Do current OELs for silica protect from obstructive lung impairment? A critical review of epidemiological data

Inhalation of respirable crystalline silica (RCS) can lead to serious health effects such as silicosis and lung cancer. There also seems to be a general consensus to consider that RCS exposure is associated with obstructive lung impairment or chronic obstructive pulmonary disease (COPD), a leading cause of mortality, morbidity, and disability worldwide. It is, however, not clear whether occupational exposure levels (OELs), generally set to prevent silicosis, also protect workers from developing an obstructive impairment. This review aims at clarifying the potential relationship between RCS exposure and obstructive lung impairment as defined by spirometry. Eleven studies that reported both silica exposure levels and spirometry results were identified. We systematically extracted data pertaining to (a) the population studied, (b) level of exposure to RCS and other pollutants, (c) spirometry procedure and interpretation, and (d) methodology used to investigate the relationship between RCS exposure and spirometry. These studies add supporting evidence in favor of a qualitative association between occupational activities exposing to RCS and obstructive lung dysfunction. However, no well-founded quantitative estimate can be drawn from these investigations; the available relevant literature does not allow defining a RCS exposure threshold associated with an increased risk of obstructive lung dysfunction, as defined by spirometry, in workers without silicosis. Further research is needed, but, as highlighted in this review, conducting epidemiological studies with both valid exposure and outcome measurements is a real challenge.