With passage of the Food Quality Protection Act (FQPA), the RfD has been analogized to the brim of a "risk" cup. Potential exposures that are added to the cup were formerly only from food but now include water and residential use. When the cup is full, or in other words, when the RfD is reached for all possible exposures, no more exposures will be allowed.
EPA has estimated that food contributes 80% of all exposures to pesticide residues over a lifetime, and water and residential use each contribute 10%. When a new pesticide is first proposed for registration, and extensive residue data are not yet available, the EPA will assume that the residues will occur at the level of the registrant's proposed tolerance. Furthermore, EPA will assume that all the crops to be registered have the residues. This estimation process is known as the Theoretical Maximum Residue Contribution (TMRC). Thus, for compounds of comparatively high toxicity and correspondingly low RfD, high tolerances will tend to fill the risk cup very rapidly when subjected to the TMRC. More space can be made in the risk cup by lowering proposed tolerances.
*Excerpted from “Will Fiddling with Pesticide Tolerances Fine-Tune Dietary Exposure?” by Dr. Allan S. Felsot, Environmental Toxicologist, WSU, AENews Issue No. 146, June 1998. http://aenews.wsu.edu/June98AENews/TolerancesandDiet.html