What are the effects of ionising radiation?
The process of ionisation necessarily changes atoms and molecules, at least temporarily, and thus may damages cells. If cellular damage does occur and is not adequately repaired, it may prevent the cell from surviving or reproducing, or it may result in a viable, but modified, cell. The two outcomes have profoundly different implications for the organism as a whole. Most organs and tissues of the body are unaffected by the loss of even a substantial amount of cells, but if the number lost is large enough, there will be observable harm to the individual, reflecting a loss of organ or tissue function. The likelihood of causing such harm will be zero at low doses but, above some level or threshold dose, the damage will occur almost with certainty. Above the threshold, the severity of the harm increases with increasing dose. This type of outcome, which includes acute radiation syndrome, is called deterministic, because the harm is almost bound to occur in exposed individuals if the dose exceeds the threshold dose. The adverse effects first observed in the early use of radiation were deterministic effects. Threshold doses are substantially higher than the doses to workers and members of the public expected from practices and sources in normal operation. Only an accident involving a source capable of delivering high doses is likely to cause deterministic effects.
The situation is very different if the irradiated cell is modified rather than killed. Despite the existence of highly effective defence mechanisms, the cloning of cells resulting from the reproduction of a modified, but viable, cell may result, after a prolonged and variable delay, called the latency period, in the manifestation of a malignant condition (e.g., cancer). The probability of a cancer resulting from radiation increases with increasing dose. This probability is assumed for protection purposes to be without a threshold and to be proportional to dose for doses below the thresholds for deterministic effects. Since only the probability, but not the severity, of the cancer is affected by the amount of dose, the outcome is called stochastic, meaning of a random or aleatory nature.
If the radiation damage occurs in a cell whose function is to transmit genetic information to later generations, it is presumed that some harm, which may be of many different kinds and severity, might be expressed in the progeny of the exposed person. This type of stochastic outcome is known as hereditary. The probability of hereditary harm also is taken to be proportional to the dose received. In addition, irradiation in utero can lead to effects in children, principally an increase in the stochastic risk of childhood leukaemia and a reduction in IQ (intelligence quotient) following irradiation, mainly during the eighth to fifteenth weeks of gestation.
Stochastic effects of radiation are only detectable in epidemiology studies having sufficient statistical power, and usually require large populations and years of follow-up to cover the latency period of the exposed individuals studied. The estimated risks of a radiation dose resulting in a stochastic outcome are derived from a number of epidemiology studies, the most important being the study of survivors of Hiroshima and Nagasaki atomic explosions. Radiation protection standards based on stochastic risk estimates employ assumptions about risk which are seen to be conservative in line with the degree of scientific Knowledge about the risk.
Based upon the wealth of scientific knowledge to date and employing conservative assumptions, the likelihood of stochastic outcome due to normal levels of radiation exposure is estimated to be very small. For average exposure to natural background radiation, the chance is of the order of 1 in 10,000 per year. For average exposures in the population from many current practices, it is very much lower than it is for background radiation.