Quantitative relationships between exposure to respirable quartz and risk of silicosis at one Scottish colliery

The British coal industry’s Pneumoconiosis Field Research included intensively detailed characterisation of exposure to respirable dust and quartz. Adverse mining conditions in one of the collieries studied led to some unusually high quartz exposures, resulting in atypically rapid radiological progression. The availability of detailed research data from this colliery has permitted a close re-examination of the form of the exposure-response relationship between silicotic appearances on radiographs and exposure to respirable quartz. Statistical analyses based on logistic regression compared the predictive ability of a variety of exposure metrics, with different weightings of concentration and time since exposure. Indices based on the square of concentration gave the best fits. Additional models treating separately exposures from quartz concentrations <2 mg.m-3 and >2 mg.m-3 also gave better predictions than with simple cumulative exposure. Improvements from differential weightings of time were negligible, yielding no evidence for an additional effect of residence time. This may be because of the narrow range of residence times for the important silica exposures. The results demonstrate the high risks associated with even brief exposures to high airborne concentrations, which must be avoided. For lower concentrations, logistic regression models predicted a risk of category 2/1+ opacities of about one quarter of one per cent (0.25%) for 15 years’ exposure to an average concentration of 0.02 mg.m-3 respirable quartz. Predictions differed slightly depending on choice of regression model and associated assumptions. A range of such predictions, combined with knowledge of the typical relationships between maximum exposure limits and mean concentrations, can inform the debate over choice of suitable exposure limits for quartz. “”