Tolerance Setting

When the use of a pesticide may have the potential to leave a residue on a food in the United States, a tolerance, representing the maximum allowable residue permitted, is usually established. Tolerances represent the maximum expected residues of a pesticide on a specified commodity resulting from legal applications of the pesticide under established conditions for its use. When such conditions are followed, it is highly unlikely that residues in excess of tolerances would be detected. Tolerances are established for specific pesticide/commodity combinations; as such, the same pesticide may have different tolerance levels established for different commodities, and the tolerances of a variety of pesticides on a single commodity frequently vary considerably.

It is important to emphasize that the tolerance level is not established based on safety, but rather represents the maximum residue anticipated from the legal use of the pesticide (10). Nevertheless, before granting a tolerance, the EPA makes assessments of potential human exposure resulting from all registered (and proposed) uses of the pesticide to calculate the theoretical maximum residue contribution (TMRC). The TMRC assumes that the specified pesticide is always applied to all acreage planted, that it is used on all commodities for which it is registered, that residues are always present at the tolerance level, and that there is no reduction on pesticide levels from the plant to the postharvest stage, up to the table. It is a typical worst-case scenario using highly conservative assumptions that may overestimate exposures by factors of 100 to 100,000 times (2).

The TMRC value is compared with the reference dose (RfD), which is a daily exposure level not considered to represent any appreciable level of risk. A pesticide is considered to pose a negligible risk and tolerances are usually approved when the TMRC is below the RfD, provided that the carcinogenic risk at the TMRC is below the level of one excess cancer per million. When the TMRC is found to be higher than the RfD, or when exposure at the TMRC leads to a cancer risk greater than one excess cancer per million, the EPA may adopt a more refined risk assessment to more accurately calculate exposure estimates. Such refinements commonly represent the anticipated residue contribution (ARC) and may include adjustments of actual pesticide use, more realistic residue data, and consideration of potential pesticide levels reduction through washing, peeling, cooking, processing, and so on. The tolerance is established if the ARC is below the reference dose, and the carcinogenic risk at such an exposure is below the negligible risk of one excess cancer per million.

The passage of the FQPA in 1996 has served to make the process for establishing tolerances for pesticides more complicated. Prior to FQPA, tolerances were established on a chemical-by-chemical basis and considered only dietary exposure to the chemical. FQPA stipulates that the EPA may establish tolerances only when the EPA assesses that the risks posed by pesticides represent a "reasonable certainty of no harm" with respect to both carcinogenic and noncarcinogenic risks. In determining whether the pesticides satisfy the reasonable certainty of no harm criteria, the EPA considers the aggregate exposure to the chemicals from dietary, drinking water, and residential sources as well as cumulative exposure from pesticides possessing a common mechanism of toxic action (such as the organo-phosphates), meaning that determinations may be made on entire families of chemicals rather than on a chemical-by-chemical basis. In addition, the EPA is required to consider applying an additional 10-fold uncertainty factor in cases where infants and children may be more susceptible than adults to specific pesticides; this effectively reduces the RfD by a factor of 10. It is clear that the new FQPA requirements will require EPA scientists and others to develop significantly more robust models for assessing human pesticide risks and that the more stringent re-registration requirements posed by FQPA may significantly reduce the amounts and types of pesticides that may be used on food crops in the near future.

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