"WASHINGTON — The government issued warnings on 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 in disposable foam plastic cups and plates, may cause cancer but is generally found in such low levels in consumer products that risks are low. ""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). ""Consumers can reduce their exposure to formaldehyde by avoiding pressed-wood products or buying only those that are labeled as U.L.E.F. (ultra-low-emitting formaldehyde), N.A.F. (no added formaldehyde) or C.A.R.B. (California Air Resources Board) Phase 1 or Phase 2 compliant. "