Carbamate Insecticides Overview
N-methyl carbamate insecticides (carbamates) have been widely used in the U.S. and throughout the world. In agricultural applications, the use of the carbamate insecticides has decreased, being replaced by pyrethroid and other insecticides. Carbamates have been used on residential lawns, ornamentals, in nurseries, and on golf courses. Carbamates do not persist in the environment and have a low potential for bioaccumulation. Some other types of carbamates, thiocarbamates and dithiocarbamates, are used as herbicides and fungicides.
General population exposure to carbamates occurs during contact with residential uses and less commonly, from ingesting contaminated foods. Agricultural workers can be exposed when they re-enter areas recently treated. Exposures to workers also can occur during the manufacture, formulation, or application of these chemicals. Carbamates can be absorbed through the skin, via inhalation, or by ingestion. Criteria for allowable levels of specific carbamates in food, the environment, and the workplace have been developed by the U.S. FDA, U.S. EPA, and OSHA, respectively.
Carbamate insecticides act by inhibiting acetylcholinesterase enzymes, which leads to an increase of acetylcholine in the nervous system. At high doses, toxic symptoms include nausea, vomiting, cholinergic signs, weakness, paralysis, and seizures. The mechanism of toxicity of carbamate insecticides is similar to organophosphate pesticides; however, carbamate insecticides generally are reversible inhibitors of acetylcholinesterase activity, acting for a shorter time than organophosphate pesticides. Carbamate insecticides are rapidly eliminated from the body. In the National Biomonitoring Program, two carbamate metabolites that are measured relate to two (carbofuran and propoxur) of about nine carbamate insecticides still used in the U.S.
CAS No. 1563-66-2
Carbofuranphenol is a metabolite of four different carbamate insecticides: benfuracarb; carbofuran; carbosulfan; and furathiocarb. Of these, only carbofuran is registered in the U.S. Carbofuran is a broad spectrum restricted-use insecticide and nematicide applied to a variety of field, fruit, and vegetable crops for control of beetles, borers, nematodes, weevils and similar pests. Recently, registered uses of carbofuran were cancelled except for the following: field corn; potatoes; pumpkins; sunflowers; pine seedlings; and spinach grown for seed (U.S.EPA, 2009). About 1 million pounds have been used annually (U.S.EPA, 2007). Carbofuran is not registered for use in residential settings or food-handling establishments. In soils of varying composition, carbofuran has a half-life ranging from one to three months. It can leach into ground waters, but has been detected infrequently in either surface or ground waters (Gilliom, 2007; USGS, 2007). Carbofuran is very highly toxic to fish and aquatic invertebrates, and it is highly toxic to birds where granular applications are used, but these have been restricted since 1991 (U.S.EPA, 2007).
General population exposure can occur through consumption of food contaminated with carbofuran. Because estimated acute intakes from some dietary components in young children may exceed recommended intake limits, U.S.EPA is in the process of revoking current regulations that allow carbofuran residues in food (U.S.EPA, 2009). Pesticide handlers and applicators are at greater risk for exposure and a number of incidents of systemic poisoning have been reported. After absorption, carbofuran is metabolized to phenolic metabolites and 3-hydroxycarbofuran, which are quickly eliminated in the urine.
Human health effects from carbofuran at low environmental doses or at biomonitored levels from low environmental exposures are unknown. Carbofuran was very highly acutely toxic in animal studies, causing effects related to acetylcholinesterase enzyme inhibition. In contrast, carbofuranphenol is not an inhibitor of acetylcholinesterase enzymes. Carbofuran was not teratogenic, but high chronic doses in animals produced nonspecific developmental effects, such as reduced weight gain and pup survival (WHO, 2004). Testicular toxicity at subacute doses was reported in adult rats, rat pups, and dogs (Pant et al., 1995, 1997; WHO, 2004). Carbofuran was not mutagenic or carcinogenic in animals (U.S.EPA, 2007). It is not rated by IARC with regard to human carcinogenicity. Additional information is available from U.S.EPA at: http://www.epa.gov/pesticides/.
Urinary carbofuranphenol levels reflect recent exposure. The level of this metabolite in urine may reflect exposure to carbofuran or to carbofuranphenol as a degradation product in the environment or food. In the U.S. representative subsamples from NHANES 1999-2000 and 2001-2002, most urinary levels of carbofuranphenol were below the limit of detection (CDC, 2009). In a nonrandom subsample from NHANES III (1988-1994), the 99th percentile level of carbofuranphenol was 2.1 µg/L (Hill et al., 1995). In a previous study of U.S. farmers and their families, carbofuranphenol was detected in 6.7% of urine samples (Shealy et al., 1997); the 95th percentile value in that study was 0.73 µg/L. Urinary levels of carbofuranphenol in two applicators were three and sixfold higher than the detection limit for the NHANES 2003-2004 subsample (Petropoulou et al., 2006).
Finding a measurable amount of carbofuranphenol in urine does not imply that the level of carbofuranphenol causes an adverse health effect. Biomonitoring studies on levels of carbofuranphenol provide physicians and public health officials with reference values so that they can determine whether people have been exposed to higher levels of carbofuran or related carbamates 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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Hill RH Jr, Head SL, Baker S, Gregg M, Shealy DB, Bailey SL, et al. Pesticide residues in urine of adults living in the United States: reference range concentrations. Environ Res 1995;71(2):99-108.
Pant N, Prasad AK, Srivastava SC, Shankar R, Srivastava SP. Effect of oral administration of carbofuran on male reproductive system of rat. Hum Exp Toxicol 1995;14(11):889-894.
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U.S. Environmental Protection Agency (U.S.EPA) Carbofuran cancellation process. March 28, 2009. Available at URL: http://www.epa.gov/pesticides/reregistration/carbofuran/carbofuran_noic.htm. 12/28/12
U.S. Environmental Protection Agency (U.S.EPA). Reregistration Eligibility Decision for Carbofuran. September 2007. Available at URL: http://www.epa.gov/pesticides/reregistration/REDs/carbofuran_red.pdf. 12/28/12
U.S. Geological Survey (USGS). The Quality of Our Nation's Waters Pesticides in the Nation's Streams and Ground Water, 1992-2001. Circular 1291. Supplemental Technical Information (available on-line only). March 2006, revised February 15, 2007. Available at URL: http://water.usgs.gov/nawqa/pnsp/pubs/circ1291/supporting_info.php. 12/28/12
World Health Organization (WHO). Carbofuran in Drinking-Water. Background document for development of WHO Guidelines for Drinking-water Quality. 2004. Available at URL: http://www.who.int/water_sanitation_health/dwq/chemicals/carbofuran.pdf. 12/28/12