Ionizing radiation hazard symbol
2007 ISO
radioactivity danger logo, designed in part for long-term
radioactive waste depositories which might survive into a far future time in which all knowledge of the meaning of present common radiation danger symbols and signs will have been lost
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 ions tend to be especially chemically reactive, and the reactivity produces the high biological damage per unit of energy that is a characteristic of all 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). These radiations are termed "ionizing." In general, particles or photons with energies above about 10 electron volts (eV) (some sources require 33 eV)[1] are considered ionizing, no matter what their intensity. This particle-energy occurs in electromagnetic waves in for X-rays and gamma rays, and also the extreme part of the ultraviolet spectrum (not all ultraviolet is ionizing).
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 the threshold of 10 or 33 eV, and commonly exceed thousands or even millions of eV of energy.
Lower-energy radiation, such as visible light, infrared, microwaves, and radio waves, are not ionizing.[2] 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 produces free radicals, such as reactive oxygen species, even at room temperatures and below. Free radical production is 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 ionizing radiation may also directly damage DNA by ionizing or breaking DNA molecules.
Radiobiologists see a degree of overlap between truly ionizing radiation and the lower ultraviolet (UV) spectrum from 3.1 to 10 eV that contains molecularly-damaging radiation that is not formally ionizing, but has somewhat similar biological effects that are larger than are predictable from thermal considerations alone. Non-ionizing ultraviolet can also cause damage to skin as a result of photoreactions in collagen. DNA molecules may also be directly or indirectly damaged by UV radiation carrying enough energy to excite certain molecular bonds to form thymine dimers (pyrimidine dimer)s (this causes sunburn).
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,[3] 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).[4] Roughly, this corresponds to both the first ionization energy of oxygen, and the ionization energy of hydrogen, both about 14 eV [5] In some Environmental Protection Agency references, the ionization of a typical water molecule at an energy of 33 eV is referenced [6] 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,[7] which combines ionization energy plus the energy lost to other processes such as excitation.[8] 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 into all regions of UV and sometimes visible light in some systems (such as photosynthetic systems in leaves). 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 close to 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 mid and lower ultraviolet electromagnetic spectrum is damaging to biological tissues as a result of electronic excitation in molecules which falls short of ionization, but produces similar non-thermal effects. To some extent, visible light and also ultraviolet A (UVA) which is closest to visible energies, have been proven to result in formation of reactive oxygen species in skin, which cause indirect damage since these are electronically excited molecules which can inflict reactive damage, although they do not cause sunburn (erythema).[9] Like ionization-damage, all these effects in skin are beyond those produced by simple thermal effects.
Alpha (
α) radiation consists of a fast moving
helium-4 (
4
He) nucleus 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 that 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 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.
Types of radiation - 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 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 radiation, 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 [10]) 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.
Human biology resists germline mutation by either correcting the changes in the DNA or inducing apoptosis in the mutated cell.
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 (radioactivity 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 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).
- The sievert (Sv) is the SI unit of equivalent dose. Although it has the same units as the gray, J/kg, it measures something different. For a given type and dose of radiation(s) applied to a certain body part(s) of a certain organism, it measures the magnitude of an X-rays or gamma radiation dose applied to the whole body of the organism, such that the probabilities of the two scenarios to induce cancer is the same according to current statistics.
- 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 an average person's background radiation exposure the dose is equivalent to. This unit is 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 μSv.
For comparison, the average 'background' dose of natural radiation received by a person per day, based on 2000 UNSCEAR estimate, makes BRET 6.6 μSv (660 μrem). However local exposures vary, with the yearly average in the US being around 3.6 mSv (360 mrem),[11] and in a small area in India as high as 30 mSv (3 rem).[12][13] The lethal full-body dose of radiation for a human is around 4–5 Sv (400–500 rem).[14]
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.[15]
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;[16] though several high profile nuclear accidents have released dangerous levels of radioactivity.
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.
- 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.
-
- 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.
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.[citation needed] 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." [17]
A schematic diagram showing a rectangle being irradiated by an external source (in red) of radiation (shown in yellow).
A schematic diagram showing a rectangle being irradiated by radioactive contamination (shown in red) which is present on an external surface such as the skin; this emits radiation (shown in yellow) which can enter the animal's body
External exposure is exposure which occurs when the radioactive source (or other radiation source) is outside (and remains outside) the organism which is exposed. Examples of external exposure include:
- A person who places a sealed radioactive source in his pocket
- A space traveller who is irradiated by cosmic rays
- A person who is treated for cancer by either teletherapy or brachytherapy. While in brachytherapy the source is inside the person it is still external exposure because the active part of the source never comes into direct contact with the biological tissues of the person.
