Several definitions have been given for human exposure assessment (HEA). And to many people, there is a distinction between the terms exposure measurement and exposure assessment. The former typically is limited to the quantification of exposure, whereas the latter extends to considering the implication or impact of the exposure level that has been measured.

As noted in the U.S. EPA guidelines (Federal Register, 1992), exposure assessment in various forms dates back at least to the early 20th century, particularly in the field of epidemiology and those dealing with occupational exposure. The federal agency further defines exposure as the (physical) contact between a chemical and a human's outer boundary (primarily the skin, the gastrointestinal tract, and the lung). The applied dose, on the other hand, is the amount of a chemical present at one or more of these outer boundaries which typically serve as absorption barriers. Another closely-related term is the internal dose from exposure, which is the amount of a substance that has been absorbed through these barriers, and is available for biological interaction with receptors located in any of the various tissue compartments.

HEA is supposed to be as important to some toxicologists as to many epidemiologists. It is a well-known toxicologic concept that the magnitude, the frequency, and the route of dosing all have a significant impact on the nature, the severity, and the potential that an adverse effect is induced. Often times, repeated exposures at lower dosage may lead to the same or other (local or systemic) effects induced by a single acute exposure at higher dosage.