ionizing radiation: Definition from Answers.com

X-rays were discovered by Röntgen in Germany in November 1895. This caused tremendous public interest and very rapidly they started to be used for many medical diagnostic purposes. Within just a few years it was realized that high radiation doses from X-rays could cause severe skin burns, cancer in exposed tissues, and even death. This resulted in steps being taken to reduce exposures, although in the early years after their discovery it was assumed that only high radiation doses from X-rays could cause cancer. By the late 1920s it had become apparent, from the studies of Müller on the fruit fly, Drosophila, that radiation damage from X-rays could cause effects in future generations, and through to the 1950s this was the principle cause for concern at lower radiation doses. By the early 1950s, however, follow-up studies of the A-bomb survivors in Japan and other exposed populations were showing that radiation-induced leukaemia and other cancers could arise even at low levels of exposure. The principle long-term effect of exposure to low doses of ionizing radiation is now considered to be the induction of cancer.

Ionizing radiations include X-rays, neutrons, cosmic rays, and radiation from radioactive materials, such as alpha particles, beta particles, and gamma rays. When ionizing radiations pass through matter, energy is deposited in the material concerned. Alpha and beta particles, being electrically charged, deposit energy through electrical interactions with electrons in the material. Gamma rays and X-rays lose energy in a variety of ways, but each involves liberating atomic electrons, which then deposit energy through interactions with other electrons. Neutrons also lose energy in various ways, an important means being through collisions with hydrogen nuclei, which are single protons. The protons are set in motion and, being charged, they again deposit energy through electrical interactions. So in all cases, these radiations ultimately produce electrical interactions in the material and this can give rise to ionizations when a neutral atom or molecule becomes charged as a result of a loss of an electron. Once removed from an atom, an electron may in turn ionize other atoms or molecules. When ionizing radiation passes through cellular tissue, it produces charged water molecules. These break up into entities called free radicals, such as the free hydroxyl radical (OHl) composed of an oxygen atom and a hydrogen atom. These free radicals are highly reactive chemically and can themselves alter molecules in the cell. One molecule of particular importance in relation to radiation damage is deoxyribonucleic acid (DNA), found in the nucleus of the cell. Radiation may ionize a DNA molecule, leading directly to a chemical change, or the DNA may be changed indirectly when it interacts with a free radical produced in the water of the cell by radiation. In either case, the chemical change can cause a harmful biological effect, leading ultimately to the development of cancer or inherited genetic defects. The quantification of these effects has provided the basis for radiation protection standards.

The principal quantity used to assess exposure to radiation is the absorbed dose, with the unit of the gray, Gy (equivalent to a deposition of energy of 1 Joule/kg). The gray can be multiplied by a ‘weighting factor’ to take account of the effectiveness of different radiations in causing damage to tissues. Thus X-rays, gamma rays, and beta particles have a ‘radiation’ weighting factor of 1; for alpha particles it is 20. The unit of this weighted dose is the sievert, Sv, and it is termed the equivalent dose.

Exposure to ionizing radiation comes from a variety of sources. There are sources of natural origin, such as cosmic rays from the atmosphere, gamma rays from radioactive materials in the ground and in our own bodies, and inhalation of radon; we can be exposed as a result of medical diagnostic procedures and treatment; radiation is also present in the environment as a result of nuclear weapons testing and as a consequence of discharges from nuclear sites and from nuclear accidents. For most people the main source of radiation exposure is natural background, which, in the UK, gives a radiation dose of about 2.2 millisieverts (mSv) a year. There can be substantial differences between individuals, mainly reflecting differences in exposure to radon gas and its decay products.

Early radiation effects

The effects of ionizing radiation soon appear if a person receives a sufficient radiation dose. A very high radiation dose to the whole body can cause death within a matter of weeks. For example, an absorbed dose of 5 Gy (5000 mGy) or more received instantaneously by the whole body would probably be lethal unless treatment were given. Death would occur because of damage to the bone marrow and the gastrointestinal tract, both of which have rapidly-dividing and hence sensitive cell populations. If the same dose were instead restricted to a limited part of the body, it might not prove fatal but early effects could still occur. Thus an instantaneous absorbed dose of 5 Gy or more to the skin would probably cause erythema within a week or so. Higher doses would lead to more serious damage and breakdown of the skin structure. Similar doses to the testes or ovaries might cause sterility. However, if the same radiation dose were to be received over a period of weeks or months, there would be the opportunity for body cells to repair some damage, with much less early sign of injury. Even in the absence of early signs, however, tissues could still have been damaged, with the effects becoming manifest only later in life, or perhaps in the irradiated person's descendants. The most important of these late effects is cancer, which is always serious and frequently fatal.

