Stochastic effects of ionizing radiation occur by chance, generally occurring without a threshold level of dose. The probability of occurrence of stochastic effects is proportional to the dose, but the severity of the effect is independent of the dose received. The biological effects of radiation on people can be grouped into somatic and hereditary effects. Somatic effects are those suffered by the exposed person, and hereditary effects are those suffered by the offspring of the individual exposed. Cancer risk is usually mentioned as the main stochastic effect of ionizing radiation, but also hereditary disorders are stochastic effects.
According to ICRP:
(83) based on these calculations, the Commission proposes nominal probability coefficients for detriment-adjusted cancer risk as 5.5 x 10-2 Sv-1 for the whole population and 4.1 x 10-2 Sv-1 for adult workers. For heritable effects, the detriment-adjusted nominal risk in the whole population is estimated as 0.2 x 10-2 Sv-1 and in adult workers as 0.1 x 10-2 Sv-1.
Special Reference: ICRP, 2007. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. Ann. ICRP 37 (2-4).
The SI unit for effective dose, the sievert, represents the equivalent biological effect of depositing a joule of gamma rays energy in a kilogram of human tissue. As a result, one sievert represents a 5.5% chance of developing cancer. Note that the effective dose is not intended as a measure of deterministic health effects, which is the severity of acute tissue damage that is certain to happen, measured by the quantity absorbed dose.
There are three general categories of stochastic effects resulting from exposure to low doses of radiation. These are:
- Genetic effects. The genetic effect is suffered by the offspring of the individual exposed. It involves the mutation of specific cells, namely the sperm or egg cells. Radiation is an example of a physical mutagenic agent. Note that there are also many chemical agents as well as biological agents (such as viruses) that cause mutations. One very important fact to remember is that radiation increases the spontaneous mutation rate but does not produce any new mutations.
- Somatic effects. Somatic effects are those suffered by the exposed person. The most common impact of irradiation is the stochastic induction of cancer with a latent period of years or decades after the exposure. Since cancer is the primary result, it is sometimes called the carcinogenic effect. Radiation is an example of a physical carcinogenic, while cigarettes are an example of a chemical cancer-causing agent, and viruses are examples of carcinogenic biological agents.
- In-Utero effects involve the production of malformations in developing embryos. However, this is a special case of the somatic effect since the embryo/fetus is exposed to radiation.
Most thought somatic effects from exposure to radiation are thought by most to occur in a stochastic manner. The most widely accepted model posits that the incidence of cancers due to ionizing radiation increases linearly with effective radiation dose at a rate of 5.5% per sievert. This model is known as the linear no-threshold model (LNT). This model assumes no threshold point, and the risk increases linearly with a dose. If this linear model is correct, natural background radiation is the most hazardous radiation source to general public health, followed by medical imaging as a close second. The LNT is not universally accepted, with some proposing an adaptive dose-response relationship where low doses are protective, and high doses are detrimental. It must be emphasized that many organizations disagree with using the linear no-threshold model to estimate risk from environmental and occupational low-level radiation exposure.
