Exposure to radiation at high doses can cause effects such as nausea, reddening of the skin or, in severe cases, more acute syndromes that are clinically expressed in exposed individuals within a short period of time after the exposure. Such effects are termed ‘deterministic effects’ because they are certain to occur if the dose exceeds a threshold level. Radiation exposure can also induce somatic effects such as malignancies, which are expressed after a latency period and may be epidemiologically detectable in a population; this induction is assumed to take place over the entire range of doses without a threshold level (ICRP, 2007). Also, hereditary effects due to radiation exposure have been statistically detected in other mammalian populations and are presumed to occur in human populations also. These epidemiologically detectable effects — malignancies and hereditary effects — are termed ‘stochastic effects’ because of their random nature.

Deterministic effects are the result of various processes, mainly cell death and delayed cell division, caused by exposure to high levels of radiation. If extensive enough, these can impair the function of the exposed tissue. The severity of a particular deterministic effect in an exposed individual increases with the dose above the threshold for the occurrence of the effect. From reviews of biological and clinical data it is concluded that in the range of absorbed dose up to and around 100 mGy no tissues express clinically relevant functional impairment. This applies to both single acute doses and to situations where the doses are experienced in a protracted form as repeated annual exposures (ICRP, 2007).

Stochastic effects may ensue if an irradiated cell is modified rather than killed. Modified cells may, after a prolonged process, develop into a cancer. The body’s repair and defence mechanisms make this a very improbable outcome at small doses; nevertheless, there is no evidence of a threshold dose below which cancer cannot result. The probability of occurrence of cancer is higher for higher doses, but the severity of any cancer that may result from irradiation is independent of the dose.

If the cell damaged by radiation exposure is a germ cell, whose function is to transmit genetic information to progeny, it is conceivable that hereditary effects of various types may develop in the descendants of the exposed individual. The likelihood of stochastic effects is presumed to be proportional to the dose received, without a dose threshold. For the purposes of practical radiological protection the ICRP recommends the use of a risk coefficient for stochastic effects of around 0.05 per Sv (effective dose) for exposure to radiation at low dose rate and at doses below about 100 mSv; this is a combined coefficient of detriment that includes all cancers and hereditable effects recognizing that the risks may be different for different organs and tissues (ICRP, 2007). In recent years, there has also been considerable discussion on such things as cellular adaptive responses, genomic instability and bystander signalling (ICRP, 2005). However, the ICRP considers that any contribution from these biological mechanisms is implicitly included in its estimated risk coefficient because it is based on human epidemiological data.

I n addition to the aforementioned health effects, other health effects may occur in infants due to exposure of the embryo or foetus to radiation. These effects include a greater likelihood of leukaemia and, for exposure above various threshold dose values during certain periods of pregnancy, severe mental retardation and congenital malformations. However, the ICRP concludes that the doses received from properly conducted prenatal diagnostic tests would lead to no measurably increased risk of prenatal or postnatal death or developmental damage but that higher doses from therapeutic procedures have the potential to result in developmental harm (ICRP, 2007).

In recent years evidence has accumulated that the frequency of non-cancer diseases is increased in some irradiated populations particularly for heart disease, stroke, digestive disorders, and respiratory disease. These effects have been seen at high doses but, at present, the ICRP considers that there is insufficient information to determine if these effects occur at low doses and without a threshold (ICRP, 2007).