Radiation-induced cancer

Although the cause of most cancers remains unknown or poorly understood, exposures to a wide range of agents, such as tobacco smoke, asbestos, ultraviolet radiation, chemicals, and ionizing radiations, are all known to induce them. The development of cancer is a complex cellular process that occurs in several stages, usually taking many years. Radiation appears to act principally at the initiation stage by causing mutations in the DNA of normal cells in tissues. It is usually considered that damage is caused by double-strand breaks (DSBs) in DNA, which are not readily repaired. The production of DSBs can result in a cell entering a pathway of abnormal growth that can sometimes lead to development of a malignancy. In recent years, much has been learned about the processes by which radiation exposure leads to DNA damage, and also about the cellular systems that act to repair, or misrepair, such damage and the way mutations can arise. This information provides supporting evidence for the long-standing belief that, although the risk of cancer after low doses of radiation may be very small, there is no dose, no matter how low, at which we can completely discount the risk. For radiation protection purposes it is therefore assumed that the risk of cancer increases progressively with the dose, with no threshold.

Advances in knowledge also indicate that a person's genetic constitution influences the risk of cancer after irradiation. At present we can identify only rare families who may carry increased risk. In future, techniques may become available that allow the identification of more groups of individuals with increased sensitivity to irradiation. This is an important factor in medical exposures, as individuals treated medically — for example by radiotherapy — might have quite different responses to radiation exposure.

It is also known that tissues vary in their response to radiation-induced cancer, thus the lung and the gastrointestinal tract are particularly sensitive, whereas the brain and muscles are very insensitive. In assessing the risks of exposure to radiation we therefore have to allow for these differences in sensitivity.

How can we calculate the risk of cancer from exposure to radiation? Suppose we know the number of people in an irradiated group and the doses they have received. By observing the incidence of cancer in the group and analyzing it in relation to the size of the radiation dose and the number of cases expected, in another similar but unirradiated group, we can estimate the raised risk of cancer per unit radiation dose. This is commonly called a risk factor. It is most important to include data for large groups of people in these calculations so as to minimize the statistical uncertainties in the estimates and to take account of factors such as age and gender that can effect the spontaneous development of the disease. For this reason, the main source of information on risks of radiation-induced cancer comes from studies on nearly 100 000 survivors of the atomic bombs dropped at Hiroshima and Nagasaki in 1945. Other risk estimates for the exposure of various tissues and organs to X-rays and gamma rays can be obtained from studies of people exposed to external radiation for the treatment of non-malignant or malignant conditions or for diagnostic purposes, and also from people in the Marshall islands in the Pacific who were exposed to severe fallout from atmospheric nuclear weapons testing. Information on the effects of internally incorporated alpha-emitting radionuclides comes from miners exposed to radon and its decay products, from workers exposed to radium in luminous paint, from patients treated with radium for bone disease, and from other patients given an X-ray contrast medium containing thorium oxide, which tended to concentrate in the liver. Long-term follow-up studies on these groups of people have allowed both national and international bodies, such as the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) and the International Commission on Radiological Protection (ICRP), to estimate risks of radiation-induced cancer in various tissues as a function of time and age after exposure. A striking observation from these studies has been that whilst radiation-induced leukaemia starts to occur within a few years, and most have occurred by about thirty years following radiation exposure, in the case of most solid cancers the so-called latent period, before any induced cancers appear, may be five, ten, or even twenty years. Subsequently cancers may then occur over the remaining lifespan of the individual. For this reason, exposed groups must be followed for very many years to obtain accurate estimates of the total risk of cancer. In practice, as no populations have yet been followed for their entire lifespan, it is necessary to predict how many excess cancers will have been found by the time all the survivors have died. Various mathematical methods are used for this purpose, which inevitably introduces some uncertainty in the risk estimates.

Not all cancers are fatal. Mortality from radiation-induced thyroid cancer is about 10%, from breast cancer about 50%, and from skin cancer only about 1%. Overall, the risk of inducing any cancer by uniform irradiation of the whole body is about half as great again as the risk of inducing a fatal cancer. In radiological protection, the risk of fatal cancer is naturally of most concern. Present estimates of the risk of radiation-induced fatal cancer provided by ICRP are 1 in 20 per Sv for exposure of a population of all ages and 1 in 25 per Sv for a working population. These values apply to a mixed population of all ages; there will be differences in sensitivity in males and females as well as between individuals of different ages.

