Frequently Asked Questions about Health Effects of Low Doses of Radiation
What is radiation?
What are examples of ionizing radiation?
What are examples of non-ionizing radiation?
Are there beneficial health effects from exposure to ionizing radiation?
What is background radiation?
What is a low dose of radiation?
What is LET?
What is radiation dose?
Will I get cancer if I am exposed to ionizing radiation?
What are the genetic effects of ionizing radiation?
How can cell and molecular research help in understanding the hazards from radiation?
What is depleted uranium?
Is depleted uranium a serious radiation problem with potential to produce cancer?
How do we detect the presence of radiation?
What is Radiation?
Radiation is a form of energy. There are two basic types of radiation-ionizing and non-ionizing radiation. The difference between these two types is the amount of energy they have. Ionizing radiation is high-energy radiation that has the ability to break chemical bonds, cause ionization, and produce free radicals that can result in biological damage. Non-ionizing radiation does not have enough energy to cause ionization but disperses energy through heat and increased molecular movement. Health effects from radiation have been a concern since their discovery.
Reference: E.J. Hall (2000) Radiobiology for the Radiologist, Fifth Addition, Lippincott Williams and Wilkins, Health Physics Website:
Health and Physics Society: What is Radiation?
» Return to top
What are examples of ionizing radiation?
Ionizing radiation consists of both particles and electromagnetic radiation. The particles are further classified as electrons, protons, neutrons and alpha particles, depending on their atomic characteristics. The most common electromagnetic radiation with enough energy to produce ions, break chemical bonds, and alter biological function comes from x-rays and gamma rays. Exposure to such radiation can cause cellular and molecular changes such as mutations, chromosome aberrations, and cell-killing.
At high doses, it is well-established that ionizing radiation is capable of increasing the cancer rate in exposed populations; at low doses, it is not possible to detect changes in cancer frequency. Studies on the health effects of ionizing radiation have been conducted with each of these types of radiation, delivered over a very wide range of exposures and doses, and the effects have been evaluated at every level of biological organization from the molecular to whole human populations. These studies have resulted in extensive information on the health effects produced by high doses from ionizing of radiation.
Studies at lower doses are currently being conducted as new tools become available to both deliver the radiation and study the response of cells and molecules. This research has resulted in some very interesting results, suggesting that low doses of ionizing radiation with matter triggers many biological responses that were not predicted from past experience. These results include bystander effects, changes in the spectrum of gene activation, adaptive responses, and genomic instability. All these are discussed further in the following cited review papers.
Morgan, W.F. (2003). Non-targeted and delayed effects of exposure to ionizing radiation: I. Radiation-induced genomic instability and bystander effects in vitro. Radiation Research 159:567-580.
Morgan, W.F. (2003). Non-targeted and delayed effects of exposure to ionizing radiation: II. Radiation-Induced genomic instability and bystander effects In vivo- Clastogenic factors and transgenerational effects. Radiation Research 159:581-596.
Redpath, J.L., Lu, Q., Lao, X., Molloi, S., and Elmore, E. (2003) Low doses of diagnostic energy X-rays protect against neoplastic transformation In vitro. International Journal of Radiation. Biology. 79(4):235-240.
Health and Physics Society: What types of radiation are there?
» Return to top
What are examples of non-ionizing radiation?
Examples of non-ionizing radiation include radio-waves, microwaves, radar, and low-energy light. Because such radiation is not able to break chemical bonds, the biological responses to this exposure are not related to events such as mutations or chromosome aberrations but are related to physiological changes. There is extensive research on these types of radiation and discussion as to whether they can cause adverse biological changes that result in the formation of cancer. Biological changes are induced by such radiation, but it has not been possible to establish a strong link between these exposures and an increased cancer risk.
National Academy of Sciences Report, Possible health effects of residential electric and magnetic fields.
» Return to top
Are there beneficial health effects from exposure to ionizing radiation?
There are many beneficial effects from the use of ionizing radiation. Radiation is widely used in medicine for diagnosis and treatment of many diseases. It has been estimated that there are more than 200 million x-rays given each year in the United States to diagnose and evaluate diseases or accidents. Worldwide, this number has been estimated at about 2 billion/year. Because of the ability of radiation to kill cells, it has also been widely used in cancer therapy. About 5.5 million people are treated with x-rays each year to cure cancer.
The use of radiation is a very important tool in our health-care system and has produced great health benefits. However, excessive use of radiation in medicine and the development of new techniques that deliver more radiation than may be needed for proper diagnosis remains as a concern for potential increased cancer risk. Extensive research is being conducted to determine if there are health benefits as well as adverse health effects from exposure to low doses of ionizing radiation. To date the assumptions remain that there is a linear relationship between the induction of adverse health effects and exposure to radiation regardless of the radiation dose. This assumption is the subject of active discussion in the scientific community, with some scientists suggesting that there are hormetic, or adaptive or beneficial, effects that may decrease cancer risk below that predicted by the linear extrapolation from high doses. If such effects exist it could change the perception of risk associated with very low doses of radiation. This is an area of active research.
