Pyrethroid Pesticides Overview
Pyrethroid pesticides are synthetic analogues of pyrethrins, which are natural chemicals found in chrysanthemum flowers. Pyrethroid pesticides are used to control a wide range of insects in public and commercial buildings, animal facilities, warehouses, agricultural fields, and greenhouses. They are also applied on livestock to control insects. In agriculture, cypermethrin, cyfluthrin, and deltamethrin have been used frequently on cotton. Pyrethroid insecticides are the most common active ingredient in commercially available insect sprays and are also used as structural termiticides. Certain pyrethroid insecticides (such as permethrin, resmethrin, and sumithrin) are also registered for use in mosquito-control programs in the United States. Outside the U.S., deltamethrin has been used for indoor protection against mosquitoes that carry malaria, in some situations replacing the use of DDT. About two million pounds of permethrin and one million pounds of cypermethrin have been applied annually (U.S. EPA, 2006a, 2006b). Permethrin is also used in skin lotions and shampoos as medical treatments for lice and scabies. Pyrethroid pesticides are generally formulated as complex mixtures of different chemical isomers, solvent oils, and synergists, such as piperonyl butoxide. Pyrethroid pesticides have low volatility, bind to soils, and are rarely detected in ground waters (USGS, 2007). Generally, they are not persistent in the environment due to their rapid degradation within days to several months. This class of pesticides has low toxicity in birds and mammals, but pyrethroids are highly toxic to fish and some aquatic invertebrates, so usage is restricted near water (U.S.EPA, 2012).
The general population may be exposed to pyrethroid insecticides primarily from the ingestion of food or from residential use. Estimated intakes from diet and drinking water are below recommended limits. Dermal exposure with the potential for inadvertent ingestion may occur when lotions or shampoos are applied to treat lice or scabies. Pesticide applicators can be exposed to pyrethroid pesticides via dermal and inhalation routes from powders and liquid formulations. Pyrethroids are not well absorbed through the skin (ATSDR, 2003; Woollen et al., 1992). After absorption from inhalation or ingestion, pyrethroids are rapidly metabolized, by either ester hydrolysis or hydroxylation, followed by conjugation, and then eliminated over several days in urine and bile (Kuhn et al., 1999; Leng et al., 1997; Soderlund et al., 2002; Woollen et al., 1992). Unmetabolized pyrethroids have been measured in breast milk, but may be poorly transferred across the placenta (ATSDR, 2003; WHO, 2005).
Human health effects from pyrethroid pesticides at low environmental doses or at biomonitored levels from low environmental exposures are unknown. Compared with other classes of insecticides such as organochlorines, organophosphorus, or carbamate pesticides, pyrethroid pesticides have less acute toxicity in animals and people. They are ranked as having moderate acute oral toxicity. Adverse effects from large doses are related to the action of pyrethroids on the nervous system, where these chemicals prolong sodium channel opening when a nerve cell is depolarized (Shafer et al., 2005; Soderlund et al., 2002). Possible other additional actions on neuroreceptors and other ion channels may also explain some pyrethroid effects. Human cases of systemic poisoning are rare and usually result from accidental exposure or intentional ingestion of pyrethroid insecticides. Signs and symptoms of acute pyrethroid poisoning after massive ingestions include agitation, hypersensitivity, tremor, salivation, choreoathetosis, and seizures (ATSDR, 2003; Ray et al., 2000; Soderlund et al., 2002). Concomitant exposure to organophosphorus insecticides may increase pyrethroid toxicity by slowing metabolic clearance of the pyrethroid. In California, cyfluthrin was the most frequent pyrethroid associated with symptomatic effects (irritant respiratory and dermal effects, paresthesias) reported in agricultural workers from 1996 to 2002 (Spencer and O'Malley, 2006). Transient dermal paresthesias have been reported among pesticide applicators after direct contact with certain types of pyrethroid pesticides. No relationship of indoor air or housedust concentrations of permethrin and irritant symptoms was found in a study of urban residents in 80 private homes (Berger-Preiss et al., 2002).
