An Evaluation of Chlorpyrifos
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Pesticides and Breast Cancer Risk: Chlorpyrifos
Fact Sheet #26, March 1999

This fact sheet reviews the information currently available on whether or not chlorpyrifos affects the risk of breast cancer. It also includes information on how chlorpyrifos is used, different ways by which people can come in contact with it, and how you can minimize your exposure to this chemical.

What is chlorpyrifos?

Chlorpyrifos is a synthetic chemical used to kill insects (insecticide). It belongs to the group of chemicals called "organophosphate pesticides." DursbanTM and LorsbanTM are two common trade names of insecticides that contain chlorpyrifos.

Why was chlorpyrifos chosen for an evaluation?

Chlorpyrifos was chosen to be evaluated because it is widely used in both agricultural and home settings. It has been found to be present in the air and dust of treated homes. While chlorpyrifos is known to be toxic to the nervous system, whether or not it causes cancer has not been well studied.

Does chlorpyrifos cause breast cancer?

There have been no studies of the incidence of breast cancer in women with past exposure to chlorpyrifos. Chlorpyrifos fed to experimental animals over a long time did not cause an increase in the incidence of mammary (breast) tumors. There are gaps in our knowledge, but the evidence available so far does not suggest an increase in breast cancer risk from exposure to chlorpyrifos.

Are there other ways by which chlorpyrifos may affect breast cancer risk?

Chlorpyrifos was not found to act like the female hormone estrogen in animals or in cells growing in a laboratory. Some studies have observed that it causes harmful changes, called mutations, in cells growing in a laboratory, but others have not observed such an effect. One study found disturbances in the immune system of eight people who had been exposed to different amounts of chlorpyrifos. Our immune system helps us fight infections as well as cancer and we recommend that the effects of chlorpyrifos on the immune system be studied in more detail.

Does chlorpyrifos cause any other types of cancer?

While studies have shown that chlorpyrifos may affect the nervous system of people exposed to this insecticide through their work, there have been no studies of cancer risk in these people. Experimental animals fed chlorpyrifos over a long time were not found to have an increased number of tumors than untreated animals. However, flaws in these studies do not allow a definite answer about whether chlorpyrifos does or does not cause cancer. Considering its toxicity, unnecessary exposure should be avoided and people who were exposed to chlorpyrifos in the past should be followed for cases of cancer.

How is chlorpyrifos used in farming?

Chlorpyrifos is used to kill insects that would otherwise damage crops and seeds. It can kill insects upon contact, or when it enters the insect's body. The major use of chlorpyrifos in farming is to protect corn. It is also used to protect cotton crops and fruit trees against insects. Chlorpyrifos protects crops against a wide variety of insect pests such as aphids, corn borers and cutworms. It is also used to treat soils and seeds to protect seeds and young plants against insect damage.

The farm use of chlorpyrifos in the 1990s doubled compared to the 1980s. In the years 1990 to 1993, it was estimated that over 14 million pounds of chlorpyrifos was used for agriculture each year in the US, ranking it as the tenth most highly used insecticide in the country. In the same time period, 218 thousand pounds of chlorpyrifos was used for agriculture each year in New York State (NYS). It is the eighth most highly used insecticide for agriculture in NYS.

How is chlorpyrifos used in non-farm settings?

The amount of chlorpyrifos used for non-agricultural purposes is even higher than its use for agriculture. Commercial manufacture of chlorpyrifos started in 1969. Since then, chlorpyrifos has been used in many different indoor areas such as homes, offices, schools, hotels, hospitals and restaurants. Its use in homes and workplaces has increased in the 1990s.

Chlorpyrifos is used in spot-treatments of cracks and crevices in homes against cockroaches, and in ant traps. Until recently, it was common to spray chlorpyrifos over a carpeted area, or use chlorpyrifos-containing flea bombs to get rid of fleas and ants in homes. There was a concern that chlorpyrifos sprayed indoors could settle on floors, carpets, toys, clothes, drapes and furniture and lead to children coming in contact with this insecticide. Toddlers frequently put their hands into their mouths and chew on toys and may thus take in more of the chlorpyrifos that settles in a room after treatment, than an adult.

In response to this concern, since 1998 chlorpyrifos is no longer available for use in sprays and foggers inside homes and on pets. It is still available for spot treatments of cracks and crevices in homes and in flea collars for pets. Chlorpyrifos is also used in the treatment of foundations of homes and in pressure-treated wood to protect homes against fire ants and termites. In outdoor areas, chlorpyrifos is sprayed on turfgrass and ornamental plants to control insect pests.

How do federal agencies regulate chlorpyrifos to protect the consumer?

Who might be exposed to chlorpyrifos?

People most likely to be exposed to this insecticide are:

Small amounts of chlorpyrifos can pass through the skin, but the levels of exposure are highest when it is breathed in through the air, or eaten through food and water.

Is chlorpyrifos found in food and water?

The very small amounts of chlorpyrifos found in food and water are not considered to be a cause for concern. Chlorpyrifos does not last long in water, and its levels fall to half within a day. It has been found in groundwater around cornfields and orchards where it is most used, but at levels that are much lower than the health advisories set by EPA.

Where is more research needed?

Is more research being done?

A large group of female and male agricultural workers are being surveyed for pesticide exposure and cancer in an ongoing study at the National Cancer Institute. A study at the University of Wisconsin, Madison seeks to understand how organophosphate pesticides can alter the immune system. A study at the University of California, Davis, will investigate if there is an association between exposure to organophosphate pesticides and frequency of injuries among 500 Hispanic migrant farmworkers. Another study, at the University of California, Riverside, will compare different clothing materials for the protection that they offer to applicators.


How can I minimize exposure to chlorpyrifos?

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Prepared by Renu Gandhi, Ph.D., BCERF Research Associate
and Suzanne M. Snedeker, Ph.D., Research Project Leader, BCERF

When reproducing this material, credit the authors and the Program on Breast Cancer and Environmental Risk Factors in New York State.