The National Research Council described human exposure to a contaminant as an event consisting of contact with a specific contaminant concentration at a boundary between the human and the environment for a specific interval. Total exposure is determined by multiplying the concentration by the exposure time. The exposure assessment is intended primarily to estimate a dose which is combined with dose-response data to estimate risk. However, exposure assessments can support an array of decisions ranging from priority setting to regulatory control. The end use of the exposure assessment dictates the quality and quantity of information used. Indoor concentrations are measured or estimated. The choice of method is dictated by such factors as the pollutant, the sources, the breadth of the area or population under consideration, the use of the information, and the cost. Direct measurements can be taken by using personal monitors and by determining the presence of biological markers in the exposed population.