Chlorpyrifos is an organophosphate insecticide. Pure chlorpyrifos is made up of white or colorless crystals. It has a slightly skunky odor, like rotten eggs or garlic. Chlorpyrifos is used to control many different kinds of pests, including termites, mosquitoes, and roundworms. Chlorpyrifos was first registered as an insecticide in 1965 and the United States Environmental Protection Agency (US EPA) re-registered it in 2006. The only legal indoor use for chlorpyrifos is in containers with treated baits.

Products with chlorpyrifos in them are used in agriculture in cattle ear tags. They may be used on golf courses, and to control fire ants and mosquitoes for public health purposes. Products containing chlorpyrifos are also used to treat wood fences and utility poles. Use of chlorpyrifos on most food crops was revoked in 2022 by the U.S. EPA because of concerns related to dietary exposure, especially for children. However, the following year certain food uses were reinstated following legal challenges. Regulatory updates are ongoing.

Chlorpyrifos can be harmful if it is touched, inhaled, or eaten. Chlorpyrifos works by blocking an enzyme which controls messages that travel between nerve cells. When the enzyme is blocked, the nervous system can't send normal signals. This causes the nervous system to malfunction and this is how it eventually kills the pest.

People can be exposed to pesticides by eating them, breathing them in, or getting them on the skin or in the eyes. You could be exposed to chlorpyrifos if you apply products containing chlorpyrifos either as part of your job or outside of your own home. If a bait station in the house contains chlorpyrifos, people or pets could be exposed if the bait station is broken. People could be exposed to chlorpyrifos if their well water has been contaminated. This can happen if products containing chlorpyrifos were used near the well for termite control. Risks can be reduced by always reading the entire label and following all instructions.

Chlorpyrifos moves to all parts of the body after exposure. Chlorpyrifos itself is not toxic, but when the body tries to break it down, it creates a toxic form. This toxic form, called chlorpyrifos oxon, binds permanently to enzymes which control the messages that travel between nerve cells. When chlorpyrifos binds to too many of the enzymes, nerves and muscles do not function correctly. The body then must make more enzymes so that normal nerve function can resume. The body can break down and excrete most of the unbound chlorpyrifos in feces and urine within a few days. Chlorpyrifos that finds its way into the nervous system may stay there much longer.

Researchers have exposed rats and mice to chlorpyrifos in laboratory studies to see if chlorpyrifos would cause tumors. Chorpyrifos was not linked to cancer by any of these studies. The U.S. EPA has decided that there is "evidence of non-carcinogenicity for humans" for chlorpyrifos.

Scientists exposed laboratory rats, mice, and dogs to low levels of chlorpyrifos for long periods of time. Some animals lost weight, and some of their internal organs got bigger and they had other tissue changes that were not normal. Many studies have shown nervous system effects that were similar to those seen in short-term exposures. Sometimes the test animals seemed to be able to tolerate the effects of chlorpyrifos over time. We don't know why or how this happens.

Long-term exposure to chlorpyrifos in people caused the same nervous system effects as short-term exposures did, but did not cause any additional health problems. Some people have suffered delayed nervous system damage if they were exposed to very large amounts of chlorpyrifos. This is very rare, and scientists and doctors do not understand it very well.

Chorpyrifos exposure was linked to changes in social behavior and brain development as well as developmental delays in young laboratory animals. Other studies showed that chlorpyrifos affected the nervous system of young mice, rats, and rabbits more severely than adult animals.

Researchers studied the blood of women who were exposed to chlorpyrifos and the blood of their children from birth for three years. Children who had chlorpyrifos in their blood had more developmental delays and disorders than children who did not have chlorpyrifos in their blood. Exposed children also had more attention deficit disorders and hyperactivity disorders.

In general children may be more sensitive to pesticides than adults. One reason for this is that their bodies may break down pesticides differently. Children are also more likely to be exposed to pesticides when playing and may put their hands in their mouths more often than adults. Children may also be more sensitive to exposures because they have more surface area of skin for their body size than adults.

Chlorpyrifos is very toxic to many bird species such as grackles and pigeons, and it is moderately toxic to others such as mallard ducks. Mallard ducks fed chlorpyrifos laid fewer eggs and raised fewer ducklings. The eggshells were thinner than normal, and many of the young ducklings died. Of all birds, robins are most often found dead following accidents involving chlorpyrifos use.

Chlorpyrifos is also very toxic to fish and aquatic invertebrates. It may build up in the tissues of fish and other animals that eat smaller animals. This is known as bioaccumulation.

Chlorpyrifos is very toxic to bees. It can poison non-target insects for up to 24 hours after it is sprayed. Chlorpyrifos can be toxic to earthworms for up to 2 weeks after it is applied to soil.