Persistent pesticides, which primarily consist of organochlorine pesticides, are considered persistent because they are stable in the environment and resist being broken down. The ability of organochlorine pesticides to persist in the environment made them highly effective and therefore widely used in agriculture and insect control efforts during the 1940s-1970s. The organochlorine DDT also was used to control typhus and malaria. The properties that made these chemicals such effective insecticides also made them environmental hazards: they are stored in human fat tissue and slowly broken down by the body.
Some of the organochlorine pesticides have been banned for use in the United States, but others are the active ingredients of some home and garden products and some agricultural and environmental pest-control products.
There are four broad groups of organochlorine pesticides:
- Hexachlorocyclohexane (lindane)
- DDT and related compounds DDE and DDD
- Cyclodienes (aldrin, heptachlor, and others)
- Mirex and chlordecone
Eleven specific persistent pesticides within these groups were measured during the Churchill County leukemia study:
- Gamma-hexachloro-cyclohexane (lindane)
- Heptachlor epoxide
People are exposed to persistent pesticides in various ways:
- The primary route of exposure for the general population is by eating foods such as root and leafy vegetables, fatty meat, fish, and poultry that have been contaminated.
- Eating foods imported from countries that still allow the use of persistent pesticides.
- Breathing contaminated air or drinking contaminated water. Levels in air and water are generally low and of little concern. Air and water near waste sites and landfills may contain higher levels of persistent pesticides than air and water in other areas.
- Infants fed on human breast milk may ingest persistent pesticides from mothers who have been exposed.
- Exposure can also occur as a result of absorption through the skin. One organochlorine, gamma-hexachlorocyclohexane, also known as lindane, is used to treat lice and scabies and readily absorbed where contact is made with skin.
Acute large dose exposures to the organochlorines typically affect the nervous system, causing problems such as tremor, numbness and tingling of the extremities, and seizures.
Early signs of acute poisoning include disturbances of sensation of the face and extremities, headache, dizziness, nausea, vomiting, incoordination, and tremors. More severe poisonings may cause seizures and possibly coma.
Organochlorines can accumulate in a person’s body over time but the health effects associated with such exposure are not well defined. People who work with organochlorine pesticides for a long time have shown biochemical changes consistent with liver injury.
Whether the effects of organochlorine pesticides cause health problems in the general population is unknown. Whether exposure to organochlorine pesticides at the levels reported among the participants of the Churchill County leukemia study can cause health problems also is not known.
Studies of organochlorine exposure have not definitively established a link between exposure and the development of cancer. Early studies of the persistent pesticide DDT found an association between levels in the body and breast cancer. However, more recent, larger studies have shown no increased risk of breast cancer due to exposures to DDT. Further, workers heavily exposed to DDT have not been shown to have an increased incidence of cancer.
Some reports suggest a weak association between chlordane and heptachlor exposures and leukemias.