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Pyrethroid Insecticides

Pyrethroids are chemicals that kill insects, including mosquitoes. They can be an important tool in helping to prevent the spread of West Nile virus. Mosquito control professionals mix pyrethroids with water or oil and apply it as an ultra low-volume spray that kills flying adult mosquitoes. When used properly, pyrethroids have been found to pose very little risk to human health and the environment. If you want to reduce your exposure to pyrethroid spray, stay indoors during spraying and for about 30 minutes afterwards.

What are pyrethroid insecticides and how are they used?

Pyrethroids are a group of man-made pesticides similar to the natural pesticide pyrethrum, which is produced by chrysanthemum flowers. Although more than 1,000 pyrethroids have been made, only a few are used in the United States. These include permethrin (Biomist®), resmethrin (Scourge®) and sumithrin (Anvil®). Pyrethroids are found in many commercial products used to control insects, including household insecticides, pet sprays and shampoos. Some pyrethroids also are used as lice treatments applied directly to the head and as mosquito repellents that can be applied to clothes.

How are pyrethroids used in adult mosquito control?

Most pyrethroid mosquito control products can be applied only by public health officials and trained personnel of mosquito control districts. Mosquito control professionals apply pyrethroids as an ultra low-volume (ULV) spray. ULV sprayers release very tiny aerosol droplets that stay in the air and kill adult mosquitoes on contact. Pyrethroids are often mixed with water or oil and applied at rates less than 1/100th of a pound of active ingredient per acre. These pesticides are approved by the U.S. Environmental Protection Agency (USEPA) for control of adult mosquitoes.

What happens to pyrethroids after they are sprayed?

After spraying, pyrethroids settle onto the ground and flat surfaces. Because pyrethroids are mixed with water or oil before being applied, the amount of residue left on surfaces is very small. Pyrethroids are broken down by sunlight and other chemicals in the atmosphere. Often, they last only one or two days in the environment.
Pyrethroids are not easily taken up by the roots of plants because they bind to the soil. Because of this, pyrethroids usually do not get into groundwater and do not contaminate drinking water supplies. Pyrethroids are eventually broken down in the soil.

How can pyrethroid exposure affect my health?

Pyrethroids are applied at very low levels to control mosquitoes. USEPA has evaluated these chemicals for this use and they have been found to pose very little risk to human health and the environment when used according to label directions. Exposure to the spray may aggravate existing respiratory conditions or affect sensitive individuals. Pyrethroids that enter the body leave quickly, mainly in the urine, but also in feces and breath.

Persons who apply pyrethroids and are accidentally exposed to very large amounts of these chemicals may experience dizziness, headache, nausea and diarrhea. Children exposed to large amounts of pyrethroids would be expected to be affected in the same way as adults. Adverse health effects would not be expected when pyrethroids are used according to label directions.

There is no evidence that pyrethroids cause birth defects in humans or affect the ability of humans to have children. Pyrethroids do not cause cancer in people.

How can I reduce my exposure to pyrethroids?

The main way to reduce your exposure to pyrethroids is to stay indoors during spraying and for about 30 minutes afterwards. Generally, there is no need to leave the area during mosquito control spraying. If you are outside during or shortly after spraying, you can breathe in air that contains pyrethroids. Toys that small children may place in their mouths should be taken indoors before spraying occurs to avoid exposure. Toys that are sprayed should be washed with soap and water. Skin contact with pyrethroids does not pose a risk because these chemicals are not well absorbed.

You also can be exposed by using household products and insecticides containing pyrethroids. If you decide to use these products, carefully follow the instructions on how to apply them properly and how long to wait before re-entering the treated area. Do not apply more than the recommended amount. The amount of pyrethroids used in gardening products can be two to three times more than what is used when fogging for insects.

Because pyrethroids and other chemicals are present in gardening products, they may be on the skins of fruits and vegetables. Make sure you wash fruits and vegetables thoroughly before eating them.

What about my pets?

To reduce exposure, keep pets indoors during and for about 30 minutes after spraying. Pets that remain outdoors could be exposed to small amounts of pyrethroids, but would not be expected to experience adverse health effects from spraying.

What about my swimming pool?

Pyrethroids are broken down in water and in sunlight. No special precautions are necessary; however, to reduce pyrethroids entering your pool water, cover your pool before spraying.

Can we run air conditioners at night?

Yes. Air conditioners may remain on. If you want to reduce possible exposure to pyrethroids in the air, set the air conditioner vent to the closed position, or choose the recirculate option.

Where can I get more information about pyrethroids and mosquito control?

More information about pyrethroid insecticides may be found on the USEPA Web site:

Information about mosquito control methods may be found at:

Where can I get more information on West Nile virus?

Call your local health department, the Illinois Department of Public Health at 217-782-5830, or visit the Department's Web site,

The U.S. Centers for Disease Control and Prevention's West Nile virus Web site is at