The Labor Department yesterday tightened standards for occupational exposure to formaldehyde, but only a fraction of the 2 million workers exposed to the cancer-causing chemical are expected to benefit.

Workers may not be exposed to more than one part per million of formaldehyde vapors in an eight-hour shift under the new standard, and employers must ventilate factories and conduct medical surveillance when levels reach .5 parts per million. Since 1970, the maximum permissible workplace level of the toxic gas has been three parts per million.

Assistant Labor Secretary John A. Pendergass said the new standard "will eliminate significant risks" of diseases among workers in foundries, medical laboratories and the pressed wood and apparel industries, where the chemical is widely used.

But union leaders branded the regulation too lax, charging the department's Occupational Safety and Health Administration was failing to protect hundreds of thousands of workers exposed to levels less than .5 parts per million. At those lower levels, formaldehyde is known to cause serious eye, skin and lung irritation.

According to OSHA, 200,000 of the nation's 2.2 million workers who have contact with formaldehyde are exposed to levels exceeding .5 parts per million, including 117,000 in the apparel industry, where the chemical is used in permanent-press fabrics.

Challenging OSHA's estimate of apparel-industry beneficiaries, Eric Frumin, safety and health director of the Amalgamated Clothing and Textile Workers, said no more than 1,000 apparel workers are exposed to more than .5 parts per million. But more than 500,000 union members work in levels of .2 parts per million to .3 parts per million, he said.

He called the new rules "a complete failure in preventing formaldehyde-related cancer and other health problems for half a million workers."

Formaldehyde is a major industrial chemical used chiefly as an ingredient in other chemicals and resins and as a preservative of biological tissue. Nearly 6 billion pounds of the pungent gas, which causes nasal cancer in laboratory rodents, were produced in 1985. According to Frumin, the Environmental Protection Agency identified increased cancer risks at levels below .5 parts per million.

Fifteen unions petitioned OSHA to lower the 3 parts per million standard in 1981, were rebuffed by the agency, sued and finally obtained a court order last month directing OSHA to draft new regulations. Yesterday's announcement was OSHA's response.