Human exposure to environmental contaminants can be via many routes of entry, with some paths apparently being more common and more important than others. In this lecture, students will be given the basic algorithms for calculating the indirect exposure estimates for the more common pathways. They will also learn that there are other critical variables that need to be considered when taking the indirect measurement methods (IMM) approach.

The dynamics and complexity of the IMM are demonstrated here again with the four hypothetical exposure cases presented in the last lecture. Because many environmental contaminants are omnipresent, and exposure to them can take on many forms or avenues (e.g., various routes of entry), the calculations involved in the IMM are necessarily numerous and complex. Many exposure assessment models thus have been developed to facilitate the required calculations. One of the objectives of this lecture is to introduce the students to some of these assessment models.

In addition to the computer models, regulatory agencies have drafted and revised guidance documents to assist in harmonizing the various IMM adopted by the human exposure assessment community. As to be discussed later, the basic algorithm for calculating an indirect exposure estimate, regardless of the pathway involved, is: [human exposure] = [environmental concentration] x [human contact]. However, the way in which the two independent variables (i.e., environmental concentration and human contact) are defined and quantified varies, in some cases greatly, with the pathway involved.