What do you mean by "exposure"?

Human exposure to an environmental air pollutant is defined as contact between molecules of the pollutant and a given biological boundary, such as the breathing zone or the human lung. For the purposes of this website, we consider exposure as the concentration of airborne particles from secondhand smoke in the breathing zone of a person. We DO NOT consider any uptake or dose of the chemical components of secondhand smoke. You can learn more about the science of exposure on various websites and in scientific articles.