NIOSH recently visited the site of a prescribed burn near a former vermiculite mine in Montana to evaluate wildland firefighters’ exposures to asbestos. Management at the federal agency responsible for managing approximately 28,000 acres of Kootenai National Forest, including 10,000 acres of land centered on the former mine, was concerned about firefighters’ exposures during prescribed burn activities. According to NIOSH, the geological deposit where the vermiculite was mined contains amphibole asbestos. The prescribed burn observed by agency staff occurred just outside Operable Unit 3 of the Libby Asbestos Superfund Site, which surrounds the former vermiculite mine and areas impacted by the releases from the mine. The agency stresses that exposure to Libby amphibole results in the same types of adverse health effects that are seen with exposure to other asbestos fibers, including lung cancer, mesothelioma, and asbestosis. On the day of the prescribed burn, NIOSH personnel observed work processes including briefing and preparation, fire line construction, burn, mop-up, and decontamination. Agency staff also reviewed the respiratory protection program and collected air samples for asbestos and total fibers. NIOSH’s health hazard evaluation report states that exposures to total fibers in air were less than the lowest occupational exposure limit. For asbestos, the OSHA permissible exposure limit, the NIOSH recommended exposure limit, and the ACGIH threshold limit value are 0.1 fibers per cubic centimeter of air. However, NIOSH was not able to determine fiber concentrations in air during the burn tasks due to overloading of the filters. “It is likely that the filters were overloaded due to an excess of organic material resulting from the burning vegetation,” the agency’s report reads. “It is also very likely that the greatest potential for exposure to asbestos fibers would be during the fire line construction and mop-up tasks, as these activities have the greatest direct disruption of soil.” NIOSH’s sampling results confirmed that the highest concentrations of total fibers in air were found during tasks such as fire line construction and dry mop-up, which involved greater plant and soil disturbance and where water was not used. These same tasks were associated with the most asbestos fibers detected in the air samples collected by NIOSH. Fire line construction involves constructing a perimeter around the proposed burn site by digging into the forest floor to create a break in the fuel, and dry mop-up involves physically disrupting soil and roots to completely put out a fire. NIOSH’s report urges the agency to use wet mop-up procedures whenever possible to help reduce the amount of dust and potential asbestos exposures. NIOSH personnel also identified aspects of the respiratory protection program that could be improved. For example, agency staff observed firefighters with facial hair, which can compromise the seal of the respirator to the worker’s face. They also observed that the fit-testing equipment used was not calibrated according to the manufacturer’s specifications. Further details are available in NIOSH’s full report (PDF).