Effects of Exposure

For the purposes of radiological protection, harmful effects of radiation exposure are grouped into two categories:

Deterministic Effects Effects, such as skin burns, that only appear at relatively high doses. Deterministic effects are also referred to as harmful tissue reactions. They include, for example, skin burns and damage to the lens of the eye. These effects do not appear below a dose threshold. Above this threshold, the higher the dose the more severe the effect. No deterministic effects would be expected below an absorbed dose of 100 mGy (above the natural background exposure), and thresholds for most effects are much higher. Because of this, deterministic effects are rare, although they can occur as a result of sophisticated medical procedures, or accidents. In extremely rare cases, such as in severe accidents, very high doses received in a very short time can lead to acute radiation syndrome and even death.

Stochastic Effects Effects, such as cancer, that are assumed to pose some risk even at low doses. Stochastic effects include cancer and heritable effects. There is reliable scientific evidence that doses above 100 mSv can increase the risk of cancer. Below this dose the evidence is less clear, but for purposes of radiological protection it is assumed that even small doses might result in small increased risk. An extra effective dose of 200 mSv (above the natural background exposure) increases the risk of fatal cancer from the typical worldwide average of about 25% to about 26%. Although heritable (genetic) effects have been seen in animals, none have ever been seen in humans. Even so, for protection purposes, a small risk of heritable effects is assumed.

Quotes from ICRP Publications

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Types of effects: ICRP Publication 103 paragraph 55 Most adverse health effects of radiation exposure may be grouped in two general categories: deterministic effects (harmful tissue reactions) due in large part to the killing/ malfunction of cells following high doses; and stochastic effects, i.e., cancer and heritable effects involving either cancer development in exposed individuals owing to mutation of somatic cells or heritable disease in their offspring owing to mutation of reproductive (germ) cells. ICRP Publication 103 paragraph 58 The induction of tissue reactions is generally characterised by a threshold dose. The reason for the presence of this threshold dose is that radiation damage (serious malfunction or death) of a critical population of cells in a given tissue needs to be sustained before injury is expressed in a clinically relevant form. Above the threshold dose the severity of the injury, including impairment of the capacity for tissue recovery, increases with dose. ICRP Publication 118 paragraph 10 The manifestations of tissue injury vary from one tissue to another depending on cellular composition, proliferation rate, and mechanisms of response to radiation, which may be highly tissue specific. Examples ... include cataracts of the lens of the eye, non-malignant damage to the skin, cell depletion in the bone marrow causing haematological deficiencies, and gonadal cell damage leading to impairment of fertility. Tissue reactions, especially late reactions, also depend on damage to blood vessels or elements of the extracellular matrix, which are common to most organs of the body.

ICRP Publication 103 paragraph 62 In the case of cancer, epidemiological and experimental studies provide evidence of radiation risk albeit with uncertainties at doses about 100 mSv or less. In the case of heritable diseases, even though there is no direct evidence of radiation risks to humans, experimental observations argue convincingly that such risks for future generations should be included in the system of protection. The 'LNT' model: ICRP Publication 103 paragraphs 65 and 66 ... the practical system of radiological protection ... will continue to be based upon the assumption that at doses below about 100 mSv a given increment in dose will produce a directly proportionate increment in the probability of incurring cancer or heritable effects attributable to radiation. This dose-response model is generally known as ‘linear-non-threshold’ or LNT. ... the adoption of the LNT model ... provides a prudent basis for the practical purposes of radiological protection, i.e., the management of risks from low-dose radiation exposure. ... whilst the LNT model remains a scientifically plausible element in its practical system of radiological protection, biological/epidemiological information that would unambiguously verify the hypothesis that underpins the model is unlikely to be forthcoming ... Because of this uncertainty on health effects at low doses, ... it is not appropriate, for the purposes of public health planning, to calculate the hypothetical number of cases of cancer or heritable disease that might be associated with very small radiation doses received by large numbers of people over very long periods of time ... Genetic effects: ICRP Publication 103 paragraph 74 There continues to be no direct evidence that exposure of parents to radiation leads to excess heritable disease in offspring. However, ... there is compelling evidence that radiation causes heritable effects in experimental animals. Therefore, the Commission prudently continues to include the risk of heritable effects in its system of radiological protection.