What it is: Chlordane belongs to a chemical class known as the cyclodienes, which also includes heptachlor, aldrin and dieldrin.

How it is used: The chemicals have been widely used against termites for almost 40 years, largely because cyclodienes do not break down easily in the soil and can keep killing insects for many years. Most commercial termiticide products contain a mixture of chlordane and heptachlor.

Health concerns: The chemicals have been shown to cause cancer in laboratory rats and liver damage in other animal species tested. The EPA classifies chlordane as a "probable human carcinogen." Cyclodienes also are suspected of causing neurological damage. At high levels, usually the result of improper application, vapors can cause headaches, dizziness, muscle spasms and nausea.

Previous actions: Chlordane and the other cyclodienes were banned for most agricultural uses in the mid-1970s. The chemical was left on the market as a termiticide because there were no effective substitutes and the EPA believed the chemical would not contaminate the air inside houses.

Why the new controls: Studies show that chlordane can contaminate the air even in houses where it has been properly applied. EPA officials consider the health risk unacceptable because effective substitutes for chlordane have been developed.

Yesterday's action: No chlordane products can be sold, but pest control companies can continue to use chemicals in stock. The chemical has not been banned and may return to the market if its manufacturer can demonstrate ways to apply it without contaminating houses.