The Food and Drug Administration plans to ban use of a potentially cancer-causing chemical in cosmetics but to allow its continued use in decaffeinating coffee. The chemical is methylene chloride, a solvent found in paint removers, aerosols and other manufacturing processes.

The FDA argues that its use in cosmetics, primarily in aerosol hair sprays, "may pose a significant risk to the public health" sufficient to take it off the market. But the agency maintains that the risk from ingestion of the chemical in tiny amounts in decaffeinated coffee "is so low as to be essentially nonexistent."

Rep. Ted Weiss (D-N.Y.), head of a House subcommittee that oversees FDA, thinks otherwise. He considers the FDA proposal to allow continued use of methylene chloride in coffee a violation of the law governing FDA's actions, particularly the Delaney Clause.

For more than two decades, the congressionally-mandated Delaney Clause has forbidden the use of food or color additives known to cause cancer in animals or humans. Weiss, chairman of a House Government Operations subcommittee, said Congress intended Delaney as an "absolute prohibition. There is no amount of a cancer-causing additive which is permitted under the law."

But in recent years, there has been increasing debate about the interpretation of the Delaney provision. Although Congress has thus far failed to tackle head on the issue of whether the FDA should have more room for judging the safety of a given chemical, the agency has begun to move on its own through administrative actions.

In a notice in the Dec. 17 Federal Register, FDA proposed to ban methylene chloride as an ingredient in all cosmetic products, citing studies showing that inhalation of the chemical causes cancer in animals -- rats and mice -- and poses a possible cancer risk to humans.

Using animal studies, the agency estimated that the lifetime cancer risk to hair-care workers may be about one in 1,000, about l0 times greater than for consumers who use aerosol hair sprays each day. Many manufacturers have already eliminated the chemical from their products.

FDA's review of studies in which methylene chloride was fed in lesser amounts in drinking water to rats and mice showed no statistically significant incidence of tumors.

The FDA concluded that when the chemical is used in decaffeinating coffee, the low residue in the beverage poses a risk no greater than one cancer per million people over a lifetime of consumption and "probably closer to one in 100 million."

"Given such a low level of risk, FDA has concluded that there would be no safety gain to the public if it interpreted the Delaney Clause to require a ban on this use of methylene chloride," said the proposal.

Although various processes may be used to remove caffeine from coffee, the FDA said that the majority of decaffeinated coffee uses methylene chloride. There is no labeling requirement for its use.

FDA is taking public comments until Feb. 18.

POLICE ACTIONS. . . In addition to approving new products, the FDA is responsible for policing foods, drugs and cosmetics to make sure manufacturers are obeying the law. About 25 cents of every consumer dollar spent comes under FDA's purview.

Consumer groups have frequently accused the Reagan administration of lax enforcement and complained that the agency's stagnant budget has meant too few people in the field. FDA officials, in turn, have argued that they are as vigilant as ever, but pursue voluntary approaches before turning to punitive measures.

The agency's year-end review of 1985 enforcement actions shows a mixed record -- a dramatic jump in recalls last year, with other actions largely steady or dropping slightly:

*Product recalls rose to 2,085 in 1985, up from 1,408 in 1984 and higher than in any year since at least 1977. FDA spokesman Faye Peterson said the rise was due in part to large-scale actions against products that failed to label the presence of yellow dye No. 5 or sulfites, both of which produce allergic reactions in some consumers.

*Recommendations to the Department of Justice for criminal prosecutions or injunctions to stop operations were both down slightly. Requests to seize products were slightly higher in 1985, but much lower than during the Carter administration. Peterson had no statistics on how often Justice followed the FDA's advice.

*Plant inspections dropped to the lowest level since at least 1977. Regulatory letters sent by FDA field offices -- a sort of warning letter before further action -- were also down over 1984 but higher than in many previous years.

*The FDA has started recording voluntary corrective actions requested of manufacturers that made further mandatory actions unnecessary. They numbered above 4,000 in 1985, up from 1983 and 1984, but lower than in 1982.

Allen Greenberg, of the Nader-founded Public Citizen Health Research Group, which in past years has attacked the FDA's enforcement record, said his group had not studied the new numbers. But he said that, with the exception of recalls, the levels "still appear much lower than in the pre-Reagan era . . . . The general antiregulatory attitude still seems to be the prevailing one at FDA."