The Environmental Protection Agency announced yesterday that it has launched reviews of four pesticides believed to pose unreasonable health risks, including alachlor, the nation's most widely used corn and soybean herbicide.

The Monsanto Chemical Co., which sells alachlor under the trade name Lasso, voluntarily canceled the chemical's use on potatoes last year and agreed to train farmers in its safe use before this spring's planting season.

But the EPA, which calls alachlor "a probable human carcinogen," said it remains concerned about potential health effects of the chemical.

According to risk documents released yesterday, persons who eat foods treated with alachlor, including meat and dairy products from animals that eat treated grains, may face lifetime cancer risks as high as one in 10,000. Farmers who mix and spray the chemical may face substantially higher risks.

But the EPA said that its conservative risk estimates may, in this case, not be conservative enough. Recent studies suggest that current testing methods may be finding as little as 10 percent of the chemical's residue in foodstuffs.

Alachlor also has been found in streams and groundwater in some intensive corn-growing areas of the Midwest, posing a potential additional threat to drinking water supplies, the agency said.

The other pesticides put under review are TPTH (triphenyltin hydroxide), used primarily on pecans; captafol, used to control foliage diseases on apples, cherries, citrus fruit and tomatoes; and dinocap, used primarily on apples. Dinocap and TPTH have been shown to cause birth defects in laboratory animals; captafol is a suspected carcinogen.

The agency also announced that it would ban the last remaining use of the pesticide DBCP -- on pineapple fields in Hawaii -- as soon as stocks are depleted. The soil fumigant is leaching into Hawaiian drinking water supplies, the agency said.