Carbaryl discussion

Pesticides are also a major source of concern as water and soil pollutants. Because of their stability and persistence, the most hazardous pesticides are the organochlorine compounds such as DDT, aldrin, dieldrin, and chlordane. Persistent pesticides can accumulate in food chains for example, shrimp and fish can concentrate some pesticides as much as 1000- to 10,000-fold. This bioaccumulation has been well documented with the pesticide DDT, which is now banned in many parts of the world. In contrast to the persistent insecticides, the organophosphorus (OP) pesticides, such as malathion, and the carbamates, such as carbaryl, are short-lived and generally persist for only a few weeks to a few months. Thus these compounds do not usually present as serious a problem as the earlier insecticides. Herbicides, because of the large quantity used, are also of concern as potential toxic pollutants. Pesticides are discussed in more detail in Chapter 5. 

In Section 17.3, rodenticidal thioureas, insecticidal thiocyanates, and fungicidal dithiocarbam-ates were discussed. Sulfur is a common constituent of other classes of insecticides. These prominently include the organophosphate insecticides discussed in Chapter 18. Mobam (Figure 17.10) is a contact insecticide of the carbamate type, closely related in structure and function to the well-known carbamate insecticide carbaryl (see Figure 15.8). Mobam has been found to have a relatively high toxicity to laboratory mammals and is considerably more toxic than carbaryl. 

The carbamates, like the organophosphorus insecticides, are cholinesterase inhibitors. However, the reaction is rapidly reversible. Carbaryl has a half life in the soil of about 8 days and is decomposed by ultraviolet light. The carbamates metabolize rapidly in animals and show little, if any, propensity for storage in animal tissues. Additional properties and reactions of carbamates are discussed in the section on fungicides. 

As it has been discussed previously, for the 2 suns CPC the collected UV radiation is about 75% of the available solar radiation, while a nonconcentrating CPC captures in principle all the available radiation. However, as the aperture area of the 2 suns CPC is twice the aperture of the one sun system, the net result is that it collects 1.5 more energy. In the same way, the 1.5 and 1.75 suns reactors are able to capture 1.4 and 1.25 times more energy than the 1 sun system, respectively. The effect of these differences in the energy accepted by each system can be clearly seen in the degradation of carbaryl (Figure 14), where degradation proceeds faster as the concentration ratio increases. 

Dick and Scott (1992) measured skin permeability of several lipophilic (aldrin, carbaryl, and fluaziflop-butyl) and hydrophilic (water, maimitol, and paraquat) compounds in pig ear skin, rat dorsal skin, and hiunan abdominal skin. Although different concentrations and vehicles make a comparison of the compounds difficult, all the data can be used for interspecies comparison because the vehicle and penetrant concentration were the same for all species studied. There is a discrepancy between values shown in Figure 4 and Table 1 that appears to be an error in the typesetting of Table 1 (see discussion in Appendix B). The quahtative conclusion that pig ear skin and rat dorsal skin are both more permeable than human abdominal skin is not affected by this error. 

Table V shows the amounts of active ingredient in the amounts of the formulations applied by volunteers. These data were taken directly from Table IV and are presented to give the reader some impression of the amounts of carbaryl used in these trials. The data can be used when comparing the use of the three formulations providing the exposures discussed below. They do show that far more carbaryl was used In 15 minutes when applied as a 5% dust than as either of the two formulations applied as a sprayed-on suspension.

Post-column hydrolysis of carbamate esters, such as carbaryl, was discussed in Chapter 4, Section 2. Further consideration of anticholinesterase insecticides can be found in Chapter 7, Section 4. 

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