Radiation-induced hereditary disease

Apart from cancer, the other main late effect of radiation is hereditary disease. As with cancer, the probability of hereditary disease, but not its severity, depends on the dose. Genetic damage arises from irradiation of the testes and ovaries, which produce sperm cells in males and the egg cells in females. Ionizing radiation can induce mutations in these cells or in the stem cells that form them. Mutations occur as a result of structural changes to the DNA in single germ cells, which subsequently carry the hereditary information in the DNA through to future generations. The hereditary diseases that may be caused vary in severity from early death and serious mental defects to relatively trivial diseases such as skeletal abnormalities and minor metabolic disorders.

ICRP has assessed the risk of severe hereditary disease in a general population of all ages exposed to low doses and dose rates. It estimated a risk factor of 1 in 100 per Sv for such diseases appearing at any time in all future generations. Mutations leading to diseases that are strictly heritable, such as haemophilia, make up about half of the total, with the remainder mainly coming from a group of so-called multifactorial diseases, such as diabetes and asthma. Estimating the risk of multifactorial diseases is complex, as there is interplay of the genetic and environmental factors that influence the development of these disorders.

In genetic terms, irradiation of the testes and ovaries is potentially harmful only if it occurs before or during the reproductive period of life. For people who will not subsequently have children, there is, of course, no hereditary risk. Since the proportion of a working population that is likely to reproduce is lower than that in the general population, the risk factor for workers is smaller. ICRP estimates 1 in 170 per Sv for hereditary disease in all future generations.

Summary and conclusions

At high radiation doses, significant effects can occur in exposed individuals within a short time of exposure, and in severe cases this can lead to early death. At low radiation doses, the principal concern is the risk of radiation-induced cancer in exposed individuals and hereditary disease in their descendants. The risks of these late effects have been quantified and this provides the basis for recommendations on limits for exposure.

— John W. Stather

This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (October 2008)

Radiation hazard symbol.

Ionizing radiation hazard symbol (recently introduced).^[1]

Ionizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number. An intense flood of particles or waves will not cause ionization if these particles or waves do not carry enough energy to be ionizing. Roughly speaking, particles or photons with energies above a few electron volts (eV) are ionizing.

Examples of ionizing particles are energetic alpha particles, beta particles, and neutrons. The ability of electromagnetic waves (photons) to ionize an atom or molecule depends on their wavelength. Radiation on the short wavelength end of the electromagnetic spectrum — ultraviolet, x-rays, and gamma rays — is ionizing.

Ionizing radiation comes from radioactive materials, x-ray tubes, particle accelerators, and is present in the environment. It 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, 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 radiation causes damage to living tissue, resulting in skin burns, radiation sickness and death at high doses and cancer,^[2] tumors and genetic damage at low doses.

Types of radiation

Alpha (α) radiation consists of a fast moving Helium-4 (⁴He) nuclei and is stopped by a sheet of paper. Beta (β) radiation, consisting of electrons, is halted by an aluminium plate. Gamma (γ) radiation, consisting of energetic photons, is eventually absorbed as it penetrates a dense material. Neutron (n) radiation consists of free neutrons which are blocked using light elements, like hydrogen, which slow and/or capture them.

Various types of ionizing radiation may be produced by radioactive decay, nuclear fission and nuclear fusion, and by particle accelerators.

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) is in the ultraviolet region of the electromagnetic spectrum; sunburn is one of the effects of ionization. Charged particles such as electrons, positrons, and alpha particles also interact electromagnetically with electrons of an atom or molecule. 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, 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 charged, and interact with the electrons in matter. A neutron can also interact with an atomic nucleus, 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, then they can produce chain reactions in the mass that is decaying, sometimes causing a larger effect of ionization.

In the picture at left, gamma rays are represented by wavy lines, charged particles and neutrons by straight lines. The little circles show where ionization processes occur.

An ionization event normally produces a positive atomic ion and an electron. High-energy beta particles may produce bremsstrahlung when passing through matter, or secondary electrons (δ-electrons); both can ionize in turn.

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 a neutron capture photon.

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. 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 National Research Council (the BEIR VII report, summarized in [2]) indicated that the overall cancer risk associated with background sources of radiation was relatively low.