In developing rodents, neurochemical changes in cholinergic, dopaminergic, and catecholaminergic pathways and behavioral changes have been demonstrated at subacute and subchronic doses for some pyrethroid pesticides (Aziz et al., 2001; Elwan et al., 2006; Eriksson and Fredriksson, 1991; Lazarini et al., 2001; Shafer, et al., 2005). The pyrethroids in general use are not considered teratogenic or to have significant reproductive toxicity (ATSDR, 2003; WHO, 2005), though a few pyrethroid pesticides and some metabolites have shown weak or inconsistent estrogenic effects on standardized assays (ATSDR, 2003; Garey and Wolff, 1998; Go et al., 1999; Hu et al., 2003; Kim et al., 2004; Kunimatsu et al., 2002; McCarthy et al., 2006; Moniz et al., 2005). Generally, the pyrethroids are not considered genotoxic in in vitro testing or carcinogenic in animal testing (WHO, 2005). IARC considers deltamethrin and permethrin as not classifiable as to their human carcinogenicity. Additional information about pesticides is available from U.S. EPA at https://www.epa.gov/pesticides and from ATSDR at https://www.atsdr.cdc.gov/toxprofiles/index.asp#H.
There are about 30 different pyrethroid pesticides in use. The table shows the urinary pyrethroid metabolites measured in the National Biomonitoring Program.
Phthalates and Urinary Metabolites Measured in the National Biomonitoring Program
|Pyrethroid (CAS number)||Urinary metabolite (CAS number)|
|Cyfluthrin (68359-37-5)||4-Fluoro-3-phenoxybenzoic acid (77279-89-1)|
|cis-3-(2,2-Dichlorovinyl)-2,2-dimethylcyclopropane carboxylic acid (63597-73-9)
trans-3-(2,2-Dichlorovinyl)-2,2-dimethylcyclopropane carboxylic acid (59042-50-1)
|Deltamethrin (52918-63-5)||cis-3-(2,2-Dibromovinyl)-2,2-dimethylcyclopropane carboxylic acid (59042-49-8)|
|3-Phenoxybenzoic acid (3739-38-6)|
CAS No. 52918-63-5
Cis-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropane carboxylic acid is a metabolite of the pyrethroid insecticide deltamethrin. Outside the U.S., deltamethrin has been used against mosquitoes that carry malaria, in some situations replacing the use of DDT. Deltamethrin can degrade to cis-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropane carboxylic acid in the environment (IPCS, 1990). Thus, in detection of cis-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropane carboxylic acid in the urine may reflect exposure to deltamethrin or to cis-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropane carboxylic acid formed in the environment.
Urinary levels of cis-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropane carboxylic acid reflect recent exposure to deltamethrin or its environmental degradate. In the NHANES 2001-2002 subsample, urinary levels of cis-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropane carboxylic acid were below the limit of detection (CDC, 2009). In an analysis of 217 urine specimens from a nonrandom sample ofUnited States residents, Baker et al. (2004) reported a geometric mean concentration of cis-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropane carboxylic acid of 0.39 µg/L. Studies in Germany of 396 children and adolescents (Becker et al., 2006) and 1177 urban adults and children (Heudorf et al., 2001) showed that urinary levels of cis-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropane carboxylic acid at the 95th percentile ranged slightly higher (0.3-0.5 μg/L) than the detection limit (0.1 μg/L) for the NHANES 2001-2002 subsample (CDC, 2009). Urinary levels for adults and children in these studies were similar (Heudorf et al., 2001, 2006) and estimated daily intakes based on urinary levels in children were considered to be below acceptable daily intakes (Heudorf et al., 2004).
Following residential spraying with deltamethrin for malaria protection in Mexico, mean peak urinary levels of cis-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropane carboxylic acid in children increased at least 450-fold relative to the non-detectable background levels for several days and mean levels remained slightly above background levels 45 days after the spraying (Ortiz-Perez et al., 2005). The peak mean levels in these children were more than 800-fold higher than the detection limit in the 2001-2002 NHANES subsample.
Finding a measurable amount of cis-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropane carboxylic acid in urine does not imply that the level will result in an adverse health effect. Biomonitoring studies provide physicians and public health officials with reference values so that they can determine whether other people have been exposed to higher levels of deltamethrin than levels found in the general population. Biomonitoring data can also help scientists plan and conduct research on exposure and health effects.
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