Health Effects

Chlorpyrifos is an organophosphorous pesticide known for its damaging effects on the human nervous system. Like all organophosphates, chlorppyrifos blocks an enzyme (called acytyl cholinesterase) that our brains need to control nerve impulses. These neurological effects pose especially elevated risks for children because their nervous systems are still developing.

Symptoms of low-dose exposure may include headaches, agitation, inability to concentrate, weakness, tiredness, nausea, diarrhea and blurred vision. Higher doses can lead to respitory paralisis and death. Pregnant women may also be more sensitive to chlorpyrifos toxicity according to the Agency for Toxic Substances and Disease Registry.

In addition to the immediate effects of exposure, chlorpyrifos is linked to a number of serious health impacts:

  • Nervous System Damage: Exposure to low levels of chlorpyrifos may interfere with healthy development of the human nervous system. In addition to inhibiting crucial brain enzymes, research suggests that chlorpyrifos disrupts the basic cellular machinary that manages brain cell and synapse growth. A study of expectant mothers in New York City found that those with the highest chlorpyrifos exposure were the most likely to give birth to children with attention deficit hyperactivity disorder (ADHD) and who score poorly on tests of cognitive development.
  • Reduced Birth Weight: Studies on pregnant women exposed to chlorpyrifos through involuntary home pesticide use demonstrate a link between in utero exposure to chlorpyrifos and low birth weights and reduced head circumference of newborns. These effects were most significant for individuals whose genetic makeup is such that they produce low levels the enzyme responsible for detoxifying chlorpyrifos in the body.
  • Endocrine Disruption: Chlorpyrifos is also a suspected endocrine disrupting compound; moderate doses have been shown to alter hormone levels in animal studies.
  • Lung and Prostate Cancer: While the U.S. EPA lists chlorpyrifos as providing evidence of no carcinogenicity, recent studies suggest possible links to both lung and prostate cancer.

Exposure & Body Burden

Human exposure to chlorpyrifos is evidenced by its concentration in our bodies. Data from the Center for Disease Control and Prevention showed chlorpyrifos breakdown products in 93% of U.S. residents sampled between 1999 and 2000. Humans can be exposed to chlorpyrifos that drifts from nearby fields, or from residue in common foods such as apples, peaches, and sweetpeppers.

Children carry particularly high levels of chlorpyrifos---almost twice those of adults, the CDC study showed. Chronic exposure levels were 4.6 times the “acceptable” level for children (6-11 years) and 3.0 times the “acceptable” level for youth (12–19 years) (See: Chemical Trespass). Farmers, pesticide applicators, and chlorpyrifos manufacturing workers likewise carry a greater body burden of the neurotoxic insecticide (CDC).

Poisonings & Drift

Like most organophosphates, chlorpyrifos is prone to drift. The semi-volatile chemical  readily evaporates from leaf and soil surfaces to become airborne, especially when outdoor temperatures are high. Once in gas form, the neurotoxin can migrate to nearby homes and schools---puting residents and their children at risk.

A drift study in Lindsay, California, demonstrated the presence of chlorpyrifos in the air near homes in this agricultural community. Over 100 air samples were collected near homes and three- quarters of the samples had detectable levels. Only 11 percent of the samples were above the levels determined to be “acceptable” for a 24-hour exposure by children. The highest concentration observed was nearly eight times the allowable level.

Among pesticide poisoning cases, chlorpyrifos is a frequent culprit. From 1997 to 2000, chlorpyrifos drift from agricultural fields resulted in group poisonings in California's Ventura, Tulare, Merced and Madera counties. For more on chlorpyrifos poisonings & drift see: Fields of Poison 2002: California Farmworkers and Pesticides and Second-Hand Pesticides: Airborne Pesticide Drift in California.


For animals that are highly sensitive to chlorpyrifos, exposure to minute concentrations can be lethal. The U.S. EPA indicates that a single application of chlorpyrifos poses significant risks--especially to endangered species. Fish, amphibians, birds, reptiles, small mammals, as well as bees and other beneficial insects are vulnerable to the potent insecticide.

Chlorpyrifos is moderately persistent in soil and can take weeks to years to break down. The insecticide can also reach rivers, lakes and streams, where it concentrates in the fatty tissue of fish. According to the National Water Quality Assessment Program, chlorpyrifos contaminates surface water in urban and agricultural streams at levels potentially harmful to aquatic life.

Chlorpyrifos can also travel long distances to remote areas far from its source. The Arctic Monitoring and Assessment Program 2009 Arctic Pollution reports the presence of chlorpyrifos in a number of locations:

  • Surface water, ice, & fog from the Bering & Chukchi seas
  • Alaskan snow and fish from Alaskan parks
  • Arctic & Subarctic Canadian lakes


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