Ionising radiations can originate from equipment such as x-ray machines and CT scanners, from outer space and from the ground beneath our feet. This radiation passes through tissues to an extent that depends on its type (alpha, beta, gamma or neutron) and its energy. In addition, radioactive substances present in the environment can be taken into the body by ingestion and inhalation, be retained and give rise to internal exposure of various organs and tissues.
Adverse effects on health can arise from both external and internal exposure. The degree of exposure is measured by the radiation dose, which is the amount of energy absorbed in a unit amount of tissue (Joules per kilogram). At high doses and dose rates, deterministic effects occur. These have a severity that increases with dose, which implies that there is a threshold dose below which they are not clinically detectable and have no adverse impacts on health. Effects of this type include radiation-induced skin burns and suppression of the functioning of the red bone marrow. Such deterministic effects are not generally of concern in normal workplace conditions or in environmental exposures of members of the public. Rather, attention is focused on the potential for induction of cancer or hereditary disease. The severity of such effects does not vary with dose, but the probability that they will be induced increases as the dose received increases. Although various shapes have been proposed for the relationship between dose and probability of effect, a linear relationship without threshold is considered to be cautiously realistic and is adopted for regulatory purposes. However, the UNSCEAR has recently advised against applying the linear relationship without threshold to estimate numbers of radiation-induced health effects within a population exposed to incremental doses at levels equivalent to or lower than natural background. No excess of genetic defects has ever been observed in the offspring of any irradiated human population.