BACKGROUND: Although general population studies of air pollution suggest that particulate matter - diesel exhaust emissions in particular - is a potential risk factor for cardiovascular disease, direct evidence from occupational cohorts using quantitative metrics of exposure is limited. In this study, we assess counterfactual risk of ischemic heart disease (IHD) mortality under hypothetical scenarios limiting exposure levels of diesel exhaust and of respirable mine/ore dust in the Diesel Exhaust in Miners Study (DEMS) cohort. METHODS: We analyzed data on 10,779 male miners from 8 non-metal, non-coal mines - hired after diesel equipment was introduced in the respective facilities - and followed from 1948 to 1997, with 297 observed IHD deaths in this sample. We applied the parametric g-formula to assess risk under hypothetical scenarios with various limits for respirable elemental carbon (a surrogate for diesel exhaust), and respirable dust, separately and jointly. RESULTS: The risk ratio comparing the observed risk to cumulative IHD mortality risk at age 80 under a hypothetical scenario where exposures to elemental carbon and respirable dust are eliminated was 0.79 (95% confidence interval (CI): 0.64, 0.97). The corresponding risk difference was -3.0% (95% CI: -5.7, -0.3). CONCLUSION: Our findings, based on data from a cohort of non-metal miners, are consistent with the hypothesis that interventions to eliminate exposures to diesel exhaust and respirable dust would reduce IHD mortality risk.

We commend the overall evaluation by Cherrie et al. (2013) of the current occupational exposure limits (OELs) for respirable poorly-soluble low toxicity (PSLT) particles. As described in that paper, the epidemiological studies provide compelling evidence that exposure to PSLT at the current OELs has been associated with adverse health effects, including pulmonary fibrosis and lung function deficits. In contrast to Cherrie et al. (2013), we discuss here that the chronic inhalation studies in animals also provide evidence of the adverse pulmonary effects of PSLT. | For example, we would like to clarify or correct the following statements (p. 688, 2nd column): | (1) …the phenomenon of rat lung overload has little relevance for human lung response at high lung burden of low-toxicity dust. | This statement is not entirely supported by the scientific evidence. While it is correct that differences have been observed in the rat and human lung clearance and retention kinetics for respirable particles, these differences have been well described, and can be accounted for, using lung dosimetry models in humans (Kuempel et al., 2001a,b; Gregoratto et al., 2010, 2011) and rats (Tran et al., 1999, 2000; Anjilvel and Asgharian, 1995). Because of the slower pulmonary clearance in humans, particles can build up in the lungs at exposures below those that would cause overloading in rats (Snipes, 1989; Kuempel et al., 2000; Kuempel and Tran, 2002). This build up is associated with the movement of particles into the alveolar interstitium of the mammalian lungs (Nikula et al., 1997, 2001). Only at overloading do the particle lung burdens in rats reach the higher levels that have been reported in coal miners, i.e., up to 10 mg g−1 lungs or more in rats (Morrow, 1988; Muhle et al., 1990; Bellmann et al., 1991; Oberdörster et al., 1992) and in humans (Attfield et al., 1994; Kuempel et al., 1997; Tran and Buchanan, 2000).

Because our original publication (1) generated considerable interest over the past two years, we now provide additional data from our nested case-control study that are valuable in evaluating the relation between diesel exhaust exposure and lung cancer. Here we include results based on alternative exposure metrics (Table 1) and alternative approaches for adjusting for cigarette smoking as a confounder (Table 2), for comparison with results that appeared in Table 3 of Silverman et al. (1).

OBJECTIVES: To evaluate respiratory related mortality among underground coal miners after 37 years of follow-up. METHODS: Underlying cause of death for 9033 underground coal miners from 31 US mines enrolled between 1969 and 1971 was evaluated with life table analysis. Cox proportional hazards models were fitted to evaluate the exposure-response relationships between cumulative exposure to coal mine dust and respirable silica and mortality from pneumoconiosis, chronic obstructive pulmonary disease (COPD) and lung cancer. RESULTS: Excess mortality was observed for pneumoconiosis (SMR=79.70, 95% CI 72.1 to 87.67), COPD (SMR=1.11, 95% CI 0.99 to 1.24) and lung cancer (SMR=1.08; 95% CI 1.00 to 1.18). Coal mine dust exposure increased risk for mortality from pneumoconiosis and COPD. Mortality from COPD was significantly elevated among ever smokers and former smokers (HR=1.84, 95% CI 1.05 to 3.22; HRK=1.52, 95% CI 0.98 to 2.34, respectively) but not current smokers (HR=0.99, 95% CI 0.76 to 1.28). Respirable silica was positively associated with mortality from pneumoconiosis (HR=1.33, 95% CI 0.94 to 1.33) and COPD (HR=1.04, 95% CI 0.96 to 1.52) in models controlling for coal mine dust. We saw a significant relationship between coal mine dust exposure and lung cancer mortality (HR=1.70; 95% CI 1.02 to 2.83) but not with respirable silica (HR=1.05; 95% CI 0.90 to 1.23). In the most recent follow-up period (2000-2007) both exposures were positively associated with lung cancer mortality, coal mine dust significantly so. CONCLUSIONS: Our findings support previous studies showing that exposure to coal mine dust and respirable silica leads to increased mortality from malignant and non-malignant respiratory diseases even in the absence of smoking.

