Workers who processed vermiculite tainted with asbestos-like fibers 25 years ago show high prevalence of scarring and thickening of the membrane that lines the chest wall, including people exposed below legal levels.

In 1970 workers who had handled "Libby vermiculite" (so named after the Libby, Montana mine where the tainted fibers were first noted), exhibited a cluster of bloody pleural effusions. The Libby mine at one time produced up to 80 percent of the vermiculite used around the world.

513 former workers at a plant that processed Libby vermiculite were checked by researchers in 1980, and pleural changes or interstitial fibrosis were found in 2.2 percent of them. A team led by James Lockey, M.D., the principal investigator of this report, found that the unadjusted prevalence in the still-living members of the original cohort was 28.7 percent for pleural changes and 2.9 percent for interstitial fibrosis. The findings are published in the second issue for March of the American Thoracic Society’s American Journal of Respiratory and Critical Care Medicine.

"I expected to see a higher rate of x-ray changes, but was surprised at the percentage," said. Dr. Lockey. "We found that even low levels of exposure to asbestos-like fibers can cause thickening of the membrane that lines the chest wall."

280 workers of the 431 workers stlll living were interviewed for the follow-up study, to determine their lung health and work history, as well as their exposure level and the number of years worked. Chest X-rays were reviewed by professional radiologists for pleural plaques, thickening and interstitial changes.

A significant trend of increasing changes with increased exposure was detected by researchers analyzing workers with pleural changes. Those with the highest exposure levels had an average of 6 to 16 times the risk of pleural changes compared with those minimally exposed. Equally important, changes were significant at levels permitted by law.

The findings indicate that "a significant number of workers exposed at the current limit would experience pleural abnormalities," wrote Gregory Wagner, M.D., of the National Institute for Occupational Safety and Health, in an accompanying editorial. Furthermore, regulations governing legal exposure limits to hazardous materials apply only to specific fibers, not to all types of fibers that have similar and predictable biological effects.

"When humans are exposed to any mineral fibers that are long, thin and durable in human tissue and can reach the pleural membrane, these fibers can cause health problems," said Dr. Lockey. "Six types of asbestos are currently regulated, but other existing types of fibers that share similar characteristics are not."Perhaps most importantly, the research highlights the need to anticipate the health implications of occupational exposures.

"The initial Lockey investigation found a relatively modest prevalence of pleural abnormalities," wrote Dr. Wagner. But the current study "found over 10 times that level, despite the fact that contaminated vermiculite had been removed from the production process by 1980."

Consumers encountered minimal risk from most products containing Libby vermiculite, said Dr. Lockey, but advised that any home improvements that involve contact with vermiculite insulation should be left to professionals.

Source: American Journal of Respiratory and Critical Care Medicine, March, 2008 (second issue).