The government is reviewing the risks of dioxin, spurred partly by studies that suggest small amounts of the chemical may not be as dangerous to humans as was believed.

William K. Reilly, administrator of the Environmental Protection Agency, said Wednesday it was important not to prejudge the results of the reassessment of dioxin, long considered one of the most toxic substances in the world."There has been much speculation about the effect of these new developments on our revised dioxin risk assessment," Reilly said in a letter launching the review. "Some factors may decrease the level of concern. Others may result in estimates of increased risk."

Dioxin is an unwanted byproduct of certain chemical reactions. It has contaminated some weed and insect killers, including the herbicide Agent Orange used extensively in Vietnam, and is produced in tiny amounts by paper mills that use chlorine bleaches.

The substance appears to be the most powerful cancer-causing agent ever tested in lab rodents. Recent studies have suggested that exposure to small amounts of dioxin may not be associated with a substantial cancer risk in humans, but large amounts might be.

Exposure to dioxin also can produce a skin disorder, chloracne.