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Environmental Backgrounder: Pesticides and Food Safety |
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Federal and state inspectors commonly sample food and feed produce for the purpose of tolerance enforcement soon after a farmer markets the treated commodity, so that any tolerance violations may be traced to their source. Thus, tolerances are intended to apply to treated raw agricultural commodities as soon as they enter into commerce, starting when the produce leaves "the farm gate."
Pesticide residues tend to dissipate or break down as time passes after harvest, and in the majority (but not all) cases, residues are further reduced by washing, peeling, cooking, and processing of foodstuffs. Almost by definition, tolerances represent levels of pesticide residue that are not expected to occur as actual residues in food commodities that reach the consumer.
In general, the data required for a tolerance are of two kinds -- residue chemistry data and toxicology study results. Such data are not generated by EPA laboratories; rather, EPA uses its data-collection authorities under the law to require the pesticide manufacturer to sponsor testing to produce these data.
All of EPA's tolerance data requirements are designed to answer three key questions. First, what is the chemical residue? Second, how much residue is there? The "what" and "how much" information, derived from residue chemistry data, is then used by EPA toxicologists to answer the third question Does the residue represent an acceptable dietary level of exposure? In other words, is there a reasonable assurance that under the prescribed conditions of use of the pesticide, no unreasonable adverse effect will result in humans even after a lifetime of exposure?
EPA evaluates this product chemistry data set to determine whether impurities could constitute a significant component of the residue in food or animal feed. This is an important consideration because impurities created in the manufacture of a pesticide may become a residue problem, if they are not identified before tolerances are established.
Plant metabolism studies are required for a minimum of three diverse crops, usually a root crop, an oilseed, and a leafy vegetable. If the metabolism in each of these crops is similar, then the metabolism in other crops is assumed to be similar. At the end of this process, EPA has enough information to answer the question, what is the residue in plants? Whenever a proposed use of a pesticide may result in residues in animal feed, or when a pesticide is intended for treatment of livestock, animal metabolism studies are required in addition to plant metabolism data. Animal metabolism studies are generally carried out on ruminants (cows or goats) and poultry (chickens).
Like plant metabolism tests, animal metabolism studies use radiolabelled pesticides. The animals are dosed, and the level of radioactivity resulting in potential meat or poultry products (muscle, liver, kidney, milk, and eggs) is analyzed. If significant activity is found, then the chemical identity associated with the activity is determined. This process answers the question, what is the residue in animals?
The toxicological effects of concern here are not the severe and immediate poisoning symptoms that could result from accidental massive ingestion of a pesticide, or skin and eye irritation characteristics. Rather, these long-term feeding tests are designed to reveal potential adverse effects which may result from continuous low-level ingestion of a pesticide -- such as bone marrow damage, cancer, blood effects, and other chronic effects. Special test procedures also determine the potential of the chemical to cause birth defects, adverse developmental effects, reproductive effects, neurotoxicity, and gene damage. In addition, the multi-generation reproduction study looks at the reproductive effects on animals exposed to pesticides in the womb and during nursing.
In general, the ADI or reference dose can be defined as an estimate of a daily exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of adverse effects.
However, as a routine practice, before making tolerance decisions on a pesticide, EPA uses the TAS to calculate TMRC and risk estimates for the two most sensitive subgroups identified by the system as well as the population as an average. In some cases, peak exposures to certain subgroups, such as infants and children, may exceed the ADI by some percentage even though this is not the case for the composite average of the population. Since the ADI is not a precise indicator of risk, exceeding the ADI by a small factor is not necessarily a cause for concern. However, if the TAS analysis indicates that exposure and thus risk to such a subgroup is so high that adverse effects are likely to occur, the Agency will not approve a tolerance even if the risks to the average population are acceptable.
Examples of cases where dietary concerns for infants and children in particular have been the overriding consideration include:
> A decision in 1985 not to approve tolerances for the pesticide pyrdrin for proposed new uses on alfalfa and sorghum, due to risks to children from secondary residues in milk.
> An action level for the occurrence of heptachlor epoxide in milk from cows fed pineapple greens that had been treated with heptachlor. Based on short- term liver toxicity in non-nursing infants and small children, EPA issued a lower action level than would have been required to protect adults from adverse effects.
> EPA's recent announcement to expedite a notice of cancellation for all uses of Alar (daminozide), meanwhile extending a temporary tolerance for Alar residues in apples and urging voluntary restraint on its use. In the case of Alar, EPA is particularly concerned about the upper-limit cancer risk for Alar exposure to infants and toddlers for the first year and one-half of childhood.
It is important to note that the TMRC is a very rough-hewn tool of the tolerance process. Taken out of context, and applied literally, the TMRC can be used to make calculations that misrepresent risks from pesticides as artificially high. It is necessary to go beyond the TMRC approach, using real data on pesticide usage rates and-anticipated residue levels, in order to evaluate actual dietary exposure and risk to consumers from pesticide residues in the food supply. Certain standard TMRC assumptions tend to greatly exaggerate the dietary risks attributable to pesticides; for example: (l) that 100% of each crop that may legally be treated with a pesticide is in fact treated with the pesticide, and (2) that pesticide residues in each commodity that may be treated with the pesticide are always present at tolerance levels at the time of consumption.