Radioactive materials usually release alpha particles, which are the nuclei of helium, beta particles, which are quickly moving electrons or positrons, or gamma rays. 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, and 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. Human biology resists germline mutation by either correcting the changes in the DNA or inducing apoptosis in the mutated cell.

Non-ionizing radiation is thought to be essentially harmless below the levels that cause heating. Ionizing radiation is dangerous in direct exposure, although the degree of danger is a subject of debate. Animals (including humans) can also be exposed to ionizing radiation internally: if radioactive isotopes are present in the environment, they may 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 often leads to thyroid cancer. Some radioactive elements also bioaccumulate.

Units

Weighting factors W_R for equivalent dose
Radiation	Energy	w_R
x-rays, gamma rays, electrons, positrons, muons		1
neutrons	< 10 keV	5
	10 keV - 100 keV	10
	100 keV - 2 MeV	20
	2 MeV - 20 MeV	10
	> 20 MeV	5
protons	> 2 MeV	2
alpha particles, fission fragments, heavy nuclei		20

The units used to measure ionizing radiation are rather complex. The ionizing effects of radiation are measured by units of exposure:

The coulomb per kilogram (C/kg) is the SI unit of ionizing radiation exposure, and measures the amount of radiation required to create 1 coulomb of charge of each polarity in 1 kilogram of matter.
The roentgen (R) is an older traditional unit that is almost out of use, which represented the amount of radiation required to liberate 1 esu of charge of each polarity in 1 cubic centimeter of dry air. 1 Roentgen = 2.58×10⁻⁴ C/kg

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.

The gray (Gy), with units J/kg, is the SI unit of absorbed dose, which represents the amount of radiation required to deposit 1 joule of energy in 1 kilogram of any kind of matter.
The rad (Roentgen absorbed dose), is the corresponding traditional unit which is 0.01 J deposited per kg. 100 rad = 1 Gy.

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 W_R which is different for each type of radiation (see above table).

The sievert (Sv) is the SI unit of equivalent dose. Although it has the same units as grays, J/kg, it measures something different. It is the dose of a given type of radiation in Gy that has the same biological effect on a human as 1 Gy of x-rays or gamma radiation.
The rem (Roentgen equivalent man) is the traditional unit of equivalent dose. 1 sievert = 100 rem. Because the rem is a relatively large unit, typical equivalent dose is measured in millirem (mrem), 10^-3 rem, or in microsievert (μSv), 10^-6 Sv. 1 mrem = 10 μSv.
A unit sometimes used for low level doses of radiation is the BRET (Background Radiation Equivalent Time). This is the number of days of average background radiation exposure the dose is equivalent to. This unit is apparently not standardized, and depends on the value used for the average background radiation dose. Using the 2000 UNSCEAR value (below), one BRET unit is equal to about 6.6 microsieverts.

For comparison, the average 'background' dose of natural radiation received by a person is around 2.4 millisieverts (240 mrem) per year. The lethal full-body dose of radiation for a human is around 4 - 5 sieverts (400 - 500 rem).

Uses

Ionizing radiation has many uses, such as to kill cancerous cells. 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 was dangerous.^[3]

Power production

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 products. The safe disposal of this waste in a way that protects future generations from exposure to its radiation is currently a worldwide unsolved problem of this technology. However, further research into fuel reprocessing and alternative fuel cycles (e.g. thorium cycle) can provide ways to drastically reduce the amount of true "waste" from the fission reactions.

Industrial measurement

Since ionizing radiations can penetrate matter, they are used for a variety of measuring methods.

Industrial radiography

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.

Gauges

Gauges use the exponential absorption law of gamma rays

Level indicators: Source and detector are placed at opposite sides of a container, indicating the presence or absence of material in the horizontal radiation path. Beta or gamma sources are used, depending on the thickness and the density of the material to be measured. The method is used for containers of liquids or of grainy substances
Thickness gauges: if the material is of constant density, the signal measured by the radiation detector depends on the thickness of the material. This is useful for continuous production, like of paper, rubber, etc.

Applications using ionization of gases by radiation

To avoid the build-up of static electricity in production of paper, plastics, synthetic textiles, etc., a ribbon-shaped source of the alpha emitter ²⁴¹Am can be placed close to the material at the end of the production line. The source ionizes the air to remove electric charges on the material.
Smoke detector: 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.
Radioactive tracers for industry: Since radioactive isotopes behave, chemically, mostly like the inactive element, the behavior of a certain chemical substance can be followed by tracing the radioactivity. Examples:
- Adding a gamma tracer to a gas or liquid in a closed system makes it possible to find a hole in a tube.
- Adding a tracer to the surface of the component of a motor makes it possible to measure wear by measuring the activity of the lubricating oil.

