Parathion continued

Table 3-3. Levels of Significant Exposure to Methyl Parathion - Oral (continued)... 

In the male offspring whose treatment was continued through 11-12 weeks of age, however, dose-related effects were seen on all the above end points, and these effects were significantly different from controls at all three dose levels (Desi et al. 1998) (see also Section S.2.2.4). The study did not determine the critical period (if any) and duration of exposure for these neurological effects. A limitation of this study is that results specifically for methyl parathion were shown only for the somatosensory electrocortico-gram the other results for this chemical were stated in the text, but not shown. 

Table 5-2. Methyl Parathion Use in Crop Production in 1992 (continued)...

Table 7-2. Analytical Methods for Determining Methyl Parathion in Environmental Samples (continued)... 

Rider JA, Moeller HC, Puletti EJ. 1966. Continuing studies on anticholinesterase effect of methyl parathion in humans and determination of level of incipient toxicity of OMPA [Abstract]. Fed Proc 25 687. 

Because of continued interest in the in situ degradation products of both DDT and parathion, preliminary spectrophotometric evidences, which are also indicative of structural alterations and/or changes in composition, have been secured with extracts of navel oranges previously treated with standard formulations of these compounds. 

The hydration status of the clay or earth material may affect the adsorption capacity of nonpolar (or slightly polar) toxic chemicals. Continuing with parathion as a case study, Fig. 8.33 shows the increase adsorbed parathion on attapulgite from a hexane solution, as the adsorbed water on the clay surface decreases. This behavior may be explained by the competition for adsorption sites between the polar water and the slightly polar parathion. Possibly, however, the reduction in adsorption due to the presence of water is caused by the increased time required for parathion molecules to diffuse through the water film to the adsorption sites. 

Fig. 16.36 Change in bacteria populations in remoistened Gilat soil after application of 10-160 pg parathion per g dry soil. Plotted points are means of three replicates standard error. Continuous curves represent model simulations. Values obtained in control soils to which hexane alone was added have been subtracted. Reprinted from Nelson LM, Yaron B, Nye PH (1982) Biologically-induced hydrolysis of parathion in soil. Soil Biol Biochem 14 223-227. Copyright 1982 with permission of Elsevier...

There seems to be no limit to the number of toxic organophosphorus compounds that can be synthesized and that exhibit insecticidal activity. While many compounds of this type have been marketed, only about 20 of them comprise the bulk of the total tonnage. Parathion, methyl para-thion, and malathion are perhaps the best known and most widely used of this class of insecticides. The relatively low cost of the first two, combined with their good performance against a broad spectrum of insects, probably accounts for their continuing popularity. The low order of toxicity of malathion to mammals has made it acceptable under many conditions where other, perhaps more insecticidally active, insecticides are restricted. 

The stratum comeum, composed of keratinized dead cells that are continually being replaced, is the first major barrier to chemical agents. The barrier qualities of the stratum comeum depend on a number of factors, including its location on the body, which affects thickness, and how much hair is present. Thus, hair follicles and sweat glands can either provide charmels through the stratum comeum, and thereby bypass its barrier attributes, or at least provide increased surface area for penetration of compounds, since a number of compounds were shown to penetrate faster in hair follicle-rich areas (lUel et al, 1991). Maibach studied three radiolabeled pesticides -parathion, malathion, and carbaryl - for their permeability at 13 different anatomical sites in humans (Maibach et al, 1971). Variations in percutaneous penetration were observed higher penetration of the pesticides occurred at the abdomen and dorsum of the hand. 

A close examination of this usage table shows that the herbicide atrazine has held the number one spot for the last 20 years. This compound is often used in nonfood crops such as cotton. However, some compounds are notably absent from the 1993 tabulation, including toxaphene, methyl-parathion, car-baryl, propachlor, DSMA, linuron, aldrin, carbofuran, chloramben, maneb, sodium chlorate, propanil, DBCP, malathion, dinoseb, and chlordane. It should also be noted that DDT, the pesticide on which Silent Spring was focused and which continues to be used as the example of man s folly with chemicals, is not on either list. The one generalization that can be made concerning this fist of deletions is that most of these pesticides are more persistent than the compounds that replaced them and a significant number of them are chlorinated hydrocarbons. It is important to keep these compounds in mind since popular press accounts of pesticide contamination often quote incidences of exposure to unnamed pesticides and then focus most discussion... 

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