The National Cancer Institute said in a study released today that it has found "little evidence" that formaldehyde causes cancer among the 1.3 million workers who are exposed to it in more than 50,000 factories.

"When compared to the general U.S. population, deaths from all causes and from all cancers among the exposed workers were about as expected," the NCI said in a mortality study of 26,000 workers, the largest investigation undertaken of the widely used chemical. " . . . Cancer overall was not related to formaldehyde exposure."

The four-year, $1 million study was strongly criticized by the United Auto Workers and the Amalgamated Clothing and Textile Workers and by a former top government epidemiologist. They said NCI may be understating the cancer danger. And they criticized the involvement of the Formaldehyde Institute and officials of the Dupont and Monsanto chemical companies in designing and writing the study.

The cancer institute reported a 32 percent "excess" in lung-cancer deaths among workers after 20 years from their first exposure to the substance and found higher-than-expected rates of upper-respiratory cancer.

But NCI, in a conclusion questioned by critics, said these increased cancers could not be linked to formaldehyde, because workers with longer and heavier exposure did not show higher cancer rates than those with lesser exposure.

Nine billion pounds of formaldehyde are produced annually for use in making plastics, plywood, textiles and numerous other products, and it is found in small amounts in many household products. It has been found to cause cancer in laboratory animals. It is classified as a human irritant but has not been established as a human carcinogen.

The debate over formaldehyde has intensified as the Occupational Safety and Health Administration prepares to begin hearings in May on whether to lower the legal standards for work place exposure.

OSHA has estimated that the risk of cancer from formaldehyde is 71 to 620 cases for every 100,000 workers exposed at the legal limit. The agency said lowering exposures could substantially reduce the risk -- although the NCI study appears to cast doubt on that assumption.

"I am skeptical of the NCI study," Dr. Philip J. Landrigan, former director of epidemiology for the National Institute of Occupational Safety and Health (NIOSH), said in an interview.

"Thirty-two percent is a lot of excess death, and it is hard to dismiss that without good reason . . . . I think the NCI study needs to be studied and gone through with a fine-toothed comb," said Landrigan, now director of occupational medicine at Mt. Sinai Medical Center in New York.

NIOSH, a division of the Department of Health and Human Services, is also investigating formaldehyde and is the only agency with legal authority to require companies to participate in such health studies. NCI's study gained voluntary access to factories and company data with the assistance of the Formaldehyde Institute, a trade group.

Dr. Lawrence Fine of the University of Michigan, one of six scientists on NCI's advisory panel for the study, said he is "uncomfortable" with the wording of its conclusion: "I am uncomfortable with saying the study is essentially negative . . . or positive. My phraseology would have been different . . . . It is a gray-zone study." Other advisers were not available for comment.

The study's director, Dr. Aaron Blair, said he is "absolutely confident" about the study's methods and conclusions. "I did not say there was no evidence [linking formaldehyde and cancer]. I said there is little evidence."

Blair said the study, like all NCI projects, has been extensively reviewed by outside advisers and that they agreed with the study methods and key conclusions.

Blair said cigarette smoking is among possible explanations for the excess cancers, but said the study did not have enough data to draw a conclusion about that factor.

NCI's study was co-written by nine researchers -- two from NCI, two from Dupont, one from Monsanto and four outside consultants. The Formaldehyde Institute paid for a preliminary feasibility study, but NCI paid for and supervised the study, begun in 1981.

The study included 10 factories, including two of Dupont's, and examined company records for workers employed from 1938 to 1965, tracing the cause of death for 3,268 persons.

Among these workers, 151 died of lung cancer, an "excess" of 37 people or 32 percent above normal death rates, NCI said. Most other causes of death were well within expected rates for a sampling of 26,000, NCI said.

NCI made "historical" estimates for 1938 to 1965, based on interviews and records, to determine whether particular workers were exposed to low, medium or high doses of formaldehyde. Because the low, medium and high groupings did not show progressively higher cancer rates, Blair said, formaldehyde cannot be blamed as the cause.

The auto workers and the clothing and textile workers unions, seeking tighter controls on formaldehyde, said they are skeptical of the "NCI-Dupont-Monsanto study" because companies may have influenced selection of plants to be examined and the interpretation of data.

But NCI's Blair, the primary author, said the institute had "complete control" of the study methodology. "I write, and I interpret," he said. NCI often coordinates its studies with researchers from companies, universities and labor unions, he added, and pains are taken to have outside "peer review" to assure that sound methods are used.

Dupont's director of epidemiology, Dr. Maureen O'Berg, said the three corporate coauthors' involvement consisted of "reviewing data and interpreting and helping to write and edit the text."

"We were not blind to the potential criticism" of company participation, O'Berg said, but added that NCI clearly "took the lead" and controlled the study.

The Formaldehyde Institute issued a statement entitled "Evidence of Formaldehyde Safety Reported to Industry." President John F. Murray said the NCI report "underscores our conviction that formaldehyde can be used safely in the work place and in consumer products, as it has been in the United States for nearly 100 years."