No one can be sure how safe MMMFs are. Some of them were made 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.
Some studies suggest rockwool and slagwool can cause cancer in people.
The International Agency for Research on Cancer, part of the World Health Organization, says ceramic fibers, glasswool, rockwool, and slagwool may cause cancer. It is best to be careful. The most dangerous fibers may be so small you can't see them. If you think you may be working with MMMFs, ask your contractor. You may need to use respirators.