Workers at a plant that processed vermiculite with asbestos-like fibers have scarring and thickening of the membrane that lines the chest walls, 25 years after the plant stopped using the material. In a study published in the American Journal of Respiratory and Critical Care Medicine, researchers show similar results even for workers who were exposed to the material - from a mine in Libby, Montana - at or below current legal levels.

Vermiculite is a natural mineral that can expand to about 20 times its original size with the application of heat. The mineral is used in several consumer products, such as loose-fill home insulation, soil conditioner or additive, and packaging materials. After researchers found bloody pleural effusions (blood in the fluid-filled space surrounding the lungs) among workers who handled vermiculite in the 1970s, it was suspected that the "Libby vermiculite" caused lung damage. One of the largest and oldest vermiculite mines in the United States, the Libby mine produced about 80 percent of the vermiculite used around the world.

The study, conducted by James Lockey, M.D. (University of Cincinnati) and colleagues, is a follow-up to a 1980 investigation of 513 workers at a plant that processed Libby vermiculite. In 1980, researchers found pleural changes or interstitial fibrosis in 2.2% of the workers. The follow-up study reveals that 28.7% of the still-living members of the original cohort had pleural changes and 2.9% had interstitial fibrosis.

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

In the follow-up study, 280 of the 431 still-living workers from the original group participated. They were interviewed about lung health and work history, providing data about particular exposure levels and the numbers of years they worked. Participants were also given chest x-rays, allowing professional radiologists to identify pleural plaques, thickening, and interstitial changes.

Results of the research suggest that there exists a significant trend of increasing pleural changes as exposure levels increase. Among workers who had the highest exposure levels, there was about a 6 to 16 times increase in risk of pleural changes compared to minimally-exposed workers. In addition, the pleural changes were significant at levels of exposure that current law permits.

An accompanying editorial, Gregory Wagner, M.D. (National Institute for Occupational Safety and Health), mentions that, "a significant number of workers exposed at the current limit would experience pleural abnormalities." However, legal exposure limits to hazardous materials, as specified by government regulations, do not apply to all types of fibers that have similar and predictable biological effects. Often, they only apply to specific fibers.

Lockey adds, "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." Currently, there are six types of asbestos that are regulated, "but other existing types of fibers that share similar characteristics are not." Lockey's research emphasizes the need to further analyze how occupational exposure affects health.

"The initial Lockey investigation found a relatively modest prevalence of pleural abnormalities," writes Dr. Wagner. This follow-up "found over 10 times that level, despite the fact that contaminated vermiculite had been removed from the production process by 1980."

As a caution to consumers, Dr. Lockey suggests that home improvements that require contact with vermiculite insulation should be done by professionals; however, consumers face minimal risk from most products that contain or contained vermiculite from the Libby mine.

Low-Level Fiber-induced Radiographic Changes Caused by Libby Vermiculite: A 25-Year Follow-up Study
Amy M. Rohs, James E. Lockey, Kari K. Dunning, Rakesh Shukla, Huihao Fan, Tim Hilbert, Eric Borton, Jerome Wiot, Cristopher Meyer, Ralph T. Shipley, Grace K. LeMasters, and Vikas Kapi
American Journal of Respiratory and Critical Care Medicine (2008). 177:630-637.
doi:10.1164/rccm.200706-841OC
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: Peter M Crosta