Researchers at the University of Cincinnati say previous studies of vermiculite -- a mineral with a flaky, fluffy-looking structure -- revealed the vermiculite ore mined in Libby, Mont., contained increased levels of an asbestos-like mineral fiber that can become airborne and inhaled during manufacturing.

In the 25-year follow-up study of workers at a plant which stopped using Libby vermiculite in 1980, current chest X-rays revealed that 20 percent of workers who experienced low cumulative exposure to these fibers had changes in the lining around their lungs. In the group with the highest exposure, changes on chest X-rays were noted in 54 percent of workers.

"Workers with low-level exposures to Libby vermiculite ore may not have obvious health effects right away, but the past exposure is something of which their physicians should be aware," research investigator Dr. James Lockey says in a statement.

Until the Montana mine was closed in 1990, it provided up to 80 percent of the world's vermiculite supply -- used in home insulation, packing materials, construction materials and gardening products.

The findings are published in the American Journal of Respiratory and Critical Care.