OBJECTIVES: We examined the potential influences of certain selection factors on the utility of the Coal Workers' Health Surveillance Program (CWHSP) data for tracking disease distribution and trends. METHODS: We combined data from the CWHSP and the Energy Information Administration to examine any influence of variable worker participation on observed disease prevalence. We evaluated effects of differential participation by coal mining region, temporal changes in employment, and active surveillance efforts. RESULTS: The published findings of pneumoconiosis distribution and trends from the CWHSP were robust compared with the various participation factors that might have affected their validity for population-based estimates of disease burden. Exploration of factors that could potentially bias the findings generally led to small increases in the primary estimates, mostly for the early years of the program. CONCLUSIONS: We confirmed previously reported findings that there was a high prevalence of coal worker pneumoconiosis (CWP) around 1970-1974, a substantial decline in 1995-1999, and indications of an increase since then. Overall our findings suggest that the previously reported distribution and trends in CWP prevalence were broadly accurate. (Am J Public Health. Published online ahead of print May 16, 2013: e1-e6. doi:10.2105/AJPH.2012.301051).

OBJECTIVES: We better defined the distribution and determinants of coal workers' pneumoconiosis (CWP) among US underground coal miners. METHODS: We obtained chest radiographs from the mobile unit of an enhanced surveillance program begun in 2005 by the National Institute for Occupational Safety and Health for underground coal miners. B Readers classified them for presence of pneumoconiosis. RESULTS: Miners from 15 states participated (n = 6658). The prevalence of CWP was higher in 3 states (Kentucky, 9.0%; Virginia, 8.0%; West Virginia, 4.8%) than in 12 other states (age-adjusted risk ratio [RR] = 4.5; 95% confidence interval [CI] = 3.3, 6.1). Miners in these 3 states were younger and had less mining tenure, but advanced CWP (category ≥ 2/1; RR = 8.1; 95% CI = 3.9, 16.9) and progressive massive fibrosis (RR = 10.5; 95% CI = 3.8, 29.1) was more prevalent among them. Advanced CWP and progressive massive fibrosis were more prevalent among workers at mines with fewer than 155 miners, irrespective of mining region, than among workers at larger mines. CONCLUSIONS: Enhanced surveillance results confirmed the persistence of severe CWP among US coal miners and documented the health consequences of inadequate dust control for miners in parts of Appalachia and at smaller mines. (Am J Public Health. Published online ahead of print March 8, 2012: e1-e5. doi:10.2105/AJPH.2011.300427).

BACKGROUND: Current information points to an association between diesel exhaust exposure and lung cancer and other mortality outcomes, but uncertainties remain. METHODS: We undertook a cohort mortality study of 12,315 workers exposed to diesel exhaust at eight US non-metal mining facilities. Historical measurements and surrogate exposure data, along with study industrial hygiene measurements, were used to derive retrospective quantitative estimates of respirable elemental carbon (REC) exposure for each worker. Standardized mortality ratios and internally adjusted Cox proportional hazard models were used to evaluate REC exposure-associated risk. Analyses were both unlagged and lagged to exclude recent exposure such as that occurring in the 15 years directly before the date of death. RESULTS: Standardized mortality ratios for lung cancer (1.26, 95% confidence interval [CI] = 1.09 to 1.44), esophageal cancer (1.83, 95% CI = 1.16 to 2.75), and pneumoconiosis (12.20, 95% CI = 6.82 to 20.12) were elevated in the complete cohort compared with state-based mortality rates, but all-cause, bladder cancer, heart disease, and chronic obstructive pulmonary disease mortality were not. Differences in risk by worker location (ever-underground vs surface only) initially obscured a positive diesel exhaust exposure-response relationship with lung cancer in the complete cohort, although it became apparent after adjustment for worker location. The hazard ratios (HRs) for lung cancer mortality increased with increasing 15-year lagged cumulative REC exposure for ever-underground workers with 5 or more years of tenure to a maximum in the 640 to less than 1280 mcg/m(3)-y category compared with the reference category (0 to <20 mcg/m(3)-y; 30 deaths compared with eight deaths of the total of 93; HR = 5.01, 95% CI = 1.97 to 12.76) but declined at higher exposures. Average REC intensity hazard ratios rose to a plateau around 32 mcg/m(3). Elevated hazard ratios and evidence of exposure-response were also seen for surface workers. The association between diesel exhaust exposure and lung cancer risk remained after inclusion of other work-related potentially confounding exposures in the models and were robust to alternative approaches to exposure derivation. CONCLUSIONS: The study findings provide further evidence that exposure to diesel exhaust increases risk of mortality from lung cancer and have important public health implications.

