WASHINGTON — The government issued warnings Friday about two materials used daily by millions of Americans, saying that one causes cancer and the other might.

Government scientists listed formaldehyde as a carcinogen and said it is found in worrisome quantities in plywood, particle board, mortuaries and hair salons. They also said that styrene, which is used in boats, bathtubs and disposable foam plastic cups and plates, may cause cancer but is generally found in such low levels in consumer products whose risks are low.

Frequent and intense exposures in manufacturing plants are far more worrisome than the intermittent contact that most consumers have, but government scientists said that consumers should still avoid contact with formaldehyde and styrene along with six other chemicals that were added Friday to the government's official Report on Carcinogens. Its release was delayed for years because of intense lobbying from the chemical industry, which disputed its findings.

John Bucher, associate director of the National Toxicology Program, which produced the report, said that the evidence of formaldehyde's carcinogenicity was far stronger than for styrene and that consumers were more likely to be exposed to potentially dangerous quantities of formaldehyde.

The federal Occupational Safety and Health Administration warned in April that a hair-care product, Brazilian Blowout Acai Professional Smoothing Solution, contained unacceptable levels of formaldehyde, and salon workers have reported headaches, nosebleeds, burning eyes, vomiting and asthma attacks after using the product and other hair-straighteners.

Studies of workers like embalmers exposed to high levels of formaldehyde have found increased incidences of myeloid leukemia and rare cancers of the nasal passages and upper mouth.

Dr. Otis Brawley, chief medical officer at the American Cancer Society, said that formaldehyde is both worrisome and inescapable.

"It's the smell in new houses, and it's in cosmetics like nail polish," he said. "All a reasonable person can do is manage their exposure and decrease it to as little as possible. It's everywhere."

The report also lists aristolochic acids, found in plants and sometimes used in herbal medicines, as a known carcinogen and added to the list of probable carcinogens other substances like captafol (a fungicide no longer sold in the United States), finely spun glass wool fibers (used in insulation), cobalt-tungsten carbide (used in manufacturing), riddelliine (plants eaten by cattle, horses and sheep) and ortho-nitrotoluene (used in dyes).