NCRP 136, Belle NewsLetter, Hall presentation, World Congress on Radiation Risk, Brussels, Belgium, 2003.
Brenner, D.J., Elliston, C.D., Hall, E.J., and Berdon W.E. (2001) Estimated risks of radiation-induced fatal cancer from pediatric CT, American Journal Roentgeneology 289-296.
What are some examples of applications of constructive uses of radioactive isotopes?
» Return to top
What is background radiation?
Natural background radiation is the radiation exposure or dose that is received daily from normal sources. Natural background radiation comes from cosmic radiation, internally deposited radioactive materials in the body, and radiation such as radon from environmental sources such as soil and granite.
The average natural background radiation in the United States is about 300 mrems or 3.0 mSv per year with the radon exposure being responsible for about half of the total. There is a wide range of background radiation (about a factor of 10 from the highest to the lowest) in different regions of the United States. In some regions of the world the background radiation is more than a thousand times higher than the average background level in the United States.
To date there have been no demonstrated health effects due to the variability in background radiation. There is also anthropogenic background radiation from nuclear weapons, nuclear accidents, production of nuclear power, and the use of radioactive materials in consumer products. The estimated average human exposure from anthropogenic background radiation is about 70 mrem/year. A complete description of background radiation can be found in NCRP Report 94.
NCRP National Council on Radiation Protection and Measurements, 1987. Exposure of the population in the United States and Canada from natural background radiation. NCRP Report No. 94. Issued December 30, 1987-Bethesda, Maryland.
» Return to top
What is a low dose of radiation?
There are two realistic definitions of a low dose of radiation. One is a dose below which it is not possible to detect adverse health effects. This level has been set by the ICRP to be at 20 rads, 20,000 mrads, or 0.2 Gy, 200 mGy. Others suggest that this level is much lower and may be as low as 1 rad. Another definition of a low dose of radiation would be the level of radiation that we are exposed to from natural background radiation. In the United States. this dose ranges from 75 - 1000 mrads with a mean of 370 mrads, 0.37 rads or 3.7 mGy. This dose does not include exposure to medical procedures.
NCRP National Council on Radiation Protection and Measurements, (1987). Exposure of the population in the United States and Canada from natural background radiation. NCRP Report No. 94. Issued December 30, 1987. Bethesda, Maryland.
Low-Dose Effects and Beneficial Effects
Do studies (like the Hiroshima survivors) show a beneficial effect of exposure to low (<10 rem) amounts of radiation?
» Return to top
What is LET?
Each type of radiation has a characteristic atomic make up and can be delivered over a range of different energies. As the radiation interacts with matter, it loses its energy through interactions with the atoms with which it associates. The average amount of energy that is lost over a defined distance; for example the energy deposited in ten cells is known as the Linear Energy Transfer (LET).
The distribution of energy in cells has a marked influence on the amount of biological damage done by a fixed amount of radiation. Some types of radiation, like alpha particles, deposit a large amount of energy in a small distance and are called high-LET radiation. For alpha particles, all the particles' energy can be deposited in a few cells. Other radiation types such as x-rays or gamma rays penetrate tissues very easily and for any event deposit energy very infrequently. The energy deposition events from these low-LET sources are separated in space so that, on average, few ionizations or damaging events occur in any one cell from a single x-or gamma ray.
High-LET radiation is more effective in producing cancer and other cellular and molecular damage per unit of exposure or dose than low-LET exposure. This may be related to the concentration of energy from a single particle in a single cell and the ability of this energy concentration to trigger bystander effects that influence many more cells than are "hit" by the alpha particles.
NCRP National Council on Radiation Protection and Measurements, (1990). The relative biological effectiveness of radiations of different quality. NCRP Report No. 104. Issued December 15, 1990 Bethesda, Maryland.
» Return to top
What is radiation dose?
There are two formal definitions of dose. One is the energy deposited at a point source and is useful in determining the dose distribution during cancer therapy. The other is the amount of energy deposited in a unit of tissue. This measurement of dose can be used to represent the average dose to an organism, organ, tissue or even a cell. There are many different types of radiation dose.
ICRP Publication 60. (1990) Recommendations of the International Commission on Radiological Protection. Pergamon Press, Oxford, New York. (1990).
ICRU Publication 51. Quantities and units in radiation protection dosimetry. (1991).
ICRU Publication 60 Fundamental quantities and units for ionizing Radiation. (1998).
» Return to top
Will I get cancer if I am exposed to ionizing radiation?
