Contact of the cells of an organism with a substance, micro-organism or radiation. In the case of humans, this may involve contact with a substance or agent by swallowing, breathing, or through the skin or eyes. Exposure may be short-term [acute exposure], of intermediate duration, or long-term [chronic exposure].

Exposure can be divided into external and internal.

External exposure refers to the whole dose to which an organism is exposed.

Internal exposure refers only to that fraction of the initial chemical dose that is absorbed and distributed throughout the body via systemic circulation.

Radioactivity exposure is the level of radiation flux to which a material or living tissue is exposed.

In that context, external radiation refers to exposure to radiation outside the body, whereas internal exposure refers to exposure to radioactive material taken into the body.