Development of a chronic inhalation reference level for respirable crystalline silica

Chronic inhalation exposure of workers to crystalline silica can result in silicosis. The general public can also be exposed to lower levels of crystalline silica from quarries, sand blasting, and entrained fines particles from surface soil. We have derived an inhalation chronic reference exposure level for silica, a level below which no adverse effects due to prolonged exposure would be expected in the general public. Incidence of silicosis and silica exposure data from a cohort of 2235 white South African gold miners yielded a reference level of 3 microg/m3) for respirable silica (particle size as defined occupationally) using a benchmark concentration approach. Data from cohorts of American gold miners, Chinese tin miners, diatomaceous earth workers, and black South African gold miners yielded similar results with a range of 3-10 microg/m3. Strengths of the chronic reference exposure level include the availability of several large long-term studies of inhalation in workers at varying exposure concentrations, adequate histopathological and radiologic analysis, adequate follow-up of exposed workers, a dose-response effect in several studies, observation of a No Observed Adverse Effect Level in the key study, and the power of the key study to detect a small effect. Uncertainties include the general underestimation of silicosis by radiography alone and the uncertainties in exposure estimation.