Understanding the nature and extent of exposure is an important part of understanding the risks posed to human health from exposure to environmental contaminants. EPA defines exposure as 'contact between an agent and the visible exterior of a person (e.g. skin and openings into the body)'. Exposure assessment is the process of measuring or estimating the magnitude, frequency, and duration of human exposure to an environmental agent, or estimating future exposures for an agent that has not yet been released. An exposure assessment includes information about the size, nature, and types of human populations exposed to the agent, as well as discussion of the uncertainties in information. Exposure can be measured directly, but more commonly it is estimated indirectly by measuring concentrations in the environment, considering how chemicals are transported in the environment, and estimating human intake over time.