BACKGROUND: Most studies of the association between diesel exhaust exposure and lung cancer suggest a modest, but consistent, increased risk. However, to our knowledge, no study to date has had quantitative data on historical diesel exposure coupled with adequate sample size to evaluate the exposure-response relationship between diesel exhaust and lung cancer. Our purpose was to evaluate the relationship between quantitative estimates of exposure to diesel exhaust and lung cancer mortality after adjustment for smoking and other potential confounders. METHODS: We conducted a nested case-control study in a cohort of 12,315 workers in eight non-metal mining facilities, which included 198 lung cancer deaths and 562 incidence density-sampled control subjects. For each case subject, we selected up to four control subjects, individually matched on mining facility, sex, race/ethnicity, and birth year (within 5 years), from all workers who were alive before the day the case subject died. We estimated diesel exhaust exposure, represented by respirable elemental carbon (REC), by job and year, for each subject, based on an extensive retrospective exposure assessment at each mining facility. We conducted both categorical and continuous regression analyses adjusted for cigarette smoking and other potential confounding variables (eg, history of employment in high-risk occupations for lung cancer and a history of respiratory disease) to estimate odds ratios (ORs) and 95% confidence intervals (CIs). Analyses were both unlagged and lagged to exclude recent exposure such as that occurring in the 15 years directly before the date of death (case subjects)/reference date (control subjects). All statistical tests were two-sided. RESULTS: We observed statistically significant increasing trends in lung cancer risk with increasing cumulative REC and average REC intensity. Cumulative REC, lagged 15 years, yielded a statistically significant positive gradient in lung cancer risk overall (P(trend) = .001); among heavily exposed workers (ie, above the median of the top quartile [REC ≥ 1005 mcg/m(3)-y]), risk was approximately three times greater (OR = 3.20, 95% CI = 1.33 to 7.69) than that among workers in the lowest quartile of exposure. Among never smokers, odd ratios were 1.0, 1.47 (95% CI = 0.29 to 7.50), and 7.30 (95% CI = 1.46 to 36.57) for workers with 15-year lagged cumulative REC tertiles of less than 8, 8 to less than 304, and 304 mcg/m(3)-y or more, respectively. We also observed an interaction between smoking and 15-year lagged cumulative REC (P(interaction) = .086) such that the effect of each of these exposures was attenuated in the presence of high levels of the other. CONCLUSION: Our findings provide further evidence that diesel exhaust exposure may cause lung cancer in humans and may represent a potential public health burden.

BACKGROUND: Digital radiography systems are replacing traditional film for chest radiographic monitoring in the recognition of pneumoconiosis. METHODS: To further investigate previous findings regarding the equivalence of film screen radiographs (FSR) and storage phosphor computed radiographs (CR), FSR and CR from 172 underground coal miners were classified independently by seven NIOSH-approved B Readers, using the International Labour Office (ILO) classification of radiographs of pneumoconiosis. RESULTS: More CR were classified as "good" quality compared to FSR (prevalence ratio (PR) =1.5; 95% CI, 1.4-1.6; P<0.001). B Readers showed good overall agreement on scoring small opacity profusion using CR versus FSR, (weighted kappa =0.58, 95% CI 0.54-0.62). Significantly more irregular opacities (compared to rounded) were classified using CR images compared to FSR (PR=1.3; 95% CI=1.1-1.6; P=0.01). Similarly, the smallest size opacities (width <1.5 mm, p and s-type) were reported more frequently using CR vs. FSR images (PR=1.3; 95% CI= 1.1-1.5; P<0.001). Inter- and intra-reader agreement was lower with respect to the classification of shape/size than for small opacity profusion. Overall, inter- and intra-reader variability did not significantly differ using CR versus FSR. CONCLUSIONS: Under optimal conditions using standardized methods and equipment, reader visualization of small pneumoconiotic opacities does not appear to meaningfully differ whether using CR or FSR. Variability in ILO classifications between imaging modalities appears considerably less than variability between readers. The well-documented challenge of reader variability does not appear to be resolved through the use of digital imaging alone, and additional approaches must be evaluated.

