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Exposure assessment is a key component of health risk assessment (RA), in that there shall be no health risk or hazard in the absence of exposure. The primary objectives of this central component are to determine the source, magnitude, and duration of contact with the agent at issue. Exposure to a toxic agent typically can occur through three major routes of entry: inhalation, dermal, and oral. Exposure assessment is also frequently considered as the key area for uncertainties in the overall RA process. This is because exposure scenario is usually very dynamic, depending on several factors that cannot be easily quantified or ascertained, such as the exposure rate and duration, the contact activity, and the exact availability or location of the agent.  Assessing human exposure can be accomplished by three basic techniques: (1) measurement of environmental concentrations with which to mathematically extrapolate the ingestion, the inhalation intake, and the dermal uptake of the toxic material by a person; (2) direct measurement of the toxic agent on or in a person, such as through biological monitoring with body fluids; and (3) physiological or statistical modeling to yield a more realistic dose estimate. Pharmacokinetic data even from animal studies are essential to biomonitoring, to the uptake and intake mathematical models, and to physiologically-based pharmacokinetic (PB-PK) modeling. Otherwise, it will be unclear how much of the (absorbed) parent compound will remain unchanged and present in the human blood or urine. Epidemiology is currently about the closest discipline in which human exposure assessment is ever taught (relatively) extensively.