An analysis of occupational crystalline silica (14808607) (quartz) exposures as reported in epidemiological studies of silicosis was performed to determine airborne concentrations of quartz that have not been associated with an increased risk of silicosis (the no observable adverse effect level, NOAEL), the lowest concentrations associated with silicosis (the lowest observable effect level, LOAEL), and to identify exposure response models that could be used for risk assessments. Six epidemiologic studies that reported evidence of an exposure response for silicosis were reviewed. In four studies that were conducted in South African gold miners, Ontario hardrock miners, North Carolina dusty trades workers, and British gypsum workers, silicosis was defined on the basis of chest X-ray analyses. Three of these studies were retrospective cohort studies. The fourth study, conducted in North Carolina workers, was a case/referent study. The other two studies conducted in South Dakota gold miners and Vermont granite workers were mortality studies in which silicosis was diagnosed on the basis of silicosis or pneumoconiosis on the death certificates. The NOAELs and LOAELs determined from five studies varied from 7 to 100, and 8 to 252 micrograms per cubic meter, respectively. A NOAEL and LOAEL could not be determined from the Ontario miners study because the results were presented in terms of a dose response model that implied risk at any level of exposure. Two studies, the South African gold miners and Ontario hardrock miners, provided exposure response models suitable for silicosis risk assessments. The cumulative risk assessment from the South African gold miner study was 0.127, which was at least 20 times higher than that obtained in the Ontario hardrock miner study which varied from 0.0009 to 0.0062. The authors conclude that the difference in the risk estimates and the NOAELs and LOAELs may have reflected errors in exposure estimates, differences in the physicochemical characteristics of the silica in the dust, cohort differences, or reader variability. The results of this analysis also raise concerns about the adequacy of current standards for preventing silicosis in the United States. Further research is needed to precisely define the dose response relationship between quartz exposure and silicosis.