Biological and medical applications

In biology, radiation is mainly used for radiation therapy, sterilization, and enhancing mutations.

Electrons, x rays, gamma rays or atomic ions may be used in radiation therapy to treat malignant tumors (cancer). Furthermore, just like in industrial application, x rays can also be used in radiography to create images of hard-to-image objects, such as inside one's body.

Radiation is also useful in sterilizing medical hardware or food. The advantage for medical hardware is that the object may be sealed in plastic before sterilization. For food, there are strict regulations to prevent the occurrence of induced radioactivity. The growth of a seedling may be enhanced by radiation, but excessive radiation will hinder growth.

Tracer methods are used in nuclear medicine in a way analogous to the technical uses mentioned above.

Mutations may be induced by radiation to produce new or improved species. A very promising field is the sterile insect technique, where male insects are sterilized and liberated in the chosen field, so that they have no descendants, and the population is reduced.

Sources

Natural background radiation

Natural background radiation comes from four primary sources: cosmic radiation, solar radiation, external terrestrial sources, and radon.

Cosmic radiation

For more details on cosmic radiation, see 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.

The ionizing component of solar radiation is negligible relative to other forms of radiation on Earth's surface.

External terrestrial sources

Most materials on Earth contain some radioactive atoms, even if in small quantities. Most of the terrestrial non-radon-dose one receives from these sources is from gamma-ray emitters in the walls and floors when inside a house, or rocks and soil when outside. The major radionuclides of concern for terrestrial radiation are potassium, uranium, and thorium. Each of these sources has been decreasing in activity since the birth of the Earth so that our present dose from potassium-40 is about ½ what it would have been at the dawn of life on Earth.

Radon

Radon-222 is produced by the decay of radium-226 which is present wherever uranium is found. Since radon is a gas, it seeps out of uranium-containing soils found across most of the world and may accumulate in well-sealed homes. It is often the single largest contributor to an individual's background radiation dose and is certainly the most variable from location to location. Radon gas could be the second largest cause of lung cancer in America, after smoking.^[4]

Human-made radiation sources

Natural and artificial radiation sources are similar in their effects on matter. 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.

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 who have CAT scans. This compares with the average dose received by people in the UK of about 2.2 mSv. As already mentioned, 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.^{[citation needed]} This has led to the suggestion that high but steady levels of radiation are easier for humans to sustain than sudden radiation bursts.

Some human-made radiation sources affect the body through direct radiation, while others take the form of radioactive contamination and irradiate the body from within.

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 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. 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).^{[citation needed]} 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).^{[citation needed]}

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.

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.

Some of the radionuclides of concern include cobalt-60, caesium-137, americium-241, and iodine-131. Examples of industries where occupational exposure is a concern include:

Airline crew (the most exposed population)
Industrial radiography
Medical radiology and nuclear medicine
Uranium mining
Nuclear power plant and nuclear fuel reprocessing plant workers
Research laboratories (government, university and private)

Biological effects

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:

Cells experience DNA damage and are able to detect and repair the damage.
Cells experience DNA damage and are unable to repair the damage. These cells may go through the process of programmed cell death, or apoptosis, thus eliminating the potential genetic damage from the larger tissue.
Cells experience a nonlethal DNA mutation that is passed on to subsequent cell divisions. This mutation may contribute to the formation of a cancer.
Cells experience "Irreparable DNA Damage." Low level ionizing radiation may induce "irreparable" DNA damage (leading to replicational and transcriptional errors needed for neoplasia or may trigger viral interactions) leading to pre-mature aging and cancer.^[5]^[6]^[7]

Other observations at the tissue level are more complicated. These include:

In some cases, a small radiation dose reduces the impact of a subsequent, larger radiation dose. This has been termed an 'adaptive response' and is related to hypothetical mechanisms of hormesis.

Chronic radiation exposure

Exposure to ionizing radiation over an extended period of time is called chronic exposure. 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.

Acute radiation exposure

Acute radiation exposure is an exposure to ionizing radiation which 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

Instantaneous flashes from nuclear explosions.
Exposures of minutes to hours during handling of highly radioactive sources.
Laboratory and manufacturing accidents.
Intentional and accidental high medical doses.

The effects of acute events are more easily studied than those of chronic exposure.

