Ionising radiation has two main types of effect:

1. Deterministic effects

These are effects that occur at high doses, typically 1000 mSv and above, and are observed in anyone who receives such a dose. They have three basic characteristics:

• They are threshold effects. This is fundamental for radiation protection purposes, since it confirms that a dose below this threshold has absolutely no impact on health.
• The severity of the effect depends on the dose. The higher the dose, the more rapid and acute the clinical effects.
• They are early effects. Except in special cases such as cataracts, the effects appear rapidly, in the hours or days following exposure.

In the case of whole body exposure to radiation, the severity of the symptoms (fatigue, nausea, vomiting) increases with the dose received. At levels exceeding 2000 mSv, there is a drop in lymphocyte, red blood cell and platelet counts, which explains the haemorrhaging and infections observed.

2. Stochastic effects

These are effects that occur randomly, irrespective of the dose received, to some of the individuals in a population exposed equally to the same dose of radiation. They mainly appear in the form of so-called “radiation-induced” cancers, which in fact are absolutely identical to the various forms of cancer observed throughout the human population when there has been no exposure to anything other than natural radiation. These stochastic effects are exactly the opposite of deterministic effects:

• They are effects that have no threshold. This assumption, which is applied on the basis of the precautionary principle, is again fundamental for radiation protection purposes, since it implies that any dose, however low it may be, leads to a slight increase in the probability of an individual developing cancer and therefore to a slight increase in the frequency of occurrence of cancers in a uniformly exposed population.
• The severity of the effect does not depend on the dose. The dose received does not determine the severity of the cancer for any given part of the body.
• They are late effects. The first cancers to appear are leukaemias, typically 5 years after exposure. “Solid” cancers can take as long as several decades to appear.

These stochastic effects have been demonstrated in humans through epidemiological studies at exposure levels of over 100 mSv. The benchmark study establishing the relation between dose and effect is still the one performed on the populations exposed at Hiroshima and Nagasaki. More than 86000 individuals were or are still being studied (for those who are still alive 66 years after the explosions). These studies have not identified any significant rise in the cancer rate among subgroups with an exposure level of less than 100 mSv. This does not mean there is no effect below this dose, but simply that if such effects exist, there are too few excess cases to be statistically significant. This difficulty, and it is not the only one, is inherent in any epidemiological study, regardless of the parameter being tested. The International Commission on Radiological Protection (ICRP), whose recommendations provide the basis for all radiation protection doctrine and the resulting regulations, has therefore decided to err on the side of caution and extrapolate these dose-effect relations to low doses as a straight-line relationship without a threshold. The risk coefficient for any individual in the population is currently estimated at 0.07 per Sv received. An individual exposed to 10 mSv will therefore see his/her risk of developing a cancer increase by 0.0007 (bearing in mind that the “natural baseline” is 0.25, since one person in four develops cancer during his/her life). Finally, it should be noted that this so-called no-threshold hypothesis is currently the subject of considerable research, particularly in biology.

Many scientists believe that the hypothesis lacks credibility in molecular biology terms, given the complexity of the mechanisms involved in the cancerisation of tissue.

See also the website of the IRSN (Institut de Radioprotection et de Sûreté Nucléaire).