The physics measurement of ionizing radiation, absorbed dose, is the energy deposited per mass. The unit is the gray (Gy), defined as 1 joule per kilogram (Gy = J/kg). The milligray is one thousandth of a gray (mGy = 0.001 Gy). If the amount and type of radiation and the dimensions and composition of the portion of the body that is exposed are known, this value can be calculated very accurately. An older unit, radiation absorbed dose (rad = erg/g), times 100 is equal to one gray (100 rad = Gy).
The effective dose includes a factor which adjusts for the biological effectiveness of different types of ionizing radiation, e.g. gamma-rays versus neutrons, and sensitivity of different tissues to ionizing radiation. One sievert of ionizing radiation has a biological effect equal to exposure of the whole body to 1 gray of gamma- or X-rays. The millisievert is one thousandth of a sievert (mSv = 0.001 Sv). The sievert (Sv) is generally used for describing therapy doses; the millisievert (mSv = 0.001 Sv) is generally used for describing radiological doses; the microsievert (µSv = 0.000001 Sv) is used for very-low level doses.
Under experimental conditions, a particular biological effect can be measured accurately as a function of the type of ionizing radiation. Similarly, the acute affects from radiation therapy in patients with cancer are well known. Fortunately, there is relatively little information about the long-term biological effects of ionizing radiation on normal populations; therefore, the factor for converting from absorbed dose to effective dose is approximate. Further, the biological factor relating absorbed dose (Gy) and effective dose (Sv) may be different for different biological effects. Although the calculation of effective dose in sieverts is only an approximation, this measure provides the best overall assessment of the risk of ionizing radiation. An older unit, roentgen equivalent man (rem = erg/g) times 100 is equal to one sievert (100 rem = Sv).
Measurement of external radiation exposure is more complicated than is immediately apparent, but in the context of this description reasonably accurate measures are millisievert per hour (mSv/h) and microsievert per hour (µSv/h), where (µSv/h = 0.001 mSv/h). The effective dose can be calculated simply by multiplying the millisievert per hour times the number of hours of exposure (effective dose (mSv) = exposure (mSv/h) • exposure-time (h)).
Simple inexpensive equipment, e.g. an ionization chamber, can be used to measure low-level radiation exposures. These devices are sensitive enough to measure normal background radiation and natural sources of radiation. Bananas are a good source of potassium, an essential nutrient. Potassium has a tiny fraction of naturally occurring radioactive potassium-40. The radiation from a bunch of bananas can be easily measured without complex equipment.
Since humans cannot see or feel ionizing radiation, even high levels of ionizing radiation, most people find ionizing radiation particularly scary. However, people who work with ionizing radiation consider the ability to measure these very low levels an important safety factor – a safety factor not available for many other toxins.
Radioactivity inside the body due to ingestion, inhalation, etc. will cause internal radiation exposure. While measurement of external radiation exposure is relatively simple, measurement of internal radiation exposure is much more complicated. The effective dose will depend upon the amount of radioactivity, the physical half-life of the radioactive isotope or isotopes, the details of the decay process(es) the amount of time the isotope stays in the body (biological half-life), the distribution within the body, and the body dimensions and composition.
The amount of radioactivity is measured using the becquerel (Bq), defined as one decay each second. One becquerel is an extremely small quantity of radioactivity. The kilobecquerel, 1000 becquerel (kBq = 1000 Bq), is used for small quantities of of radioactivity. The megabecquerel, 1,000,000 becquerel (MBq = 1,000,000 Bq), is used for quantities used in diagnostic Nuclear Medicine. The gigabecquerel, 1,000,000,000 becquerel (GBq = 1,000,000,000 Bq), is used for therapeutic levels of radioisotopes. An older unit, the curie (Ci), is equal to 37,000,000,000 Bq; the millicurie (mCi) is equal to 37,000,000 Bq. A convenient conversion for diagnostic Nuclear Medicine is one millicurie equals 37 megabecquerel (mCi = 37 MBq).
The distribution of an isotope within the body depends upon the chemical form or forms of the isotope. A frequent example is radioiodine in the form of an ion (iodide). Iodide is highly concentrated in the thyroid gland. This high level of concentration allows radiation therapy of various thyroid disorders with only small affects on other organs. Large amounts of potassium iodine (KI), which can be used to block the concentration in the thyroid gland, can reduce the effective dose caused by accidental exposure to radioiodine. However, since large amounts of potassium iodine have important side effects in a small number of people, it should only be used when a significant exposure to radioiodine is expected (see http://interactive.snm.org/docs/hpra/Radiation_Risk_Joint_Statement_FINAL_Letterhead.pdf).