Radiation levels

The associations between ionizing radiation exposure and the development of cancer are mostly based 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^[8], 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 chemical carcinogens. Furthermore, National Cancer Institute literature indicates that other 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 have emerged which predict differing levels of risk.

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 [3].

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 [4].) 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.

Two widely studied instances of large-scale exposure to high doses of ionizing radiation are: atomic bomb survivors in 1945; and emergency workers responding to the 1986 Chernobyl accident.

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 accident 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.

Ionizing radiation level examples

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), 0.5–1 Sv, 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, 1/1000th of a rem (1 mrem = 0.01 mSv).

The following table includes some short-term dosages for comparison purposes.

Level (mSv)	Duration	Description
0.001-0.01	Hourly	Cosmic ray dose on high-altitude flight, depends on position and solar sunspot phase.^[9]
0.01	Annual	USA dose from nuclear fuel and nuclear power plants ^[10]
0.01	Daily	Natural background radiation, including radon ^[11]
0.1	Annual	Average USA dose from consumer products ^[10]
0.15	Annual	USA EPA cleanup standard^{[citation needed]}
0.25	Annual	USA NRC cleanup standard for individual sites/sources^{[citation needed]}
0.27	Annual	USA dose from natural cosmic radiation (0.16 coastal plain, 0.63 eastern Rocky Mountains) ^[10]
0.28	Annual	USA dose from natural terrestrial sources ^[10]
0.39	Annual	Global level of human internal radiation due to radioactive potassium^{[citation needed]}
0.46	Acute	Estimated largest off-site dose possible from March 28, 1979 Three Mile Island accident^{[citation needed]}
0.48	Day	USA NRC public area exposure limit^{[citation needed]}
0.66	Annual	Average USA dose from human-made sources ^[11]
1	Annual	Limit of dose from all DOE facilities to a member of the public who is not a radiation worker ^[11]
1.1	Annual	1980 average USA radiation worker occupational dose ^[11]
2	Annual	USA average medical and natural background [5] Human internal radiation due to radon, varies with radon levels ^[10]
2.2	Acute	Average dose from upper gastrointestinal diagnostic X-ray series^{[citation needed]}
3	Annual	USA average dose from all natural sources ^[11]
3.66	Annual	USA average from all sources, including medical diagnostic radiation doses^{[citation needed]}
few	Annual	Estimate of cobalt-60 contamination within about 0.5 mile of dirty bomb^{[citation needed]}
5	Annual	USA NRC occupational limit for minors (10% of adult limit) USA NRC limit for visitors Orvieto town, Italy, natural [6]
5	Pregnancy	USA NRC occupational limit for pregnant women^{[citation needed]}
6.4	Annual	High Background Radiation Area (HBRA) of Yangjiang, China [7]
7.6	Annual	Fountainhead Rock Place, Santa Fe, NM natural^{[citation needed]}
10–50	Acute	USA EPA nuclear accident emergency action level ^[11]
50	Annual	USA NRC occupational limit (10 CFR 20)
100	Acute	USA EPA acute dose level estimated to increase cancer risk 0.8% ^[11]
120	30 years	Exposure, long duration, Ural mountains, lower limit, lower cancer mortality rate[8]
150	Annual	USA NRC occupational eye lens exposure limit^{[citation needed]}
175	Annual	Guarapari, Brazil natural radiation sources
250	Acute	USA EPA voluntary maximum dose for emergency non-life-saving work ^[11]
250	2 hours	Whole body dose exclusion zone criteria for US nuclear reactor siting^[12] (converted from 25 rem)
260	Annual	Ramsar, Iran, natural background peak dose [9]
500	Annual	USA NRC occupational whole skin, limb skin, or single organ exposure limit
500	30 years	Exposure, long duration, Ural mountains, upper limit [10]
750	Acute	USA EPA voluntary maximum dose for emergency life-saving work ^[11]
500-1000	Acute	Low-level radiation sickness due to short-term exposure
500-1000	Detonation	World War II nuclear bomb victims^{[citation needed]}
3000	Acute	Thyroid dose (due to iodine absorption) exclusion zone criteria for US nuclear reactor siting ^[13] (converted from 300 rem)
4500-5000	Acute	LD₅₀ in humans (from radiation poisoning), with medical treatment.^[14]

Hormesis

Main article: Radiation hormesis

Radiation hormesis is the unproven theory 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.

