are promulgated by the Occupational Safety and Health Administration, and threshold limit values proposed by the American Council of Governmental Industrial Hygienists for individual pesticides are enforced.
However, there have been excessive exposures and adverse effects associated with manufacturing processes. The best known of these are related to the production and synthesis of 2,4,5-trichlorophenol, the precursor of a number of chlorinated herbicides. The synthesis of 2,4,5-trichlorophenol was associated with the unavoidable trace byproduct 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The major effects noted were chloracne and occasional liver damage (Firestone 1977, Pocchiari, F. et al. 1979, Coulston and Pocchiari 1983, Kimbrough et al. 1984) and porphyria (Bleiberg et al. 1964). Furthermore, TCDD has been shown to be carcinogenic in laboratory bioassays of rats and mice, and there is epidemiological evidence of its human carcinogenicity (Gallo et al. 1991). The herbicides that were based on 2,4,5-trichlorophenol have been removed from the market, because of the difficulties encountered in removing TCDD and related dioxin impurities.
Most of the chlorinated-hydrocarbon insecticides—such as DDT, dieldrin, aldrin, toxaphene, and chlordane, as well as TCDD—have been found to be carcinogenic in rodent bioassays. That experience has generally not been duplicated in humans. However, human exposures tend to be much lower, even under occupational conditions, than those attained during bioassays (Leber and Benya 1994). Many of the chlorinated-hydrocarbon pesticides are very fat-soluble and exhibit a tendency to bioaccumulate. For these reasons, most of the chlorinated hydrocarbon pesticides are no longer on the US market.
The Secretary's Commission on Pesticides (HEW 1969) called attention to a number of cases of poisoning due to improper storage or spillage during transportation involving the insecticides endrin, dieldrin, diazinon, mevinphos, and parathion. Another prominent case involved a shipment of dyed seed grain to Iraq; it had been treated with methyl mercury as a fungicide, was washed to remove the dye, and was used for food. The instructions on the seed bags were only in English; as a consequence there were 6,530 cases of poisoning, with 459 deaths. (Bakir et al. 1973).
In normal use, the potential for the highest exposures occurs during the handling of the concentrated pesticides in preparation for application to crops. The application process is the source of many exposure scenarios. The highest exposures are likely to occur during ground applications, especially for spot treatments when the plants being treated are