The risk of radiation exposure has been very widely studied, with literally billions of dollars invested to define and understand radiation-related health effects. The risk for the induction of cancer by ionizing radiation is summarized in a wide number of reports from national and international committees.
It is important to understand that radiation is a very good cell killer, as demonstrated in its wide use in cancer therapy. However, radiation is not a very strong carcinogen, especially at low doses. The body has many repair mechanisms that can handle the damage induced by low doses of radiation, which illustrates that at levels of radiation of environmental concern, the individual risk for developing cancer is very small. Following exposures to low levels of ionizing radiation it is not possible to detect a change in cancer incidence. This is related to the many factors that can produce cancers, the high background level of cancers and the high and variable background levels of radiation exposure in the population.
To calculate cancer risk from a specific dose:
(Dose in mrem) x (cancer risk of 0.04 cancers/Sv)* x (0.001 Sv/mSv) x (0.01 mSv/mrem) = Risk of radiation-induced cancer
*-To calculate this conservative radiation cancer risk, the regulatory bodies use the recommendations of the NCRP 116 (Limit of Exposure to Ionizing Radiation) which calculates that cancer risk increases above the background rate of cancer by 4% per Sv of radiation dose.
NCRP National Council on Radiation Protection and Measurements, (1980) Influence of dose and its distribution in time on dose-response relationships for low-LET radiations. Report No. 64. Issued April 1, 1980. Bethesda, Maryland.
NCRP National Council on Radiation Protection and Measurements (1982) Critical issues in setting radiation dose limits, In Proceedings of the Seventeenth Annual Meeting of the National Council of Radiation Protection and Measurements, Proceedings No. 3, April 1, 1982.
NCRP National Council on Radiation Protection and Measurements, (1987), Recommendations on limits for exposure to ionizing radiation. NCRP Report No. 91 (Bethesda, National Council on Radiation Protection and Measurements).
NCRP National Council on Radiation Protection and Measurements, Risk estimates for radiation protection. NCRP Report No. 115. Issued December 31, 1993. Bethesda, Maryland.
NCRP National Council on Radiation Protection and Measurements (1993) Limitation of exposure to ionizing radiation. NCRP Report No. 116.
Radiation Dose Units
Category: Environmental and Background Radiation - General
» Return to top
What are the genetic effects of ionizing radiation?
The concern about the genetic effects of radiation has been present since the first discovery of radiation and the observation that radiation produced mutations in fruit flies. This observation triggered extensive studies on radiation-induced mutation. It is well established that both internally deposited and external exposure to ionizing radiation can cause mutations, chromosome aberrations, and transmitted genetic effects.
The extent of these changes is very easy to measure in specialized cell and tissue cultures. In these cell-culture experimental systems the number of mutations increases as a linear function of radiation dose. Each unit of dose causes an increase in mutation frequency. However, research in mice demonstrated that there were changes in mutation frequency as a function of both dose and dose rate and that the number of mutations decreased as the radiation dose rate decreased. In addition, there are many steps and processes that a cell must accurately go through before a sperm can fertilize an egg and produce an offspring. Most damaged cells are eliminated by these processes and do not result in offspring. Thus, the mutation frequency in irradiated offspring is very low. This results in the property of radiation that demonstrates that radiation is a very good cell-killer, so it is used in therapy, and it is a weak mutagen and carcinogen.
The risk for mutations has been examined carefully in some very large animal studies. In these studies, male mice were given large doses of radiation and the exposed animals produced hundreds of thousands of offspring. With this large study it was possible to detect mutations in a small number of animals. These data as well as in the follow-up on the A-bomb survivors, where it has not been possible to detect an increased frequency of mutations in the offspring of exposed parents, support the conclusion that radiation is a weak mutagen. Research is continuing on transgenerational effects of radiation by mechanism other than direct DNA damage and mutation induction.
NCRP National Council on Radiation Protection and Measurements (1987). Genetic effects from internally deposited radionuclides. NCRP Report No. 89. Bethesda, Maryland.
NCRP 99, William L. Russell mouse data, NAS 1980, BEIR III Report, Effects on populations of low levels of ionizing radiation.
Baulch, J.E., Raabe, O.G., Wiley, L.M., and Overstreet, J.W. (2002) Germline drift in chimeric male mice possessing an F(2) component with a parental F(0) radiation history. Mutagenesis 17(1):9-13.
» Return to top
How can cell and molecular research help in understanding the hazards from radiation?
Recent research has been able to take advantage of the many advances in genomics, cell, and molecular biology to detect changes in cells exposed to low doses of ionizing radiation. The current DOE Low Dose Research Program, outlined on this site, is one example of an organization that is funding research to measure such effects following radiation exposures of 10 cGy and less.
