2. Environmental toxicology—The organochlorine pesticides are considered persistent chemicals. Degradation is quite slow when compared with other pesticides, and bioaccumulation, particularly in aquatic ecosystems, is well documented. Their mobility in soil depends on the composition of the soil; the presence of organic matter favors the adsorption of these chemicals onto the soil particles, whereas adsorption is poor in sandy soils. Once adsorbed, they do not readily desorb. These compounds induce significant abnormalities in the endocrine balance of sensitive animal and bird species, in addition to their adverse impact on humans. Since the early 1960s, when Rachel Carson’s work and subsequent book, Silent Spring, brought attention to the issue, the organochlorine pesticides have been recognized as pernicious environmental toxins. Their use is banned in most jurisdictions.
These agents, some of which are listed in Table 56–3, are used to combat a large variety of pests. They are useful pesticides when in direct contact with insects or when used as plant systemics, where the agent is translocated within the plant and exerts its effects on insects that feed on the plant. The many varieties currently in use are applied by spray techniques including hand, tractor, and aerial methods. They are often spread widely by wind and weather and are subject to widespread drift. The organophosphate pesticides are based on compounds such as soman, sarin, and tabun, which were developed for use as war gases. Some of the less toxic organophosphorus compounds are used in human and veterinary medicine as local or systemic antiparasitics (see Chapters 7 and 53). The compounds are absorbed by the skin as well as by the respiratory and gastrointestinal tracts. Biotransformation is rapid, particularly when compared with the rates observed with the chlorinated hydrocarbon pesticides. Storm and collaborators reviewed current and suggested human inhalation occupational exposure limits for 30 organophosphate pesticides (see References).
TABLE 56–3 Organophosphorus pesticides.
Trichlorfon 4 0.01 1Toxicity rating: Probable human oral lethal dosage for class 4 = 50–500 mg/kg, class 5 = 5–50 mg/kg, and class 6 = ≤5 mg/kg, ;—, data not found. (See Gosselin et al, 1984.) 2ADI, acceptable daily intake (mg/kg/d).
1. Human toxicology—In mammals as well as insects, the major effect of these agents is inhibition of acetylcholinesterase through phosphorylation of the esteratic site. The signs and symptoms that characterize acute intoxication are due to inhibition of this enzyme and accumulation of acetylcholine; some of the agents also possess direct cholinergic activity. Specific treatment with antidotes and useful antagonists is available. In addition, pretreatment with physostigmine and other short-acting compounds may provide protection against these pesticides or their war gas analogs if used in timely fashion. These effects and their treatment are described in Chapters 7 and 8 of this book. Altered neurologic and cognitive functions, as well as psychological symptoms of variable duration, have been associated with exposure to these pesticides. Furthermore, there is some indication of an association of low arylesterase activity with neurologic symptom complexes in Gulf War veterans.