Public release date: 14-Mar-2008
Men and women who worked in a plant that processed vermiculite tainted with asbestos-like fibers that originated from a mine in Libby, Montana, show high prevalence of scarring and thickening of the membrane that lines the chest wall some 25 years after the plant stopped using the material—even those who were exposed at or below current legal levels.
Vermiculite is a mineral that expands to nearly 20 times its original size when rapidly heated and has a number of consumer applications, from gardening products to loose-fill home insulation. The “Libby vermiculite” was first suspected of causing lung damage in the late 1970s, when researchers documented a cluster of bloody pleural effusions among workers who handled it. At one time, the Libby mine produced up to 80 percent of the vermiculite used around the world.
In 1980, researchers examined 513 individuals who worked at a plant that processed Libby vermiculite and found pleural changes or interstitial fibrosis in 2.2 percent of the overall cohort. In this follow-up study, a research team led by James Lockey, M.D., the principal investigator of the 1980 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.
The high prevalence surprised even Dr. Lockey, senior researcher at the University of Cincinnati. “I expected to see a higher rate of x-ray changes, but was surprised at the percentage,” he said. “We found that even low levels of exposure to asbestos-like fibers can cause thickening of the membrane that lines the chest wall.”
Of the 431 workers from the original group who were still living, 280 participated in the follow-up study and were interviewed about their lung health and work history, including particular exposure level and the numbers of years they worked. They were then given chest x-rays, which were assessed for pleural plaques, thickening and interstitial changes by professional radiologists.
When the researchers analyzed the workers with pleural changes by exposure levels, they found a significant trend of increasing changes with increased exposure. Workers with highest exposure levels had an average of 6 to 16 times the risk of pleural changes when compared to those who were minimally exposed. Moreover, the changes were significant even at levels of exposure currently 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.”