NONCANCER INHALATION TOXICOLOGY OF CRYSTALLINE SILICA - EXPOSURE-RESPONSE ASSESSMENT

Silicosis from inhalation of silica has long been recognized as an occ upational hazard. Concern has arisen regarding the potential risk of s ilicosis from ambient silica (primarily quartz dust). This presentatio n reviews available data regarding ambient silica levels and estimates of the risk of silicosis at low exposure levels as they relate to the cli-rent U.S. Environmental Protection Agency National Ambient Air Qu ality Standards (NAAQS) for particulate matter. Current data indicate that for individuals not compromised by other respiratory ailments and for ambient environments expected to sustain 10% or less silica fract ion in particulate matter with a mean aerodynamic diameter of less tha n or equal to 10 mu m (PM10), maintenance of the 50 mu g/m(3) annual N AAQS for PM10 is adequate to protect agaist fibrotic effects from ambi ent silica exposures. Issues such as the large divergence of risk esti mates within the occupational setting (particularly at high cumulative exposures) and factors to consider for extrapolating risk in an occup ational setting to risk from ambient exposure are discussed.