ever, exposure occurs in all countries when buildings, ships, and other structures insulated with asbestos are demolished, when asbestos is removed, or during maintenance and repair of asbestos-containing materials.

Consideration needs to be given to the different measurement methods used when interpreting and comparing the reported levels of airborne exposure in various settings. Historically, airborne asbestos in workplaces was measured with a midget impinger to collect the fibers, a standard occupational hygiene method, and concentrations were expressed as millions of particles per cubic foot (mppcf). More recently, airborne fibers have been collected on membrane filters, and concentration has been reported in terms of either mass (such as nanograms per cubic meter, ng/m3) or number of fibers (such as fibers per milliliter, f/ml). The latter measure is most commonly used. In water, concentrations may be expressed in terms of fibers per liter. In a given measurement system, fibers may qualify for counting on the basis of criteria such as length (for instance, over 5 μm) or aspect (length:diameter) ratio (for instance, over 3:1), characteristics also relevant to their potential to cause health effects.

Fibers may be counted with either phase-contrast microscopy (PCM) or transmission electron microscopy (TEM). Of the two, TEM is the more sensitive and may measure higher concentrations in the same environment than PCM, because PCM may miss very thin fibers. In addition, PCM may fail to distinguish asbestos from other types of fibers. However, workplace exposures are generally measured with PCM, which is less expensive and considered adequate by regulatory agencies. Conversion between different measures of airborne units is problematic because conversion factors vary with the distribution of fiber thickness and length in the environment of interest. The most valid approach to conversion involves obtaining measurements simultaneously under the same conditions using the different methods for which conversion factors are needed. Using that approach, Dement et al. (1983) found factors for converting PCM to TPM data within the same facilities in the textile industry that ranged from 2.5 to 7.5 f/ml :: 1 mppcf.

Occupational Exposure

Asbestos concentrations observed in occupational settings have been orders of magnitude higher than the highest concentrations observed in residential settings, but some in-home activities, such as shaking out work clothes, can produce levels that may rival those found in the workplace. The highest well-documented exposures have been among workers manufacturing asbestos products or employed in mining and milling operations. Table 4.1 provides selected summary statistics for some asbestos-product manufacturing facilities in the United States based on samples



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