One of the key points is that external exposure is often relatively easy to estimate, and the irradiated objects do not become radioactive, except for a case where the radiation is an intense neutron beam which causes activation of the object. It is possible for an object to be contaminated on the outer surfaces; assuming that no radioactivity enters the object it is still a case of external exposure and it is normally the case that decontamination is relatively easy.
Internal exposure occurs when the radioactive material enters the organism, and the radioactive atoms become incorporated into the organism. Below are a series of examples of internal exposure.
- The exposure caused by Potassium-40 present within a normal person.
- The exposure to the ingestion of a soluble radioactive substance, such as 89Sr in cows' milk.
- A person who is being treated for cancer by means of an unsealed source radiotherapy method where a radioisotope is used as a drug (usually a liquid or pill). A review of this topic was published in 1999.[18] Because the radioactive material becomes intimately mixed with the affected object it is often difficult to decontaminate the object or person in a case where internal exposure is occurring. While some very insoluble materials such as fission products within a uranium dioxide matrix might never be able to truly become part of an organism, it is normal to consider such particles in the lungs and digestive tract as a form of internal contamination which results in internal exposure.
- Boron neutron capture therapy (BNCT) involves injecting a boron-10 tagged chemical that preferentially binds to tumor cells. Neutrons from a nuclear reactor are shaped by a neutron moderator to the neutron energy spectrum suitable for BNCT treatment. The tumor is selectively bombarded with these neutrons. The neutrons quickly slow down in the body to become low energy thermal neutrons. These thermal neutrons are captured by the injected boron-10, forming excited (boron-11) which breaks down into lithium-7 and a helium-4 alpha particle both of these produce closely spaced ionizing radiation.This concept is described as a binary system using two separate components for the therapy of cancer. Each component in itself is relatively harmless to the cells, but when combined together for treatment they produce a highly cytocidal (cytotoxic) effect which is lethal (within a limited range of 5-9 micrometers or approximately one cell diameter). Clinical trials, with promising results, are currently carried out in Finland and Japan.
When radioactive compounds enter the human body, the effects are different from those resulting from exposure to an external radiation source. Especially in the case of alpha radiation, which normally does not penetrate the skin, the exposure can be much more damaging after ingestion or inhalation. The radiation exposure is normally expressed as a committed effective dose equivalent (CEDE).
[edit] Type of Radiation Exposure
The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) itemized type of exposures and reported exposure rate of each segment.
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.[19] This has led to the suggestion that high but steady levels of radiation are easier for humans to sustain than sudden radiation bursts.
Natural background radiation comes from five primary sources: cosmic radiation, solar radiation, external terrestrial sources, radiation in the human body and radon.
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, 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.[20]
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:
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[citation needed], 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[citation needed].
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).[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.
Some of the radionuclides of concern include cobalt-60, caesium-137, americium-241, and iodine-131.
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.[23][24][25]
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.[26]
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.[27][28]
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
- 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.
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,[29] 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 .[30] There is evidence that low level, brief radiation exposures are not harmful. [31]
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 [10]). 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 disaster.
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. Months after the accident, radiation levels remain high in some parts of the plant that were formerly accessible to workers: above 10,000 mSv/h (millisievert per hour) in one area,[33][34] which is a level that can cause fatal radiation poisoning from less than one hour of exposure.[35] 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.[36] 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.[37] 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.[38] 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".[39]
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.
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.[40]
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'.
Geiger counters and scintillation counters measure the exposure rate of ionizing radiation directly.
There are three 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 (in an absolute vacuum).[3]
- Shielding: Air or skin can be sufficient to substantially attenuate low-energy alpha and beta radiation. Barriers of lead, concrete, or water give effective protection from more 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 that stop beta particles, and air will stop most alpha particles. The effectiveness of a material in shielding radiation is determined by its half-value thicknesses, the thickness of material that reduces the radiation by half. This value is a function of the material itself and of the type and energy of ionizing radiation.
