Exposure limits and guidelines for particles not otherwise classified or regulated (PNOC/R) evolved from several important studies of pneumoconiosis performed early in the 20th century. These studies indicated that exposure to dust concentrations of less than 50 million particles per cubic foot (mppcf) and with low crystalline silica concentrations did not appear to lead to the development of disabling lung disease. Consequently, dusts containing less than 5 percent crystalline silica were termed "nuisance dusts," and an exposure limit of 50 mppcf was recommended. With the advent of mass sampling technology and the ability to sample directly for respirable dust, exposure limits migrated to total and respirable nuisance dust exposure limits of 15 and 5 mg/rn3, respectively, for dusts containing less than 1 percent crystalline silica. Regulatory exposure limits were established by incorporating these exposure guidelines that were in existence when the enabling legislation was enacted. Changes to regulatory limits have lagged behind threshold limit values adopted by the American Conference of Governmental Industrial Hygienists (ACGIH), which have been updated based on current research. Current ACGIH recommendations for inhalable and respirable dusts are 10 and 3 mg/m3, respectively. Recent studies of ultrafine particulate and toxicological investigation of the pulmonary overload phenomena from exposure to PNOC/R indicate that exposure to these materials at sufficiently high concentrations can cause irreversible changes to the airways. The term "nuisance dust" was inaccurate, so the terminology of PNOC/R was substituted as a better descriptor for dusts without component-specific exposure limits. Further reductions in exposure limits and guidelines have been proposed for control of occupational exposure to PNOC/R. These proposals are motivated by increasing recognition that clinically significant chronic air-ways obstruction is caused by occupational dust exposure.
Frank J. Hearl, Division of Respiratory Disease Studies, National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention, U.S. Public Health Service, 1095 Willowdale Road, Morgantown, WV 26505-2888
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