OBJECTIVE: To assess whether the recent increases in the prevalence of coal workers' pneumoconiosis (CWP) in the USA reflect increased measured exposures over recent decades, and to identify other potential causative factors. METHODS: The observed CWP prevalence was calculated for 12 408 underground coal miner participants in the Coal Workers' Health Surveillance Program for the period 2005-2009, stratified by the Mine Safety and Health Administration (MSHA) geographical districts. The predicted prevalence was estimated using a published exposure-response model from a large epidemiological study among US coal miners using dust exposure, tenure, miner's age and coal rank as predictors. chi(2) Testing was performed to compare the observed versus predicted CWP prevalence. RESULTS: Observed prevalence was significantly higher than predicted prevalence in MSHA districts 4-7 (central Appalachian region) (10.1% vs 4.2%; prevalence ratio (PR) 2.4; p<0.001) and significantly lower than predicted in other regions (1.6% vs 3.6%; PR 0.4; p<0.001). The central Appalachian region had a significantly older workforce with greater mining tenure, a lower proportion of mines with 200 or more employees, and lower seam heights. Significant lower average compliance dust concentrations were reported for this region. CONCLUSION: The observed CWP prevalence substantially exceeded predicted levels in central Appalachia. However, the increased prevalence was not explained by the measured levels of dust exposures. Likely contributing factors include mine size and low seam mining, which may be associated with higher exposure to silica. Further study is needed to characterise the responsible factors for the elevated CWP rates in central Appalachia.

We developed quantitative estimates of historical exposures to respirable elemental carbon (REC) for an epidemiologic study of mortality, including lung cancer, among diesel-exposed miners at eight non-metal mining facilities [the Diesel Exhaust in Miners Study (DEMS)]. Because there were no historical measurements of diesel exhaust (DE), historical REC (a component of DE) levels were estimated based on REC data from monitoring surveys conducted in 1998-2001 as part of the DEMS investigation. These values were adjusted for underground workers by carbon monoxide (CO) concentration trends in the mines derived from models of historical CO (another DE component) measurements and DE determinants such as engine horsepower (HP; 1 HP = 0.746 kW) and mine ventilation. CO was chosen to estimate historical changes because it was the most frequently measured DE component in our study facilities and it was found to correlate with REC exposure. Databases were constructed by facility and year with air sampling data and with information on the total rate of airflow exhausted from the underground operations in cubic feet per minute (CFM) (1 CFM = 0.0283 m(3) min(-1)), HP of the diesel equipment in use (ADJ HP), and other possible determinants. The ADJ HP purchased after 1990 (ADJ HP1990+) was also included to account for lower emissions from newer, cleaner engines. Facility-specific CO levels, relative to those in the DEMS survey year for each year back to the start of dieselization (1947-1967 depending on facility), were predicted based on models of observed CO concentrations and log-transformed (Ln) ADJ HP/CFM and Ln(ADJ HP1990+). The resulting temporal trends in relative CO levels were then multiplied by facility/department/job-specific REC estimates derived from the DEMS surveys personal measurements to obtain historical facility/department/job/year-specific REC exposure estimates. The facility-specific temporal trends of CO levels (and thus the REC estimates) generated from these models indicated that CO concentrations had been generally greater in the past than during the 1998-2001 DEMS surveys, with the highest levels ranging from 100 to 685% greater (median: 300%). These levels generally occurred between 1970 and the early 1980s. A comparison of the CO facility-specific model predictions with CO air concentration measurements from a 1976-1977 survey external to the modeling showed that our model predictions were slightly lower than those observed (median relative difference of 29%; range across facilities: 49 to -25%). In summary, we successfully modeled past CO concentration levels using selected determinants of DE exposure to derive retrospective estimates of REC exposure. The results suggested large variations in REC exposure levels both between and within the underground operations of the facilities and over time. These REC exposure estimates were in a plausible range and were used in the investigation of exposure-response relationships in epidemiologic analyses.