Some generally accepted thicknesses of attenuating material are 5 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.[41] GCR levels that might lead to acute radiation poisoning are not as well-understood.[42]
Air travel exposes people on aircraft to increased radiation from space as compared to sea level, including cosmic rays and from solar flare events.[43] Software programs such as Epcard, CARI, SIEVERT, PCAIRE are attempts to simulate exposure by aircrews and passengers.[43] 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.[43] However, dosages can vary, such as during periods of high solar activity.[43] 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.[43] In addition, many airlines do not allow pregnant flightcrew members, to comply with a European Directive.[43] The FAA has a recommended limit of 1 mSv total for a pregnancy, and no more than 0.5 mSv per month.[43] Information originally based on Fundamentals of Aerospace Medicine published in 2008.[43]
- ^ [1] Discussion of ionizing vs. non-ionizing radiation literature
- ^ "Fact Sheet on Biological Effects of Radiation". Publication date December 2004. The U.S. Nuclear Regulatory Commission (NRC). http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/bio-effects-radiation.html. Retrieved 2010-12-04.
- ^ 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.
- ^ [2] Questions and Answers about Biological Effects and Potential Hazards of Radiofrequency Electromagnetic Fields. OET Office of Engineering and Technology BULLETIN 56 Fourth Edition August 1999.
- ^ [3] elemental ionization energies.
- ^ [4] Discussion of ionizing vs. non-ionizing radiation literature
- ^ "Fundamental Quantities and Units for Ionizing Radiation (ICRU Report 85)". Journal of the ICRU 11 (1). 2011. http://www.icru.org/index.php?option=com_content&task=view&id=202.
- ^ Gas Filled Detectors, lecture note by Hao Peng at MacMaster University, Department of Medical Physics and Radiation Sciences, MED PHYS 4R06/6R03 - Radiation & Radioisotope Methodology
- ^ J Invest Dermatol. 2012 Feb 9. doi: 10.1038/jid.2011.476. Irradiation of Skin with Visible Light Induces Reactive Oxygen Species and Matrix-Degrading Enzymes. Liebel F, et al. PMID 22318388
- ^ a b Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII – Phase 2. Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation, National Research Council (2006) Free Executive Summary
- ^ Radioactivity in Nature <http://www.physics.isu.edu/radinf/natural.htm>
- ^ "Background Radiation: Natural versus Man-Made" Washington Stet Department of Health
- ^ "Monazite sand does not cause excess cancer incidence ", The Hindu
- ^ "Lethal dose", NRC Glossary (August 2, 2010)
- ^ Lewis, Leon; Paul E Caplan (January 1, 1950). "THE SHOE-FITTING FLUOROSCOPE AS A RADIATION HAZARD". California Medicine 72 (1): 26–30. PMC 1520288. PMID 15408494. //www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1520288.
- ^ Hvistendahl, Mara. "Coal Ash Is More Radioactive than Nuclear Waste". Scientific American. http://www.scientificamerican.com/article.cfm?id=coal-ash-is-more-radioactive-than-nuclear-waste. Retrieved 2012-03-05.
- ^ "IRRADIATED FOODS Fifth Edition Revised and updated by Paisan Loaharanu May 2003 AMERICAN COUNCIL ON SCIENCE AND HEALTH" (PDF). http://www.acsh.org/docLib/20040331_irradiated2003.pdf. Retrieved 2012-03-05.
- ^ Wynn, Volkert; Hoffman, Timothy (1999). "Therapeutic Radiopharmaceuticals afrtin=2+3=9000" (PDF). Chemical Reviews 99 (9): 2269–92. DOI:10.1021/cr9804386. PMID 11749482. http://pubs.acs.org/cgi-bin/article.cgi/chreay/1999/99/i09/pdf/cr9804386.pdf.
- ^ "International Congress Series : Cancer risk due to exposure to high levels of natural radon in the inhabitants of Ramsar, Iran". ScienceDirect. 2005-02-24. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B7581-4FJT8MK-5X&_user=10&_coverDate=02%2F28%2F2005&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1280797455&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=f4bab607012e40261fe6a863d029dbbd. Retrieved 2012-03-05.
- ^ "Health Risks | Radon | US EPA". Epa.gov. http://www.epa.gov/radon/healthrisks.html. Retrieved 2012-03-05.
- ^ Pattison, J.E., Bachmann, D.J., Beddoe, A.H. (1996). "Gamma Dosimetry at Surfaces of Cylindrical Containers". Journal of Radiological Protection 16 (4): 249–261. Bibcode 1996JRP....16..249P. DOI:10.1088/0952-4746/16/4/004.
