Parathion studies

For the past 2 years the output of analyses has been increased greatly, particularly with penetration studies, by utilization of wet-processing techniques and development of several new pieces of apparatus. This equipment includes the revolving drum-type stripping machine and especially adapted drill press used also for parathion studies. A detailed description of this press has been published ( ), and the stripping machine and accessory equipment have been described (5). 

Standardized procedures were adopted with regard to sample preparation, recovery of toxicant, and chemical assay. In order to determine the nature and magnitude of penetrated residues, it was necessary to disassociate all extra-surface residues. The techniques originally developed to effect this separation and which were used in most of the DDT penetration studies have been described by Gunther 11). Certain modifications which have been developed subsequently in connection with the parathion studies are described in detail below since this phase of penetration studies assumes singular importance (see also 14). 

This method proved inadequate in the parathion studies because of trace contaminations and the following procedure of sample preparation was adopted ... 

Pome Fruit Types. As with citrus fruit types, the method of sample preparation was modified for the parathion studies. In the earlier studies the DDT-treated apples and pears were scrubbed in a warm 10% solution of trisodium phosphate, and all the peel was removed from the water-rinsed fruit with a household-type potato peeler. The pooled samples of peel and pulp were then processed independently to recover the contained toxicant for subsequent estimation. 

In the parathion studies and the more recent DDT studies, the fruits were split in half by cutting axially one third through the fruit with a broad-bladed knife and then twisting the blade to one side. The core and pulp, exclusive of the part touched by the knife blade, were carefully scraped out with sharpened melon bailers. 

Ongoing Studies of Methyl Parathion Found in Federal Research in Progress... 

Methyl parathion is approved only for use on crops. The maximum amount of methyl parathion residue allowed by the Food and Drug Administration (FDA) and EPA on crops used as food is 0.1-1 ppm. The FDA has monitored the food supply for pesticides for a number of years. FDA purchases many kinds of foods through Market Basket Surveys and analyzes them for residue levels of pesticides. These FDA studies allow scientists to estimate the daily intake of pesticides. Generally, the FDA monitoring studies conclude that the U.S. food supply contains only very small amounts of pesticides that are not a concern. However, there have been some reports of the illegal use of methyl parathion inside homes. For more information, see Section 1.7 and Chapter 6. 

Animal studies show effects of methyl parathion similar to those seen in people. In addition, short-term high exposure of animals to methyl parathion caused decreased heart rate. This may be the result of methyl parathion s effects on the nerves that control the heart. Methyl parathion decreased the ability of animals to fight infections in some studies, but not in others. It is not known whether any of these effects occur in people. It is not known whether methyl parathion affects the ability of animals to reproduce. Studies in animals have not shown that methyl parathion causes cancer. 

There is no evidence in humans that methyl parathion causes birth defects. Birth defects have not been seen when methyl parathion was given to animals by mouth, but minor birth defects did occur in one study in which high doses were injected into pregnant animals. It is not known whether these effects occur in people. It is unlikely that people would be exposed by breathing, touching, or eating as much methyl parathion as was injected in the animal studies. 

The greatest potential for exposure of the general population to methyl parathion is by consumption of food containing residues from spray applieations of the inseetieide. In a 10-year study, methyl parathion was found at an average concentration of 0.0035 ppm in a few examples of ready-to-eat foods. Concentrations in the range of 0.05-2.0 ppm were reported in 0.5% of the samples of domestie and... 

Hematological Effects. No information was found regarding hematological effects in humans following exposure to methyl parathion. Repeated oral exposure to methyl parathion resulted in decreased mean corpuseular volume in one study and decreased hematocrit and erythrocyte count in another study in rats. Chronic ingestion of methyl parathion induced reduction of mean hemoglobin, hematocrit, and erythrocyte eounts in rats. 

Developmental Effects. Adverse effects of methyl parathion on hirman fetal development have not been reported. Based on studies in animals, such effects appear to be possible if pregnant women were exposed during the first trimester to high concentrations of methyl parathion that resulted in significant depression of cholinesterase levels, particularly if concomitant signs and symptoms of organophosphate intoxication occur. Such an exposure scenario may occur with occupational exposure, exposure in homes or offices illegally sprayed with methyl parathion, or accidental exposure to methyl parathion, but is less likely as a result of low-level exposure. 

