safety limits for infants had been detected at 18 water-purification plants in Tokyo and five other prefectures".
Recognized effects of acute radiation exposure are described in the article on radiation poisoning. The exact units of measurement vary, but light radiation sickness begins at about 50 - 100 rad (0.5 - 1 gray (Gy), 500 - 1000 mSv, 50 - 100 rem, 50,000 - 100,000 mrem).
Although the SI unit of radiation dose equivalent is the sievert, chronic radiation levels and standards are still often given in millirems, 1/1000 of a rem (1 mrem = 0.01 mSv).
Table A.2 presents a scale of dose levels, with an example of the type of exposure that may cause such a dose, or the special significance of such a dose.
See main article: Radiation hormesis.
Radiation hormesis is the conjecture that a low level of ionizing radiation (i.e., near the level of Earth's natural background radiation) helps "immunize" cells against DNA damage from other causes (such as free radicals or larger doses of ionizing radiation), and decreases the risk of cancer. The theory proposes that such low levels activate the body's DNA repair mechanisms, causing higher levels of cellular DNA-repair proteins to be present in the body, improving the body's ability to repair DNA damage. This assertion is very difficult to prove in humans (using, for example, statistical cancer studies) because the effects of very low ionizing radiation levels are too small to be statistically measured amid the "noise" of normal cancer rates.
The idea of radiation hormesis is considered unproven by regulatory bodies, which in general use the standard "linear, no threshold" (LNT) model. The LNT model, however, also remains unproven, and was originally created as an administrative convenience, to simplify the process of developing safety standards. The LNT states that risk of cancer is directly proportional to the dose level of ionizing radiation, even at very low levels. The LNT model is perceived to be safer for regulatory purposes because it assumes worst-case damage due to ionizing radiation. Once this assumption is made, the conclusion is that regulations based on it will ensure the protection of workers - that they might be over-protected, but never be under-protected. However, if the LNT does not apply at low levels, it is conceivable that regulations based on it will prevent or limit the hormetic effect, and thus have a negative impact on health.
Radiation has always been present in the environment and in our bodies. The human body cannot sense ionizing radiation, but a range of instruments that are capable of detecting even very low levels of radiation from natural and man-made sources exists.
Dosimeters measure an absolute dose received over a period of time. Ion-chamber dosimeters resemble pens, and can be clipped to one's clothing. Film-badge dosimeters enclose a piece of photographic film, which will become exposed as radiation passes through it. Ion-chamber dosimeters must be periodically recharged, and the result logged. Film-badge dosimeters must be developed as photographic emulsion so the exposures can be counted and logged; once developed, they are discarded. Another type of dosimeter is the TLD (Thermoluminescent Dosimeter). These dosimeters contain crystals that emit visible light when heated, in direct proportion to their total radiation exposure. Like ion-chamber dosimeters, TLDs can be re-used after they have been 'read'.
There are three standard ways to limit exposure:
Some generally accepted thicknesses of attenuating material are 5 mm of aluminum for most beta particles, and 3 inches of lead for gamma radiation.
Containment: Radioactive materials are confined in the smallest possible space and kept out of the environment. Radioactive isotopes for medical use, for example, are dispensed in closed handling facilities, while nuclear reactors operate within closed systems with multiple barriers that keep the radioactive materials contained. Rooms have a reduced air pressure so that any leaks occur into the room and not out of it.
In a nuclear war, an effective fallout shelter reduces human exposure at least 1,000 times. Other civil defense measures can help reduce exposure of populations by reducing ingestion of isotopes and occupational exposure during war time. One of these available measures could be the use of potassium iodide (KI) tablets, which effectively block the uptake of radioactive iodine into the human thyroid gland.
During human spaceflights, in particular flights beyond low Earth orbit, astronauts are exposed to both galactic cosmic radiation (GCR) and possibly solar particle event (SPE) radiation. Evidence indicates past SPE radiation levels that would have been lethal for unprotected astronauts. GCR levels that might lead to acute radiation poisoning are not as well-understood.
Air travel exposes people on aircraft to increased radiation from space as compared to sea level, including cosmic rays and from solar flare events. Software programs such as Epcard, CARI, SIEVERT, PCAIRE are attempts to simulate exposure by aircrews and passengers. An example of a measured dose (not simulated dose), is 6 μSv per hour from London Heathrow to Tokyo Narita on a high-latitude polar route. However, dosages can vary, such as during periods of high solar activity. The United States FAA requires airlines to provide flight crew with information about cosmic radiation, and an ICRP recommendation for the general public is no more than 1 mSv per year. In addition, many airlines do not allow pregnant flightcrew members, to comply with a European Directive. The FAA has a recommended limit of 1 mSv total for a pregnancy, and no more than 0.5 mSv per month. Information originally based on Fundamentals of Aerospace Medicine published in 2008.