Occupational Health Assessment of Crystalline Silica and Respirable Dust Exposure in Taconite Mine Workers in Northeastern Minnesota

University of Minnesota Ph.D. dissertation.January 2016. Major: Environmental Health. Advisors: Jeffrey Mandel, Bruce Alexander. 1 computer file (PDF); xv, 98 pages.Introduction and objectives: This research effort investigated three study areas. Firstly, the impact of ATS/ERS “acceptability” and “repeatability” criteria for spirometry on the estimates of restrictive ventilatory defect was assessed in a population of taconite miners. The estimates of restrictive ventilatory defect were made using three different pulmonary function tests (spirometry, alveolar volume and diffusing capacity). Secondly, the association between cumulative silica exposures in taconite mining and non-malignant respiratory disease (NMRD) outcomes was evaluated. Cumulative silica exposure was determined using current and historical exposure measurements while health outcomes were assessed from a cross-sectional screening study of taconite mine workers. The final study area explored the joint effects of silica dust, elongate mineral particles (EMP) and non-silica respirable dust on exposure-NMRD association in miners, also using health outcomes from cross-sectional screening. Methods: A survey of current and former taconite workers was undertaken in 2010-2011. Miners were screened with a questionnaire that focused on occupational and medical history, followed by clinic examinations including lung tests (spirometry, chest x-rays). Current and former workers who completed the survey and performed all three pulmonary function tests (spirometry, alveolar volume and diffusing capacity) were assessed (n=1084). We applied American Thoracic Society (ATS/ERS) acceptability criteria for all tests and categorized subjects into groups according to whether they fully met, partially met, or did not meet acceptability criteria for spirometry. Obstruction and restriction were defined utilizing the lower limit of normal (lower five percent) for all tests. Mixed ventilatory defect groups were also described indicating coexisting obstructive and restrictive ventilatory defects. When using alveolar volume, restriction was identified after excluding obstruction. Occupational exposure assessment was performed which measured over 1,500 onsite samples for respirable dusts including silica in 28 major job functions in taconite mining. Historical exposures to dusts were estimated with data obtained from prior onsite exposure measures by existing mining operations, and from Mine Safety and Health Administration (MSHA) data for those same operations. Individual work histories from completed questionnaires were used to determine the length of time worked (years) in these jobs. Cumulative silica exposure ((mg/m3)-years) was estimated as a product of time worked and year-specific silica dust measures for each of 28 unique job functions. Forced vital capacity (FVC) less than lower limits of normal (LLN) for age, height, race and gender was used to determine spirometric restriction in participants with “usable” spirometry. Chest x-rays were evaluated using ILO criteria for any evidence of parenchymal abnormalities of 1/0 or greater and for pleural abnormalities suggestive of pneumoconiosis by two blinded B-readers, with a third reader to arbitrate disagreements. Prevalence ratios of association (PR), with 95% confidence intervals (CI), between silica exposures and lung disease outcomes were estimated using Poisson regression models. Regression models were adjusted for smoking, gender, age, BMI and estimation of commercial asbestos exposure. The last area of study focused on exploring possible associations between combined silica, EMP, and non-silica respirable dust exposures and NMRD prevalence. Non-silica respirable dust includes iron oxides and particulate matter (PM) generated from mining and processing the ore. PRs of association with NMRD outcomes were calculated each for silica, EMP and non-silica respirable dust as continuous variables. Using dichotomous exposures (high versus low levels determined by the median cumulative exposure), we then estimated the PRs for silica NMRD-association within strata of EMP and non-silica dust. Relative excess risk due to interaction (RERI), attributable proportions (AP) and synergy index (SI) with 95% CIs, were then estimated to assess interaction on the additive scale. On the multiplicative scale, separate models each were used for assessing silica-EMP interaction and a second model for silica-non-silica interaction, using corresponding product terms within the models. Results: Estimating restrictive ventilatory defect Only 519 (47.9%) tests fully met ATS/ERS spirometry acceptability criteria. Within this group, 5% had obstruction and 6%, restriction on spirometry. In contrast, among all participants (N=1,084), 16.8% had obstruction, while 4.5% had restriction. Alveolar volume restriction showed similar results in all groups after obstruction was excluded. Impaired gas transfer (Diffusing capacity) was identified in less than 50% of restriction identified by either spirometry or alveolar volume. BMI was significantly related to spirometric restriction in all groups. Association between silica and nonmalignant respiratory disease Spirometric restriction occurred in 7.2%; chest x-ray parenchymal abnormalities occurred in 5.4%; chest x-ray pleural abnormalities consistent with pneumoconiosis were observed in 16.8%; and symptoms of shortness of breath (dyspnea on exertion) occurred in 11.4% of the study population. Silica exposure was associated with restrictive ventilatory defect prevalence (PR= 1.40; 95%CI=1.08-1.81) and the prevalence of parenchymal changes on chest x-ray (PR= 1.30; 95% CI=1.00-1.69). Exploring respirable dusts joint effects in taconite mining Assessments for silica-EMP and silica-non-silica additive and multiplicative interactions were not statistically significant. The exposure with significant association with health outcome (on spirometry), of the three exposures studied, was silica. Conclusions: Population estimates of restriction using spirometry or alveolar volume varied by spirometric acceptability criteria. Other factors identified as important considerations in the estimation of restrictive ventilatory defect included increased BMI and gas transfer impairment in a relatively smaller proportion of those with spirometric restriction. Spirometric assessment suggested a 40% increase in association with NMRD prevalence for workers given silica exposure. Silica exposure was also associated with parenchymal chest x-ray findings. However, these associations were dependent on the approach for estimating exposure. The presence of EMP and non-silica dust did not significantly modify the relationship between silica exposure in taconite mining and NMRD on either the additive or the multiplicative scales. Overall, these insights are important when interpreting population-based physiological data in occupational settings and understanding lung disease associations of silica and other respirable dust exposures