Organophosphorus Insecticides: Dialkyl Phosphate Metabolites
Organophosphorus insecticides, which are active against a broad spectrum of insects, have accounted for a large share of all insecticides used in the United States. Although organophosphorus insecticides are still used for insect control on many food crops, most residential uses have been phased out in the United States as a result of implementation of the Food Quality Protection Act of 1996. Certain organophosphorus insecticides (e.g., malathion, naled) are also registered for public health applications (e.g., mosquito control) in the United States. An estimated 73 million pounds of organophosphorus insecticides (70% of all insecticides) were used in the United States in 2001, with usage declining 45% since 1980 (U.S. EPA, 2004). Approximately 40 organophosphorus insecticides in a wide variety of formulations are registered for use in the United States by the U.S. EPA. In general, the various organophosphorus insecticides demonstrate low vapor pressures (with some exceptions), slight to moderate water solubility, moderate to high soil binding, widely varying degrees of soil leaching or runoff potential, and a low persistence in the environment.
General population exposure to organophosphorus insecticides may occur by ingesting contaminated food and from hand-to-mouth contact with surfaces containing organophosphorus insecticides; less common routes include inhalation and dermal contact. In general, the organophosphorus insecticides have better gastrointestinal than dermal absorption. Mammalian elimination half-lives can range from hours to weeks. The thiophosphate type organophosphorus insecticides (e.g., chlorpyriphos) are initially metabolized to the more toxic "oxon" form. Most organophosphorus insecticides undergo hydrolysis with excretion of major hydrolytic metabolites in the urine. Estimated intakes by the general population are usually considered below regulatory thresholds though concerns have been raised about some organophosphorus insecticides because of unique routes of exposures and intakes in infants and children (NRC, 1993). Farm workers, gardeners, florists, pesticide applicators, and manufacturers of these insecticides may have greater exposure than the general population. Many states have programs to monitor cholinesterase activity in the blood of pesticide applicators as part of monitoring exposure to organophosphorus insecticides. The U.S. FDA, USDA, U.S. EPA, and OSHA have developed criteria on allowable levels of these chemicals in foods, the environment, and the workplace.
The acute high dose effects of the organophosphorus insecticides from intentional and unintentional overdoses or from high-dose worker exposures are well known and include neurological dysfunction that results from the inhibition of the enzyme acetylcholinesterase leading to excess acetylcholine in the central and peripheral nervous systems. Acute symptoms include nausea, vomiting, cholinergic effects, weakness, paralysis, and seizures. Mild to severe peripheral neuropathies and residual deficits in neurocognitive functioning can persist following acute poisonings (London et al., 1998; Rosenstock et al., 1991; Savage et al., 1988). Chronic exposures studied in farmers and insecticide applicators, who have neither past acute poisoning or significant reduction in blood cholinesterase activity, have shown possible subtle or subclinical neurological effects, though various study results are inconsistent (Albers et al., 2004; Daniell et al., 1992.; Engel et al., 1998; Farahat et al., 2003; Fiedler et al., 1997; Jamal et al., 2002; Maizlish et al., 1987; Peiris-John et al., 2002; Pilkington et al., 2001; Rodnitzky et al., 1975; Rothlein et al., 2006; Stephens et al., 1995; Stokes et al., 1995; Young et al., 2005). Animal studies at high doses generally demonstrate the effects of inhibition of acetylcholinesterase mentioned above for acute poisoning in humans, as well as mechanistically-related neurodevelopmental and reproductive effects (Astroff et al., 1998a and 1998b; Prendergast et al., 1998). Few animal studies have addressed the potential for low environmental doses to produce non-cholinergic effects (i.e., without inhibition of acetylcholinesterase). Additional information about insecticides is available from U.S. EPA at https://www.epa.gov/pesticides and from ATSDR at https://www.atsdr.cdc.gov/toxprofiles/index.asp.
About 75% of registered organophosphorus insecticides are metabolized in the body to measurable dialkyl phosphate metabolites. The dialkyl phosphate metabolites do not inhibit acetylcholinesterase and are not considered toxic, but are regarded as markers of exposure to organophosphorus insecticides. Dialkyl phosphate metabolites can be present in urine after low level exposures to organophosphorus insecticides that do not cause clinical symptoms or inhibition of cholinesterase activity (Davies and Peterson, 1997; Franklin et al., 1981). Measurement of these metabolites reflects recent exposure, predominantly in the previous few days. Dialkyl phosphates may also occur in the environment as a result of degradation of organophosphorus insecticides (Lu et al., 2005), and therefore, the presence in a person's urine may reflect exposure to the metabolite itself.