Therefore, the idea of radiation hormesis is considered unproven by regulatory bodies, which generally use the standard "linear, no threshold" (LNT) model, which states that risk of cancer is directly proportional to the dose level of ionizing radiation. The LNT model is safer for regulatory purposes because it assumes worst-case damage due to ionizing radiation; therefore, if regulations are based on it, workers might be over-protected, but they will never be under-protected.

Monitoring and controlling exposure

Radiation has always been present in the environment and in our bodies. The human body cannot sense ionizing radiation, but a range of instruments exists which are capable of detecting even very low levels of radiation from natural and man-made sources.

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'.

Geiger counters and scintillation counters measure the dose rate of ionizing radiation directly.

Limiting exposure

There are four standard ways to limit exposure:

Time: For people who are exposed to radiation in addition to natural background radiation, limiting or minimizing the exposure time will reduce the dose from the radiation source.

Distance: Radiation intensity decreases sharply with distance, according to an inverse square law.^[2] Air attenuates alpha and beta radiation.

Shielding: Barriers of lead, concrete, or water give effective protection from radiation formed of energetic particles 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 which stop beta particles and air will stop alpha particles. The effectiveness of a material in shielding radiation is determined by its halve value thicknesses, the thickness of material that reduces the radiation by half. This value is a function of the material itself and the energy and type of ionizing 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 which 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.

References

^ This symbol is included in ISO 21482:2007. ISO International Standards are protected by copyright and may be purchased from ISO or its members (please visit www.iso.org for more information). ISO has not reviewed the accuracy or veracity of this information.
^ ^a ^b Camphausen KA, Lawrence RC. "Principles of Radiation Therapy" in Pazdur R, Wagman LD, Camphausen KA, Hoskins WJ (Eds) Cancer Management: A Multidisciplinary Approach. 11 ed. 2008.
^ Lewis, Leon; Paul E Caplan (January 1950). "The Shoe-Fitting Fluoroscope as a Radiation Hazard". California Medicine 72 (1): 27. PMID 15408494. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1520288. Retrieved 2008-03-05.
^ See this web site.
^ Acharya, PV; The Effect of Ionizing Radiation on the Formation of Age-Correlated Oligo Deoxyribo Nucleo Phospheryl Peptides in Mammalian Cells; 10th International Congress of Gerontology, Jerusalem. Abstract No. 1; January 1975. Work done while employed by Dept. of Pathology, University of Wisconsin, Madison.
^ Acharya, PV; Implicatons of The Action of Low Level Ionizing Radiation on the Inducement of Irreparable DNA Damage Leading to Mammalian Aging and Chemical Carcinogenesis.; 10th International Congress of Biochemistry, Hamburg, Germany. Abstract No. 01-1-079; July 1976. Work done while employed by Dept. of Pathology, University of Wisconsin, Madison.
^ Acharya, PV; Irreparable DNA-Damage by Industrial Pollutants in Pre-mature Aging, Chemical Carcinogenesis and Cardiac Hypertrophy: Experiments and Theory; 1st International Meeting of Heads of Clinical Biochemistry Laboratories, Jerusalem, Israel. April 1977. Work conducted at Industrial Safety Institute and Behavioral Cybernetics Laboratory, University of Wisconsin, Madison.
^ "A Nested Case-Control Study of Leukemia and Ionizing Radiation at the Portsmouth Naval Shipyard", NIOSH Publication No. 2005-104. National Institute for Occupational Safety and Health.
^ UNSCEAR 2000 report, Volume 1, Annex B, ``Exposures from natural radiation sources, pp 88. See figure 3. available online at [1]
^ ^a ^b ^c ^d ^e Oak Ridge National Laboratory (http://www.ornl.gov/sci/env_rpt/aser95/appa.htm)
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Oak Ridge National Laboratory (http://www.ornl.gov/sci/env_rpt/aser95/tb-a-2.pdf)
^ 10 CFR Part 100.11 Section 1
^ 10 CFR Part 100.11 Section 1
^ Biological Effects of Ionizing Radiation

External links

The Nuclear Regulatory Commission regulates most commercial radiation sources and non-medical exposures in the US:
Biological Effects of Low Level Exposures: Radiation Hormesis
Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2
NLM Hazardous Substances Databank – Ionizing Radiation
RISC-RAD is a European research project on assessment of low dose cancer risk
UNSCEAR 2000 Report, Volume 1: Sources
UNSCEAR 2000 Report, Volume 2: Effects
Beginners Guide to Ionising Radiation Measurement
Quantities, units and their relationships
Radiation Risk Calculator Calculate cancer risk from CT scans and xrays.
Health Physics Society Public Education Website