Thus, for example, since 92 pesticides are registered for use on tomatoes, the TMRC calculations would lead one to assume that all tomatoes contain tolerance level residues of all 92 pesticides. This is never the case in reality. To illustrate, Chart 1[*] gives an overview of the pesticides registered for use on tomatoes, by type: 54 insecticides, 20 fungicides, 7 herbicides, 4 soil fumigants, and 3 growth regulators; plus 4 additional miscellaneous pesticides (not shown). By comparison, Chart 2 shows recent pesticide usage data on tomatoes grown for processing in the State of California, which produces 86% of tomatoes grown for processing in the U.S. California processing tomatoes grown in 1986 were actually treated with very few of these 92 pesticides. Only 3% of California farms reported use of as many as 3 fungicides; 4% used as many as 4 herbicides; 4% used as many as 5 insecticides, 2% used 2 soil fumigants, and none used more than 1 growth regulator.
[*] NOTE: Charts and figures are not included in this version. They are available from the Chemicals-Pesticides Program upon request.
To illustrate the difference between a tolerance limit and actual residues present in consumer products, Chart 3 shows recent data on actual residues of the pesticide chlorothalonil on tomatoes. The established tolerance is 5 parts per million (ppm), indicated by the leftmost vertical bar. All post-harvest "farm gate" residues in tomatoes in these studies were well below 5 ppm. Moreover, there was little or no detectable residue of chlorothalonil in tomatoes by the time the commodity reached the consumer. In fact, 95% of residues were removed during washing at the packing house.
There is an important exception to the reference dose approach described above. EPA has not used the reference dose concept, which implies a threshold level of risk, in considering tolerances for pesticides that induce cancer in test animals.
Instead, the Agency takes a conservative approach, based on widely used quantitative risk assessment models, which projects upper-bound (worst-case) estimates of additional risk above the background cancer risk in the general population of 1 in 4 or 0.25 (2.5 x 10 to the minus 1). Basically, the approach involves determining a quantitative estimate of a pesticide's oncogenic potency, called a "Q star," and comparing the Q star to dietary exposure estimates. Dietary exposure estimates are based on the tolerance level of residue unless verifiable data on actual residues of the pesticide in agricultural commodities and consumer products are available.
In regulating pesticides that induce cancer in laboratory animals, EPA applies the "negligible risk" concept recommended by the National Academy of Sciences for making registration and tolerance decisions under FIFRA and FFDCA.
In sum, the tolerance process is highly protective in that it is based on the most sensitive animal tests results available and a combination of highly conservative assumptions and risk assessment practices. Tolerances are set at the lowest level necessary to accommodate the maximum application rate and frequency being proposed, even when higher levels would be safe for human consumption.
> As mentioned above, in animal studies used for human risk assessment purposes, chemicals are administered to test animals beginning with young animals (post-weanling) and continuing through adulthood (mimicking human exposure that begins in childhood and continues over a lifetime). The body dose received by the young animals may be double that of the adult animals, due to changes in their consumption patterns; however, the lower (adult) body dose is typically used in reference dose (ADI) estimates. This results in a lower (and more protective) reference dose than would otherwise be the case.
> In setting ADIs or reference doses, EPA generally uses a lO-fold safety factor to compensate for the uncertainty inherent in the process of extrapolating human dietary risk projections from animal data and, in addition, another 10-fold factor to compensate for the possibility of differing sensitivities in individuals or subgroups -- such as children -- among the general population.
> In cases where carcinogenicity is the potential effect of concern, EPA considers the upper limits of risk that may result from a lifetime exposure. Where these upper limit risks are not unreasonable from lifetime exposure, EPA concludes that the risk from a portion of lifetime exposure (e.g., between ages 1 and 12 or between 1 and 21) is likewise not unreasonable -- regardless of changes in eating habits expected to occur between infancy and adulthood. This practice is consistent with EPA's published cancer risk assessment guidelines and with current, mainstream thinking in the scientific community. EPA does not at this time have data to support specific modifications to this approach with respect to infants and children. However, the NAS is currently examining issues surrounding pesticides in the diets of infants and children. When the NAS concludes its report, EPA will be considering whether its present approach should be adjusted in light of the Academy's findings.
> Data on the comparative toxicity of pesticides in adult versus young weanling mammals are very limited and pertain to acute toxic effects only. These studies show mixed results. In studies of 37 chemicals administered to adult and weanling rats, weanlings demonstrated greater sensitivity in 8 cases. Adults showed greater sensitivity than weanlings in 23 cases. In 6 cases the sensitivity of adult and weanling rats was roughly the same.
> When estimating dietary exposure to infants and children, EPA's Tolerance Assessment System uses milligram-per-kilogram body weight (rather than body surface) comparisons with dietary intake levels for risk assessment purposes. As the FIFRA Scientific Advisory Panel has pointed out, this practice is likely to overstate dietary risk to infants.
2/23/89
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