Chronic exposure is continuous or intermittent exposure to low levels of radiation over a long period of time.
Acute exposure is exposure to a large, single dose of radiation, or a series of doses, for a short period of time. Large acute doses can result from accidental or emergency exposures or from special medical procedures (radiation therapy).
For most biological effects the relationship between dose and effect is not a linear one. For example, if arsenic were administered to a group of subjects, at low doses there would be no response, but as the dose increased the effect would be proportional to the dose. This is called a non-linear dose response as shown in the Dose Response figure.
Radiation protection adopts a linear non-threshold model to express response to radiation as shown in the Dose Response figure. Increasing effects were observed at high doses of radiation, but effects at low doses were statistically insignificant. It was felt that there must be some risks from radiation at low doses so the effects from high doses were extrapolated back to zero dose and the linear non-threshold dose response was developed for radiation protection.
Dose Response Curve
Many biological effects are categorized as non-stochastic. The magnitude of the effect increases with the size of the dose, and there is a clear relationship between exposure and the observed effect
Some biological effects occur in the unexposed as well as the exposed population and are categorized as stochastic effects. This type of effects are not unequivocally linked to the exposure. For example, lung cancer is a disease that both smokers and non-smokers get, but the more cigarettes a person smokes the higher the likelihood of lung cancer. However even heavy smokers are not guaranteed to get lung cancer.