When workers with poor health decrease their exposure but healthy workers do not, it becomes difficult to detect an association even when exposure causes disease. This healthy worker survivor bias is particularly problematic in studies of relationships between long-term occupational exposures and chronic diseases. Satisfactory solutions to the healthy worker survivor effect using conventional regression analysis do not exist. Fortunately, alternative methods have been developed that do eliminate this type of bias; only recently have these g-methods been applied in an occupational setting. Our group successfully applied them to a large mortality study and found clear evidence for relationships that had eluded researchers using more conventional methods. Recent work extends the use of these methods to a continuous exposure, and we have developed a public health approach that evaluates explicitly the impacts of enforcing new exposure limits. Workers in the diatomaceous earth industry are exposed to crystalline silica, which causes silicosis and other non-malignant respiratory diseases and is now classified as a carcinogen. Previous analyses of cohorts of diatomaceous earth workers have not adjusted for healthy worker survivor bias. Relatively weak lung cancer results and protective estimates for cardiovascular outcomes may be further biased due to censoring by competing risks. We therefore aim to apply g-methods to address these two sources of bias in hopes that doing so will enable us to uncover clearer evidence of causal relationships between crystalline silica and mortality from lung cancer, COPD, and heart disease. A general industry permissible exposure limit (PEL) of 100g/m3 has been in effect for silica since 1971 and has never been updated; however, hearings were held in March 2014 regarding a proposed federal standard that introduces a new PEL of 50g/m3. We will evaluate the potential impact of the proposed exposure limit for respirable crystalline silica by estimating years of life saved, evaluating effects of interventions rather than comparing groups of workers with different exposure histories. This project addresses the Mining Sector and the Cancer, Reproductive and Cardiovascular Disease NORA cross-sector program. Its rigorous analytical methods will provide clearer epidemiologic evidence for the effects of inhaled silica, framing results in terms of impacts of occupational exposure limits using a metric that is easy to communicate to policymakers and the general public. This is directly relevant to NIOSH's r2p initiative. Outcomes and Outputs will include scientific research papers in peer-reviewed journals and presentations at conferences. By reducing bias in etiologic studies of workplace exposure and evaluating the potential impacts of occupational exposure limits, the project will lead to better control of occupational health hazards.
This project aims to control healthy worker survivor bias in order to obtain better estimates for the effects of occupational exposure to crystalline silica on mortality from lung cancer, non-malignant respiratory diseases, and cardiovascular diseases. Application of g-estimation using a public health framework will allow us to estimate of the number of years of life that could have been saved by enforcing exposure limits in a cohort of diatomaceous earth workers.