Although a number of studies have reported the effects of inhalation exposure to methyl parathion in humans, no inhalation MRLs were derived based on human data because of the lack of adequate quantitative exposure information. Animal data were also insufficient to support the derivation of an acute-, intermediate-, or chronic-duration inhalation MRL. 

No acute oral MRL was derived for methyl parathion because data regarding the most sensitive effect that was observed after acute oral exposure are conflicting. Increased pup mortality and altered behavior occurred in offspring of rats exposed to 1 mg/kg/day methyl parathion during, but no effects on pup survival or on sensitive electrophysiological indices of neurotoxicity were seen at virtually the same dose, 0.88 mg/kg/day, in a similar developmental toxicity study. 

An intermediate-duration oral MRL of 0.0007 mg/kg/day was derived for methyl parathion based on the observation of electrophysiological effects in the central and peripheral nervous systems of male rats exposed to methyl parathion through gavage administration of 0.22 mg/kg/day to the dams on days 5-15 of gestation and days 2-28 of lactation, followed by direct administration of the same dose to the male pups for 8 weeks. More marked effects occurred at the two higher doses, 0.44 and 0.88 mg/kg/day. The effects were dose-related, and were statistically significant at all three dose levels. The MRL was derived by dividing the LOAEL from this study (0.22 mg/kg/day) by an uncertainty factor of 300 (3 for a minimal LOAEL, 10 for extrapolation from animals to humans, and 10 for human variability). 

The primary purpose of this chapter is to provide public health officials, physicians, toxicologists, and other interested individuals and groups with an overall perspective on the toxicology of methyl parathion. It contains descriptions and evaluations of toxicological studies and epidemiological investigations and provides conclusions, where possible, on the relevance of toxicity and toxicokinetic data to public health. 

No studies were located regarding gastrointestinal, hematological, musculoskeletal, or dermal effects in humans or animals after inhalation exposure to methyl parathion. Dean et al. (1984) reported that seven children exposed to methyl parathion by many routes exhibited pinpoint pupils, abdominal pain, and diarrhea. The respiratory, cardiovascular, hepatic, and renal effects reported by Fazekas (1971) that were found in humans acutely exposed to methyl parathion intoxication resulted from exposure by all three routes however, the results did not distinguish between the routes. 

No studies were located regarding hepatic effects in animals atter inhalation exposure to methyl parathion. 

Hepatic Effects. Liver lesions have been reported in humans acutely intoxicated by methyl parathion formulation (Wolfatox) (Fazekas 1971 Fazekas and Rengei 1964). These studies are discussed in detail in Section 3.2.2.1. Liver lesions were hepatocellular swelling, degeneration, and fatty change. 

Male and female rats exposed to 2.5 mg/kg/day methyl parathion in the diet for 2 years had statistically significant reduced body weights when compared to vehicle controls (Suba 1984). This effect was not consistent throughout the study and did not occur in rats exposed to either 0.025 or 0.25 mg/kg/day methyl parathion. Mean food consumption values were significantly elevated in male rats but only within the first 13 weeks of the 2-year exposure to 2.5 mg/kg/day methyl parathion (Suba 1984). Females exposed to 2.5 mg/kg/day methyl parathion had significantly reduced food intake values during the first 2 weeks of exposure, but intake was significantly elevated from week 3 to termination. Effects on food... 

Neurologic signs did not occur over a 30-day period in male prisoner volunteers in California who ingested daily doses of methyl parathion ranging from 1.0 to 19 mg. There were no uniform changes in plasma or erythrocyte cholinesterase levels at any of these doses (Rider et al. 1969). By increasing concentrations of methyl parathion administered to the same experimental population and using the same protocol, a dose that inhibited cholinesterase values was established. These additional studies were published nearly 20 years ago in abstract form only therefore, they are not discussed in this section. 

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