Through this research, it has been possible to make measurements that change our thinking on how radiation interacts with matter to result in biological damage. For example, changes in cells that are not "hit" by ionizing radiation have been observed using a wide range of biological endpoints. These "bystander effects" illustrate that contrary to conventional past wisdom, a cell does not have to have energy directly deposited in it to result in a biological change. This new knowledge results in research questions that are currently being addressed, such as: Are these bystander changes protective or a reflection of increased damage and risk?
The research also demonstrated that the types and numbers of genes activated by low doses of radiation are different from those activated by high radiation doses. These changes in gene activation seem to be able to modify the response of cells to subsequent radiation exposure, termed the "adaptive response." This adaptive response seems to be the manifestation of a protective effect that may reduce risk at very low doses. Cell and molecular research also determined that cells may lose their genetic control of many cell generations following radiation exposure. This is termed genomic instability and is thought to be an essential step in radiation induced cancer. There is a very active research effort being conducted to better understand the impact of all these cell and molecular effects on the risk for development of cancer.
Morgan, W.F. (2003). Non-targeted and delayed effects of exposure to ionizing radiation: I. Radiation-induced genomic instability and bystander effects in vitro. Radiation Research 159:567-580.
Morgan, W.F. (2003). Non-targeted and delayed effects of exposure to ionizing radiation: II. Radiation-induced genomic instability and bystander effects In vivo, Clastogenic factors and transgenerational effects. Radiation Research 159:581-596.
Redpath, J.L., Lu, Q., Lao, X., Molloi, S., and Elmore, E. (2003) Low doses of diagnostic energy x-rays protect against neoplastic transformation in vitro. International Journal Radiation. Biolology 79(4):235-240.
NCRP National Council on Radiation Protection and Measurements, 1993. Report No. 117, "Research Needs for Radiation Protection", Scientific Committee 83, Issued Nov 30, 1993, Bethesda, Maryland.
» Return to top
What is depleted uranium?
Depleted uranium is currently being used for military purposes to make shielding and projectiles. This use has raised questions about the potential long-term health effects from the radiation exposure produced by the uranium. Depleted uranium is mainly the non-fissionable 238-U with the fissionable 235U removed for making nuclear weapons or nuclear fuel. 238U has a very long half-life, the length of time that it takes for half of the radioactive material to decay, of about 4,500,000,000 years and therefore results in very few radiation disintegrations per unit of mass and time. Depleted uranium thus, delivers very low radiation doses per unit of mass. This is much less activity per unit of mass than is found in many phosphate fertilizers, smoke detectors or camping lantern mantles.
» Return to top
Is depleted uranium a serious radiation problem with potential to produce cancer?
Because 238U is an alpha emitter, and alpha particles can be stopped by a single layer of skin, it presents no radiation hazard from external exposure. The concern is when large amounts of depleted uranium are deposited in the body through wounds or inhalation. Uranium is a heavy metal and as such is toxic, especially to the kidneys. Depleted uranium has been shown to convert normal cells in tissue culture to transformed cells. This may be a function of its metal toxicity.
On the other hand, we all have uranium in our bodies from natural sources. Therefore the real concern would be from addition of high levels of uranium to our bodies. Such high levels would result from being "hit" by fragments from a uranium bullet or being very close to where a uranium bullet hit and ignited. This ignition can result in the production of small particles of uranium oxide. The hazard would be the inhalation of the uranium. Because of the high mass of the uranium, the aerodynamic diameter of the particles would tend to be large which makes it difficult for them to remain in the air for a long enough time to create a major inhalation hazard.
These facts suggest that the radiation doses from uranium in the environment is of little concern. If there are "bystander effects" produced by the alpha particles from the uranium that result in many more of the cells in the tissue responding than are "hit" by the alpha particles, it could affect the risk. To date it is not possible to determine if such bystander effects would make the tissue more or less sensitive to the toxicity of the uranium and if the risk could be increased or decreased. Research needs to be further conducted to evaluate the interaction between the know toxicity of the Uranium and the potential for a very small radiation dose. Additional questions and concerns can be addressed by the following references.
NCRP National Council on Radiation Protection and Measurements, 1997. Deposition, retention and dosimetry of inhaled radioactive substances. NCRP Report No. 125. Issued February 14,1997. Bethesda, Maryland.
» Return to top
How do we detect the presence of radiation?
The development of radiation detection equipment has been very successful. This equipment is extremely sensitive and can detect very low levels of radiation, characterize the types of isotopes involved in producing the radiation and estimate the radiation dose. This equipment can detect both high- and low-LET radiation as well as the type of radiation found during space travel. Such physical dosimeters are critical in health protection. The website for the Health Physics Society has a very good coverage on the types of instruments used to detect the different types of radiation and the sensitivity of each type of equipment.
» Return to top
If you have additional questions that you would like addressed feel free to send them to us at: Low Dose
Last Update: September 2010