As well
reflected in this lecture, it is not always practical to perform human biological
monitoring studies for some chemicals when their pharmacokinetics are not fairly well
understood. Incomplete exposure history will also be a stumbling block in interpreting
human biomonitoring data.When the internal dose cannot be measured or interpreted,
exposure assessors often retrieve to using various indirect measurement methods (IMM).
These IMM each typically consist of two critical components: (1) the concentration of the
chemical in a particular environmental medium; and (2) the magnitude and the frequency of
human contact with the environmental contaminant. The product of these two components will
yield an estimate of the applied dose received. The magnitude and the extent of contact per
se consist of several variables: transferability of residues from the media to the
skin; inhalation uptake; number of exposure days, frequency of hand-to-mouth movements,
etc.
From the applied dose, an absorbed dose can be calculated using an estimated value for
the absorption of the chemical through the human's outer boundary (the skin, the lung,
etc.). Theoretically the same individual can receive the applied doses from any product
combination of the various available environmental media by the various available exposure
pathways. There also have been assessment models designed to account for aggregate (from
multiple pathways) or cumulative (from multiple chemicals having a common mechanism of
toxicity) exposure. These exposure assessment models and the various IMM will be the focus
of discussion in the next lecture.