Although the calculation of the effective dose is quite complicated, this calculation has been performed for several isotopes for various situations. Thus, given the isotope and the amount (Bq) of internal radioactivity experts can often estimate an approximate effective dose.
Some effects of ionizing radiation are a function of the effective dose to a particular organ or system. These effects are call deterministic effects; the effective dose determines if the effect will or will not occur. These effects are generally acute or subacute, occurring within hours to weeks. There is an effective dose below which these effects are rarely seen and an effective dose above which they are almost always seen. They can be described in terms of the effective dose at which 50% of the exposed population will exhibit the effect. The severity of the effect is generally proportional to the effective dose received by an organ or system. Examples of deterministic effects are decrease in blood cell count, nausea and vomiting, seizure, and death.
Other effects either occur or do not occur, but the severity of the effect is not related to the effective dose. Rather, the incidence of these effects is a function of the effective dose. Since the incidence is described using stochastic methods (probability and statistics), these effects are called stochastic effects. These effects often occur over a lifetime. The most common of these effects is cancer induction; considerably less common are genetic effects. The regulatory assumption for doses less than 100 mSv is that stochastic effects are linearly related to the effective dose and that there is no threshold below which they do not occur, the linear, no-threshold assumption.
The cancers which are thought to arise from radiation exposure are generally similar to naturally occurring cancers and they tend to happen at the same age as naturally occurring cancer. Given the high natural frequency of cancer, it is very difficult to measure the rate of additional cancers caused by ionizing radiation. The best data in humans comes from the atom bomb survivors. This data plus other existing biological and human data are periodically reviewed by the US National Academies of Science (NAS) Biological Effects of Ionizing Radiation (BEIR) committee (see http://www.nap.edu/catalog.php?record_id=11340).
The risk of developing a cancer after exposure to ionizing radiation compared to the risk of developing that same cancer is called the relative risk. The relative risk from ionizing radiation is greater for some types of cancers than other types and it varies with the age at which a person is exposed. For example, the relative risk of developing thyroid cancer as a child is much greater than as an adult. The relative risk is also related to the dose to the tissue from which the cancer develops. Radioactive iodine is highly concentrated in the thyroid gland, so the thyroid dose will be much higher than the dose to other organs. However, in the case of exposure to a mixture of sources of ionizing radiation, the overall relative risk is much more important than the risk of any particular cancer. Thus, the effective dose should still be used as the best overall guide to ionizing radiation risk.
There is concern even about very low levels of ionizing radiation. The likelihood of an adverse effect in any one person from a low-level exposure is very small. However, if the linear, no-threshold assumption is correct, then even low-level exposure of the whole US population would result in stochastic effects in an important number of people. Obviously, there is also concern about high-level radiation especially in the range where deterministic effects are likely to occur in each individual. Thus, the range of radiation exposures which are commonly discussed is very large. Note that the list below gives gives exposures which vary by a factor of 10 million. The effective doses listed are approximate, but the list provides a general idea of which exposures are high, which are low, and which are in between.
Several caveats should be noted. This list includes exposures below 100 mSv where the linear, no-threshold assumption is used and higher level exposures where this assumption is not valid. The list includes single acute exposures and exposures over a period of times. The rate at which exposure occurs is important in some cases, especially high-level exposures. For example, a typical course of radiation therapy exposes the cancerous tissue to 50,000 mSv; when given as a single dose or using an accelerated schedule considerably lower total absorbed doses result in the same biological effect. (Note that 50,000 mSv is the dose to the cancerous tissues; normal tissues receive a much lower dose.)
mSv |
Example |
|
|---|---|---|
0.005 |
bone density measurement | |
0.1 |
chest X-ray | |
0.4 |
radiation from food per year (natural radioactivity) | |
3 |
background radiation per year | |
5 |
CT scan | |
5 |
pregnant radiation worker, limit per year | |
50 |
radiation worker, limit per year | |
500 |
threshold for decreases in blood count (single exposure) | |
1000 |
threshold for nausea and vomiting (single exposure) | |
5000 |
death with supportive medical care (50% from single exposed) | |
50,000 |
typical dose to treat cancer (course of radiation therapy) |
ALARA is an acronym for As Low As Reasonably Achievable. The ALARA principle is the major regulatory principle for dealing with ionizing radiation. This principle implies that there is no safe level of ionizing radiation. The goal is to reduce excess ionizing radiation at any level of exposure, even levels that are below normal background levels. There are variations in background radiation from location to location. Excess exposure less than typical variations in background radiation is considered reasonable in many situations.
There is no scientific difference between the risks associated with ionizing radiation and with other carcinogens, agents which cause cancer. Application of the ALARA principle to regulation of ionizing radiation provides an additional measure of safety as compared with other agents.