- ^ Pattison, J.E. (1999). "Finger Doses Received during Samarium-153 Injections". Health Physics 77 (5): 530–5. DOI:10.1097/00004032-199911000-00006. PMID 10524506.
- ^ Acharya, PVN; 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, PVN; 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 Narasimh; 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.
- ^ "RADIATION HORMESIS CHALLENGING LNT THEORY VIA ECOLOGICAL AND EVOLUTIONARY CONSIDERATIONS". Publication date 2002. Health Physics Society. http://www.groenerekenkamer.com/grkfiles/images/parsons.pdf. Retrieved 2010-12-11.
- ^ http://www.pnas.org/content/100/9/5057.full.pdf
- ^ "Repair of x-ray-induced DNA double-strand breaks in specific Not I restriction fragments in human fibroblasts: joining of correct and incorrect ends". Pnas.org. 1995-12-19. http://www.pnas.org/content/92/26/12050.long. Retrieved 2012-03-05.
- ^ "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.
- ^ Cardis E, Vrijheid M, Blettner M, et al. (July 2005). "Risk of cancer after low doses of ionising radiation: retrospective cohort study in 15 countries". BMJ 331 (7508): 77. DOI:10.1136/bmj.38499.599861.E0. PMC 558612. PMID 15987704. //www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=558612.
- ^ Werner Olipitz, etal. (April 2012). "Integrated Molecular Analysis Indicates Undetectable DNA Damage in Mice after Continuous Irradiation at ~400-fold Natural Background Radiation". Environmental Health Perspectives (National Institute of Environmental Health Sciences). DOI:10.1289/ehp.1104294. http://ehp03.niehs.nih.gov/article/fetchArticle.action?articleURI=info%3Adoi%2F10.1289%2Fehp.1104294.
- ^ Martin Fackler (June 1, 2011). "Report Finds Japan Underestimated Tsunami Danger". New York Times. http://www.nytimes.com/2011/06/02/world/asia/02japan.html?_r=1&ref=world.
- ^ "Tepco Reports Second Deadly Radiation Reading at Fukushima Plant". Bloomberg. 8 August 2011. http://www.bloomberg.com/news/2011-08-02/tepco-reports-second-deadly-radiation-reading-at-fukushima-plant.html. Retrieved 3 August 2011.
- ^ "Fukushima radiation hotspot". World Nuclear News. 2011-08-02. http://www.world-nuclear-news.org/RS_Fukushima_radiation_hotspot_0208111.html. Retrieved 19 May 2012.
- ^ Turner, James Edward (2007). Atoms, Radiation, and Radiation Protection. Wiley-VCH. p. 421. ISBN 978-3-527-40606-7.
- ^ Keith Bradsher et al. (April 12, 2011). "Japanese Officials on Defensive as Nuclear Alert Level Rises". New York Times. http://www.nytimes.com/2011/04/13/world/asia/13japan.html?_r=1.
- ^ Cresswell, Adam (March 16, 2011). "Stealthy, silent destroyer of DNA". The Australian.
- ^ Winter, Michael (March 24, 2011). "Report: Emissions from Japan plant approach Chernobyl levels". USA Today. http://content.usatoday.com/communities/ondeadline/post/2011/03/report-radioactive-emissions-from-japan-plant-approach-chernobyl-levels/1.
- ^ Michael Winter (March 24, 2011). "Report: Emissions from Japan plant approach Chernobyl levels". USA Today. http://content.usatoday.com/communities/ondeadline/post/2011/03/report-radioactive-emissions-from-japan-plant-approach-chernobyl-levels/1.
- ^ Sanders, Charles L. (2010). "The LNT assumption". Radiation Hormesis and the Linear-No-Threshold Assumption. Heidelberg, Germany: Springer. p. 3. ISBN 978-3-642-03719-1. "...a large number of experimental and epidemiological studies challenge the validity of the LNT assumption, strongly suggesting the presence of a threshold and/or benefits from low doses of ionizing radiation"
- ^ "Superflares could kill unprotected astronauts". New Scientist. 21 March 2005. http://www.newscientist.com/article/dn7142.
- ^ "Space Radiation Hazards and the Vision for Space Exploration". NAP. 2006. http://www.nap.edu/catalog.php?record_id=11760.
- ^ a b c d e f g h Jeffrey R. Davis, Robert Johnson, Jan Stepanek - Fundamentals of Aerospace Medicine (2008) - Page 221-230 (Google Books Link 2010)
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