Nuclear technology

Science

Physics · Fission · Fusion · Radiation (ionizing) · Nucleus · Chemistry · Engineering

Fuel

Fissile · Fertile · Thorium · Uranium (enriched • depleted) · Plutonium · Deuterium · Tritium · Isotope separation

Neutron

Temp · Thermal · Fast · Fusion · Cross section · Capture · Activation · Poison · Radiation · Generator · Reflector

Fission
reactors
by
moderator

Water	Pressurized (PWR) · Boiling (BWR) · Supercritical (SCWR) · Heavy (PHWR · CANDU)

Carbon	Pebble bed (PBMR) · Very high temperature (VHTR) · UHTREX · RBMK · Magnox · AGR

FLiBe	Molten salt (MSR)

None (Fast)	Breeder (FBR) · Liquid-metal-cooled (LMFR) · Integral (IFR) · Traveling Wave (TWR) · SSTAR Generation IV by coolant: (Gas (GFR) · Lead (LFR) · Sodium (SFR))

Power

By country · Economics · Safety · Fusion · Isotope thermoelectric (RTG) · Propulsion (rocket)

Medicine

Imaging	Positron emission (PET) · Single photon emission (SPECT) · Gamma camera · X-ray

Therapy	Radiation therapy · Tomotherapy · Proton · Brachytherapy · Boron neutron capture (BNCT)

Weapon

Topics	History · Design · War · Race · Explosion (effects) · Test (underground) · Delivery · Proliferation · Yield (TNTe)

Lists	States · Tests · Weapons · Free zones · Treaties · Pop culture

Waste

Products	Fission (LLFP) · Activation · Actinide: (Reprocessed uranium · Reactor-grade plutonium · Minor actinide)

Disposal	Fuel cycle · Spent fuel (pool • cask) · HLW · LLW · Repository · Reprocessing · Transmutation

Radiation (Physics)

Main articles

Non-ionizing radiation	Ultraviolet light · Near ultraviolet · Visible light · Infrared light · Microwave · Radio waves · Acoustic Radiation

Ionizing radiation	X-ray · Cosmic radiation · Gamma ray · Background radiation · Nuclear fission · Nuclear fusion · Particle accelerators · Nuclear radiation (nuclear weapons · Nuclear reactors) · Radioactive materials (Radioactive decay)

Thermal radiation · Electromagnetic radiation · Earth's radiation balance

Radiation health effects

Radiation therapy · Radiation poisoning · Skin depth · Radioactivity in biological research · List of civilian radiation accidents
Mobile phone radiation and health · Wireless electronic devices and health · Health physics

Radiation hardening · Half-life · Radiobiology · Nuclear physics

v • d • e Carcinogen - Cancer causing materials and agents

Main articles	Cancer

Major suspected carcinogens	PTFE · PFOA · Xenoestrogen · Bisphenol A · Ionizing radiation · DDT · 1,3-Butadiene · List of breast carcinogenic substances · PAH

IARC	International Agency for Research on Cancer / IARC carcinogen lists: Group 1 · Group 2A · Group 2B · Group 3 · Group 4

See also: Neoplasm · Oncology · Non-stick pan

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Donate to Wikimedia

		Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more
		Chemistry Dictionary. A Dictionary of Chemistry. Sixth Edition. Copyright © Market House Books Ltd, 2008. All rights reserved. Read more
		World of the Body. The Oxford Companion to the Body. Copyright © 2001, 2003 by Oxford University Press. All rights reserved. Read more
		Food and Nutrition. A Dictionary of Food and Nutrition. Copyright © 1995, 2003, 2005 by A. E. Bender and D. A. Bender. All rights reserved. Read more
		Dental Dictionary. Mosby's Dental Dictionary. Copyright © 2004 by Elsevier, Inc. All rights reserved. Read more
		Encyclopedia of Public Health. Encyclopedia of Public Health. Copyright © 2002 by The Gale Group, Inc. All rights reserved. Read more
		Veterinary Dictionary. Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved. Read more
		Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Ionizing radiation". Read more

	Whats the properties of ionizing radiation? Read answer...
	What happens when gases are ionized by ionizing radiation? Read answer...
	What is the difference between ionizing and non-ionizing radiation? Read answer...

	Why are Ionizing Radiations Dangerous?
	What is the mechanism of action of ionizing radiations?
	How can you protect ourselves from ionizing radiation?

ionizing radiation

Contents