What are organophosphates?
How do organophosphates work?
How are they applied?
OPs are applied to fields as solid granules or as liquids (often dissolved in organic solvents) by hand-sprayers, motorized blowers/dusters, and by aircraft.
In California it has been demonstrated that air applications result in higher systemic illness risk than ground applications.
What are organophosphates?
Organophosphates (OPs) are a class of man made chemicals that consist of organic compounds containing phosphorus and contain one or more phosphate ester groups. Majority of these compounds are used in agriculture as pesticides, several of which are highly toxic, but they are also used in homes, gardens and veterinary practices and even as nerve agents.
Several notable OPs have been discontinued for use, including parathion, which is no longer registered for any use, and chlorpyrifos, which was prohibited for home and agricultural use by the Obama administration but has since been approved for use by the current administration.
All can potentially cause acute and subacute toxicity due to their ability to bioaccumulate in the fatty tissues of organisms.
How do organophosphates work?
The underlying mechanism involves the inhibition of the acetylcholinesterase (AChE) enzyme, which breaks down the neurotransmitter acetylcholine (ACh).
Organophosphates phosphorylate AChE, inactivating it and preventing the enzyme from binding to ACh; leading to a buildup of this neurotransmitter. Accumulation of ACh results in an overstimulation of the central nervous system in organisms and could result in death in high enough doses.
AChE is critical to normal control of nerve impulse transmission from nerve fibers to smooth and skeletal muscle cells, symptoms and signs of OP poisoning become apparent soon after levels of ACh reach a critical point (varies by organism). Oxidation is required for pesticide to have a toxic effect, and occurs after consumption.
How are they applied?
OPs are applied to fields as solid granules or as liquids (often dissolved in organic solvents) by hand-sprayers, motorized blowers/dusters, and by aircraft.
In California it has been demonstrated that air applications result in higher systemic illness risk than ground applications.
Environmental Impacts:
Organophosphates have been used since in agriculture since 1937, they are the most commonly used insecticides in the world and as a result of the magnitude of their application they are the chemicals most frequently associated with toxicity to domestic animals, wildlife, and humans. They are also major contaminants of the environment and of water supplies around the globe, pollution being the worst in developing countries.
OPs are transported long distances by runoff and wind, drastically increasing the areas of exposure. OPs have even been detected in pine needles in the Sierra Nevada Mountains in CA miles away from application sites.
Degradation:
OPs are broken down by their environment by 3 major processes hydrolysis, photolysis, and biological degradation.
Hydrolysis:
chemical reaction of OP that results in the insertion of a water molecule or hydroxyl group (-OH) and usually leads to loss of function/ toxicity, main form of degradation in water but is also the slowest.
Photolysis:
Direct photolysis and indirect photolysis, both involve the absorbance of sunlight with wavelengths greater than 290 nm that result in transformations like rearrangement, dissociation, and oxidation.
In direct photolysis, OP absorbs light directly, and in indirect photolysis substances naturally present in aquatic environments absorb sunlight to form excited chemical species or radicals which then react with OP. Especially important for surface water and air pollution but less effective in OPs in soils.
Biological degradation:
Biological degradation involves enzyme-catalysed transformations and constitute the bulk of OP breakdown in the environment. Most of the microbial breakdown of OPs and other pesticide residues is considered incidental. Meaning that these chemicals, unless they are very toxic to the microorganisms, are metabolized by non specific microorganisms that already have the enzyme systems for degradation of residues that are present in soils.
Over time, the organisms that can degrade the compounds increase in population size, while those that can’t decrease. Biodegradation rates are a function of the microbial biomass present that can degrade OP and the concentration of the OP under given environmental conditions. So an increase in population of degraders leads to an increase in degradation of OPs/ pesticide residues, but this does not happen overnight.
An acclimatization period is required to induce soil microorganisms to produce necessary enzymes, to develop biodegradation organisms by mutation, and to increase the number of microbes to substantial levels. Acclimation periods are significantly slower in virgin soils that have never been exposed to OP and this is the where the issue lies.
Persistence:
Organophosphates are applied in such large quantities due to their alleged “safety”that they rapidly degrade in the environment. However, degradability is dependent on many factors and the true interactions with the environment remain unknown. Half-life of OP are estimated based on models that do not take all possible interactions into consideration (pH, temperature, interactions with SOM & minerals, OP in soils not adapted to exposure).
Although OPs do have a relatively short half-life and can be degraded somewhat quickly when compared to other pesticides, not all of if is removed from the environment and their mass application forfeits this slight benefit. In low concentrations this is not a problem but repeated use of OP will result in small amounts accumulating to dangerous levels in the environment after years of exposure.
Water Pollution:
OPs contaminate groundwater reservoirs and cause eutrophication of bodies of water, especially near sites of application sites. Studies have found that OPs are exceptionally toxic to aquatic organisms and that they can bioaccumulate in wildlife.
Laboratory values quoted for OP hydrolysis rates are most often for a pH of 7 and a temperature of 25C, however, groundwaters can vary between a pH of 5–9. Breakdown of OP drastically decreases as pH and temperature go down. Increasing the half-life of OPs in water and thus the chance of exposure.
-
K. VALA RAGNARSDOTTIR; Environmental fate and toxicology of organophosphate pesticides. Journal of the Geological Society ; 157 (4): 859–876.
-
Maximum Residue Limits (MRL) Database.” Maximum Residue Limits (MRL) Database | USDA Foreign Agricultural Service, www.fas.usda.gov/maximum-residue-limits-mrl-database.
-
Colovic, Mirjana B., et al. “Acetylcholinesterase Inhibitors: Pharmacology and Toxicology.” Current Neuropharmacology, vol. 11, no. 3, 2013, pp. 315–335.
-
Matthews, G.A., 1992. Pesticide Application Methods, 2nd Edition. Longman Scientific & Technical with John Wiley & Sons, New York.
-
Weinbaum, Zipora, et al. “Risk Factors for Systemic Illnesses Following Agricultural Exposures to Restricted Organophosphates in California, 1984-1988.” American Journal of Industrial Medicine, vol. 31, no. 5, 1997, pp. 572–579.
-
Donald, David B., Allan J. Cessna, and Nancy E. Glozier Sverko. "Pesticides in Surface Drinking-Water Supplies of the Northern Great Plains." Environmental health perspectives 115.8 (2007): 1183.