OBJECTIVE: To determine whether the prevalence of coal workers' pneumoconiosis (CWP) or progressive massive fibrosis (PMF) among United States underground miners is associated with mine size. METHODS: We examined chest radiographs from 1970 to 2009 of working miners who participated in the National Coal Workers Health Surveillance Program for the presence of small and large opacities consistent with pneumoconiosis, based upon the International Labour Organization classification system. RESULTS: A total of 145 512 miners contributed 240 067 radiographs for analysis. From the 1990s to the 2000s, the prevalence of radiographic CWP increased among miners in mines of all sizes, while miners working in mines with fewer than 50 employees had a significantly higher prevalence of CWP compared to miners who worked in mines with 50 or more employees (p<0.0001). When adjusted for age and within-miner correlation, the difference in prevalence of CWP by mine size was significant for all decades. Since 1999, miners from small mines were five times more likely to have radiographic evidence of PMF (1.0% of miners) compared to miners from larger mines (0.2% of miners) with a prevalence ratio of 5.0 and 95% CI 3.3 to 7.5. CONCLUSION: The prevalence of CWP among United States coal miners is increasing in mines of all sizes, while CWP and PMF are much more prevalent among workers from underground mines with fewer than 50 workers.

Traditional film-screen radiography (FSR) has been useful in the recognition and evaluation of interstitial lung diseases, but is becoming increasingly obsolete. To evaluate the applicability of storage phosphor digital computed radiography (CR) images in the recognition of small lung opacities, we compared image quality and the profusion of small opacities between FSR and CR radiographs.We screened 1388 working coal miners during the course of the study with FSR and CR images obtained on the same day from all participants. Each traditional chest film was independently interpreted by two of eight experienced readers using the International Labour Office (ILO) classification of radiographs of pneumoconiosis, as were CR images displayed on medical-grade computer monitors.The prevalence of small opacities (ILO category 1/0 or greater) did not differ between the two imaging modalities (5.2%, for FSR and 4.8% for soft copy CR, p>0.50). Inter-reader agreement was also similar between FSR and CR. Significant differences between image modalities were observed in the shape of small opacities, and in the proportion of miners demonstrating high opacity profusion (category ≥2/1).Our results indicate that, with appropriate attention to image acquisition and soft copy display, CR digital radiography can be equivalent to FSR in the identification of small interstitial lung opacities.

OBJECTIVES: Epidemiologic reports since 2000 have documented increased prevalence and rapid progression of pneumoconiosis among underground coal miners in the United States. To investigate a possible role of silica exposure in the increase, we examined chest x-rays (CXRs) for specific abnormalities (r-type small opacities) known to be associated with silicosis lung pathology. METHODS: Underground coal miners are offered CXRs every 5 years. Abnormalities consistent with pneumoconiosis are recorded by National Institute for Occupational Safety and Health (NIOSH) B Readers using the International Labour Office Classification of Radiographs of Pneumoconioses. CXRs from 1980-2008 of 90,973 participating miners were studied, focusing on reporting of r-type opacities (small rounded opacities 3-10 mm in diameter). Log binomial regression was used to calculate prevalence ratios adjusted for miner age and profusion category. RESULTS: Among miners from Kentucky, Virginia, and West Virginia, the proportion of radiographs showing r-type opacities increased in the 1990s (PR=2.5; 95% CI=1.7-3.7) and after 1999 (PR=4.1; 95% CI=3.0-5.6), compared to the 1980s (adjusted for profusion category and miner age). The prevalence of progressive massive fibrosis in 2000-2008 was also elevated compared to the 1980's (PR=4.4; 95% CI=3.1-6.3) and 1990's (PR=3.8; 95% CI=2.1-6.8) in miners from Kentucky, Virginia, and West Virginia. CONCLUSIONS: The increasing prevalence of pneumoconiosis over the past decade and the change in the epidemiology and disease profile documented in this and other recent studies imply that U.S. coal miners are being exposed to excessive amounts of respirable crystalline silica.

We read with interest the recent article by McCunney, Morfeld, and Payne entitled What Component of Coal Causes Coal Workers' Pneumoconiosis (CWP)?1 The intent of the article was to identify the specific substance(s) in the underground coal mining environment that are directly responsible for causing CWP. In addressing this question, the authors use a substantial portion of their work to point out that exposure to silica does not likely contribute to CWP. | The authors are not wrong in their assessment of the role of silica in the development of CWP when it occurs at low levels. Epidemiologic studies dating back to the early 1970s showed through modeling of quantitative exposure data that mixed coal mining dust was the primary factor relating to CWP development, whereas silica's role was much smaller.2–4 Anthracite workers in many parts of the world have long been known to suffer high rates of CWP, although anthracite usually contains very low levels of silica. | We believe that by downplaying silica in their desire to seek a more precise explanation for the cause of CWP, the authors have potentially done coal miners a great disservice. There is an important distinction that the authors fail to highlight in their report—silica may not be a major factor in the development of the clinical condition known as CWP, but silica does cause another form of pneumoconiosis among coal workers—silicosis.