In this context exposure is defined as the contact between an agent and a target. Contact takes place at an exposure surface over an exposure period.
Mathematically, exposure is defined as <br>
where  is exposure,  is a concentration that varies with time between the beginning and end of exposure. It has dimensions of mass times time divided by volume. This quantity is related to the potential dose of contaminant by multiplying it by the relevant contact rate, such as breathing rate, food intake rate etc. The contact rate itself may be a function of time.