Occupational exposure standards such as the Threshold Limit Values (TLVs) established by ACGIH, Permissible Exposure Limits enforceable by OSHA, and the Recommended Exposure Limits (RELs) established by NIOSH are based on workplace settings. These occupational exposure standards are not appropriate reference standards for non-occupational exposures, such as household contact exposures, since family members are often subject to take-home exposures for extended and prolonged periods, including much of the day and night.
For non-occupational settings, a common practice of evaluating indoor air quality is by using outdoor air concentrations as comparison or reference values. Small quantities of asbestos fibers are ubiquitous in outdoor air, arising from natural sources (weathering of asbestos-containing minerals), from windblown soil from hazardous waste sites, deterioration of automobile clutches and brakes, or breakdown of asbestos-containing materials such as insulation, mainly chrysotile (ATSDR, 2001).
There is general agreement that very low background environmental exposures carry only an extremely low risk (Finnish Institute of Occupational Health, 1997). There have been several efforts to establish the historical ambient background concentration of asbestos. The early attempts to quantify ambient concentrations of asbestos did so in terms of weight and not fiber counts. The EPA reported two studies of ambient concentrations in the United States. Both studies reported that 100% of the samples (n = 314) were less than 100 ng/m³ (EPA, 1986). In 1984, the National Research Council reviewed all studies available and estimated that the median outdoor air (ambient) concentration ranged from 0.00002 to 0.00075 f/cc (National Research Council, 1984). In 1971, the median asbestos concentration in outdoor air measured in 48 U.S. cities was reported by Nicholson to be 0.00005 f/cc (National Research Council, 1984).
The Agency for Toxic Substances and Disease Registry (ATSDR) in their 2001 Toxicological Profile for Asbestos estimated “average concentrations of asbestos in ambient outdoor air are within the range of 10-8 to 10-4 PCM f/mL; levels in urban areas may be an order of magnitude higher than those in rural areas.” In addition, ATSDR estimates that the general population is typically exposed to a lifetime [70 years] ambient asbestos dose of 0.000014 f-yr/mL (ATSDR, 2001). In calculating this typical cumulative exposure attributed to ambient (outdoor) air, the ATSDR assumed a typical PCM concentration of 0.000002 f/cc and 10% of the time outdoors.
References
ATSDR. (2001). Toxicological Profile for Asbestos. Atlanta, Georgia: Agency for Toxic Substances and Disease Registry.
EPA. (1986, June). Airborne Asbestos Health Assessment Update,
EPA/600/8-84/003F. Table 5-1, p. 147.
Finnish Institute of Occupational Health. (1997). Asbestos, asbestosis, and cancer: the Helsinki criteria for diagnosis and attribution. Scand J Work Environ Health, 23, 311-316.
National Research Council. (1984). Asbestiform Fibers: Nonoccupational Health Risks. Washington, D.C.: National Academy Press.
Nicholson, J. W. (1971). Measurement of Asbestos in Ambient Air. Final Report, Contract CPA 70-92. National Air Pollution Control Administration.