Chlorpyrifos is moderately toxic to humans and chronic exposure has been linked to neurological effects, developmental disorders, and autoimmune disorders. Exposure during pregnancy retards the mental development of children, and most use in homes has been banned since 2001 in the U.S. In agriculture, it remains "one of the most widely used organophosphate insecticides", according to the United States Environmental Protection Agency (EPA).
Chlorpyrifos is produced via a multistep synthesis from 3-methylpyridine, eventually reacting 3,5,6-trichloro-2-pyridinol with diethylthiophosphoryl chloride.
The crops with the most intense chlorpyrifos use are cotton, corn, almonds, and fruit trees including oranges, bananas and apples.
Chlorpyrifos is normally supplied as a 23.5% or 50% liquid concentrate. The recommended concentration for direct-spray pin point application is 0.5% and for wide area application a 0.03 – 0.12% mix is recommended (US).
First registered in 1965 and marketed by Dow Chemical under the tradenames Dursban, Lorsban and Renoban, chlorpyrifos was a well known home and garden insecticide, and at one time it was one of the most widely used household pesticides in the US.
In 1995, Dow was fined US$732,000 for not sending the EPA reports it had received on 249 chlorpyrifos poisoning incidents.
Facing impending regulatory action by the EPA, Dow agreed to withdraw registration of chlorpyrifos for almost all use (except child-proof containerized insect baits) in homes and other places where children could be exposed, and severely restricted its use on crops. These changes took effect on Dec 31, 2001. It is still widely used in agriculture, and Dow continues to market Dursban for home use in developing countries. Dow's sales literature claimed Dursban has "an established record of safety regarding humans and pets."
In 2003, Dow agreed to pay US$2 million – the largest penalty ever in a pesticide case – to the state of New York, in response to a lawsuit filed by the Attorney General to end Dow's illegal advertising of Dursban as "safe".
On July 31, 2007, a coalition of farmworker and advocacy groups filed a lawsuit against the EPA seeking to end agricultural use of chlorpyrifos. The suit claims that the continued use of chlorpyrifos poses an unnecessary risk to farmworkers and their families. The suit was still pending as of August 2012.
In August 2007, Dow's Indian offices were raided by Indian authorities for allegedly bribing officials to allow chlorpyrifos to be sold in the country.
In 2008, the National Marine Fisheries Service (NMFS) imposed 1000 ft buffer zones around salmon habitat to protect endangered salmon and steelhead species. Aerial applications of chlorpyrifos will be prohibited within these zones.
Chlorpyrifos is an organophosphate, with potential for both acute toxicity at larger amounts and neurological effects in fetuses and children even at very small amounts. For acute effects, the EPA classifies chlorpyrifos as Class II: moderately toxic. The oral LD50 for chlorpyrifos in experimental animals is 32 to 1000 mg/kg. The dermal LD50 in rats is greater than 2000 mg/kg and 1000 to 2000 mg/kg in rabbits. The 4-hour inhalation LC50 for chlorpyrifos in rats is greater than 200 mg/m3.
Chlorpyrifos poisoning has been described by New Zealand scientists as the likely cause of death of several tourists in Chiang Mai, Thailand who developed myocarditis in 2011. Thai investigators have come to no conclusion as to what caused the deaths, but maintain that chlorpyrifos was not responsible, and that the deaths were not linked.
Research indicated in 2006 that children exposed to chlorpyrifos while in the womb have an increased risk of delays in mental and motor development at age 3 and an increased occurrence of pervasive developmental disorder and ADHD. An earlier study had demonstrated a correlation between prenatal chlorpyrifos exposure and lower weight and smaller head circumference at birth.
Among 50 farm pesticides studied, chlorpyrifos was one of two found to be associated with higher risks of lung cancer among frequent pesticide applicators than among infrequent or non-users. Pesticide applicators as a whole were found to have a 50% lower cancer risk than the general public, which is attributable to the nearly 50% lower smoking rate found among farm workers. However, applicators of chlorpyrifos had a 15% lower cancer risk than the general public, which the study suggests indicates a likely link between chlorpyrifos application and lung cancer.
A 2010 study found that each 10-fold increase in urinary concentration of organophosphate metabolites was associated with a 55% to 72% increase in the odds of ADHD in children.
Studies have shown evidence of "deficits in Working Memory Index and Full-Scale IQ as a function of prenatal [chlorpyrifos] exposure [as measured when the children reach] 7 years of age." A 2012 study showed that the insecticide is more harmful to the mental development of boys than to that of girls.
A 2011 study on the neurotoxic effects of chlorpyrifos showed that chlorpyrifos and its more toxic metabolite, chlorpyrifos oxon, altered firing rates in the locus coeruleus. These results indicate that the pesticide may be involved in Gulf War Syndrome and other neurodegenerative disorders.
Effects on aquatic life and bees
Chlorpyrifos is highly toxic to amphibians, and a recent study by the United States Geological Survey found that its main breakdown product in the environment, chlorpyrifos oxon, is even more toxic to these animals.
The substance is very toxic for aquaculture fishand bees.
A body burden study conducted by the Centers for Disease Control and Prevention found TCPy, a metabolite specific to chlorpyrifos, in the urine of 91% of people tested. An independent analysis of the CDC data claims that Dow has contributed 80% of the chlorpyrifos body burden of people living in the US. A 2008 study found dramatic drops in the urinary levels of chlorpyrifos metabolites when children switched from conventional to organic diets.
Air monitoring studies conducted by the California Air Resources Board (CARB) have documented chlorpyrifos in the air of California communities. Analyses of the CARB data indicate that children living in areas of high chlorpyrifos use are often exposed to levels of the insecticide that exceed levels considered acceptable by the EPA. Recent air monitoring studies in Washington and Lindsay, CA have yielded comparable results. Grower and pesticide industry groups have argued that the air levels documented in these studies are not high enough to cause significant exposure or adverse effects, but a follow-up biomonitoring study in Lindsay, CA has shown that people there have higher than normal chlorpyrifos levels in their bodies.
A study of the effects of chlorpyrifos on humans exposed over time showed that people exposed to high levels have autoimmune antibodies that are common in people with autoimmune disorders. There is a strong correlation to chronic illness associated with autoimmune disorders after exposure to chlorpyrifos.
Before it was banned from residential use in the US, chlorpyrifos was detected in 100% of personal indoor air samples and 70% of umbilical-cord blood collected from pregnant women 18–35 years old who self-identified as African American or Dominican and living in New York City public housing.
^Cao, Jun-li , Varnell, Andrew, and Cooper, Donald.(2011) Gulf War Syndrome: A role for organophosphate induced plasticity of locus coeruleus neurons. Available from Nature Precedings <http://hdl.handle.net/10101/npre.2011.6057.1> (2011)