Generally, six urinary dialkyl phosphate metabolites of organophosphorus insecticides are measured in the National Biomonitoring Program and for other research studies: dimethylphosphate (DMP); dimethylthiophosphate (DMTP); dimethyldithiophosphate (DMDTP); diethylphosphate (DEP); diethylthiophosphate (DETP); and diethyldithiophosphate (DEDTP). The table shows the six urinary metabolites and the parent organophosphorus insecticides responsible for these metabolites. For example, chlorpyrifos is metabolized to both diethylphosphate and diethylthiophosphate. Each of the six urinary dialkyl phosphate metabolites can be produced from the metabolism of more than one organophosphorus insecticide. Therefore, the presence of one or more dialkyl phosphate metabolites without additional information cannot be linked to exposure to a specific organophosphorus insecticide.
Urinary dialkyl phosphate levels reflect recent exposure. In nationally representative subsamples of the U.S. population from NHANES 1999-2000, 2001-2002, 2003-2004, and 2007-2008 (CDC, 2012), geometric mean urinary dialkyl phosphate levels were generally lower than levels reported in smaller studies of children and adults in Italy and Germany (Aprea et al., 2000; Aprea et al., 1996; Heudorf and Angerer, 2001; Saieva et al., 2004). In these studies and the NHANES subsamples, children have slightly higher levels than adults. Diet influences the measured levels of urinary dialkyl phosphates. For example, subjects ingesting "organically-grown" foods were shown to have lower levels of urinary dialkyl phosphates than subjects eating a conventional diet (Curl et al., 2003). Also, urinary levels in children of farm workers and non-farm workers have been reported to correlate weakly with environmental dust levels of particular insecticides in some, but not all, studies (Bouvier et al., 2006; Curl et al., 2003; Rothlein et al., 2006).
Measurements of dialkyl phosphates in urine have been used to document exposure of farmers, agricultural workers, pest-control workers, and others to organophosphorus insecticides (Davies and Peterson, 1997; Franklin et al., 1981; Krieger and Dinoff, 2000; Takamiya, 1994). In some of these occupational studies, reported levels of urinary dialkyl phosphates may exceed levels seen in the general population by up to fiftyfold, though in general, worker levels are only moderately higher. Urinary levels of dialkyl phosphate metabolites vary with the type of field application, seasonal use of the parent insecticide, and demonstrate substantial variability when measured over multiple times of day and over multiple days, which may reflect changes in exposure, collection timing, and elimination kinetics (Kissel et al., 2005; Koch et al., 2002; Lambert et al., 2005; Petchuay et al., 2006).
Children and pregnant family members of farm workers were reported to have median levels of many urinary dialkyl phosphates that were either similar or slightly higher (Arcury et al., 2006; Bradman et al., 2005) than those presented in U.S. representative subsamples from NHANES 1999-2000, 2001-2002, 2003-2004 (CDC, 2012), except for one study in which DMTP levels were up to fourteenfold higher depending on the season and the type of crop application (Lambert et al., 2005). Also, estimates of dose or intake calculated from urinary dialkyl phosphate levels in studies of pregnant women in one agricultural community (Castorina et al., 2003) and in another study of workers exposed on reentry to treated orchards (Fenske et al., 2003) generally did not exceed doses considered to be safe. Estimates of dose or intake for the general U.S. population as calculated from urinary dialkyl phosphate measurements were below environmental dose estimates based on multiple routes of exposure (Duggan et al., 2003).
Information is limited with regard to associations between levels of urinary dialkyl phosphates and any health effects. Summed levels of urinary dialkyl phosphates in prenatal samples from mothers of neonates living in an agricultural community were associated with subtle changes in one of seven domains of neurophysiologic neonatal testing during one restricted postnatal period of time (Young et al., 2005). In a study of farm workers, median urinary levels of DMTP and DMDTP were more than twentyfold higher than median levels in the U.S. population (CDC, 2012), and these higher levels were associated with a few subtle neurobehavioral test results (Rothlein et al., 2006).
Finding a measurable amount of dialkyl phosphate metabolites in urine does not imply that the level of dialkyl phosphate metabolites causes an adverse health effect. Biomonitoring studies of dialkyl phosphate metabolites provide physicians and public health officials with reference values so that they can determine whether or not people have been exposed to higher levels of organophosphorus pesticides than are found in the general population. Biomonitoring data can also help scientists plan and conduct research on exposure and health effects.
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