Environmental fate and toxicology of organophosphate pesticides

Abstract

Organophosphate pesticides (OPs) are generally regarded as safe for use on crops and animals due to their relatively fast degradation rates. Their degradation varies as a function of microbial composition, pH, temperature, and availability of sunlight. Under laboratory conditions (25°C and pH 7) biodegradation is about one order of magnitude faster than chemical hydrolysis, which in turn is roughly ten times faster than photolysis. Microbial biomass often needs a lengthy adaptation period in which soil bacteria mutate to be able to metabolize OPs. Biodegradation is thus in general an order of magnitude faster in soils that have had repeated applications of OPs compared to control soils which have never had OP applications. Because OPs are relatively soluble, they often enter surface and groundwaters. In the latter OPs are primarily broken down through chemical hydrolysis, which is pH dependent. Hydrolysis half-life of an OP pesticide of 10 days in the laboratory increases to one year if the pH of the water is 6 and the temperature 5°C, suggesting that OPs can persist in the environment for long periods of time. Indeed, OPs are detected in soils years after application. Why this environmental persistence occurs is not clear, but it may be due to sorption of the OPs to soil particles, making them unavailable for microbial metabolism. Example calculations and literature data show that conditions can occur in soil where OPs are preserved and transferred to humans through food. A review of the literature shows that OPs are highly toxic and that human exposure is undesirable. Evidence suggests that OPs are mutagenic and teratogenic and that a large number of modern-day diseases of the nervous and immune system of mammals can be linked to these pesticides. These include BSE (mad cows disease), CJD, Gulf War syndrome, Parkinson’s disease and multiple sclerosis, arguing for a thorough examination of the environmental fate and toxicology of OPs as well as their use.