Health effects attributable to radiation exposure
There are two types of health effects that can be attributable to the radiation exposure due to the Chernobyl accident.
The first are early syndromes that can be clinically observed in the exposed individuals - i.e. they can be diagnosed by a specialized practitioner who can unequivocally attribute the type and severity of the effect to the amount of the individual's radiation exposure. They only occur at relatively high radiation doses, above a threshold dose, and present a distinctive pathology affecting specific organs and tissues. With large doses they affect the whole body and are diagnosed as acute radiation syndrome (ARS). At Chernobyl, these effects were suffered by a number of fire fighters and other emergency workers only.
The second are potential long-term radiation induced malignancies and - plausibly - hereditary effects, which are difficult or sometimes impossible to discern from the usually high normal incidence of these types of effects in the population. These long-term effects cannot be directly attributed to radiation from the results of individual clinical examinations but only indirectly through long epidemiological studies in large population groups. They became evident as an increase in the statistical incidence of the effects in the population. However, if the radiation dose is very small or the number of people affected is small, the effects become undetectable against the normal incidence. At Chernobyl, such effects have become evident only as an increase in the incidence of thyroid malignances in children.