Construction workers should be protected from breathing man-made mineral fibers (MMMF). The fibers — glass, ceramic, and rockwool and slagwool — are used for heat and sound insulation. Ceramic fibers are made from alumina, silica, and other metal oxides. Rockwool and slagwool are made from molten rock.

The man-made fibers are sometimes used as a "safe" replacement for asbestos. All kinds of asbestos can cause crippling lung diseases and cancers.

No one can be sure how safe MMMFs are. Some of them were produced more than 100 years ago, but they were not used much in construction until the 1970s. Many experts say it will take more time to see if work with these fibers makes workers sick. Asbestos diseases usually take 20 years or more to show up.

Studies have found that glasswool and ceramic fibers cause cancer in animals. And the International Agency for Research on Cancer, part of the World Health Organization, says refractory ceramic fibers "possibly" cause cancer in people. (See http://www-cie.iarc.fr/htdocs/monographs/vol81/81.html )

It is best to be careful. The most dangerous fibers may be ones so small you can't see them. If you think you may be working with MMMFs, ask your employer. You may need to use respirators. If you need more information, contact the National Institute for Occupational Safety and Health, NIOSH (1-800-356-4674, or www.cdc.gov/niosh), or OSHA (1-800-321-OSHA or www.osha.gov).