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• Lung cancer risk in coal miners: The need for further investigations
  Judith M Graber
  Published on: 27 April 2016
• Lung cancer excess risks after coal mine dust exposure?
  Peter Morfeld
  Published on: 27 April 2016

• Published on: 27 April 2016

  Lung cancer risk in coal miners: The need for further investigations

  • Judith M Graber, Assistant Research Porfessor
  • Other Contributors:
    • Leslie T. Stayner, Ph.D. Division of Epidemiology and Biostatistics, School of Public Health, University of Illinois at Chicago, USA; Robert A. Cohen, M.D. Division of Environmental and Occupational Health Sciences, School of Public Health, University of

  We thank Dr. Morfeld for his comments on our updated mortality study of the U.S. coal miners study.[1] However, we disagree with his assertion that the excess of lung cancer we observed must be attributed to smoking alone. Firstly, despite the smoking prevalence being higher in our cohort than in the U.S. population in 1970, smokers in our population were significantly less likely to be heavy smokers (> 24 cigarettes...

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  We thank Dr. Morfeld for his comments on our updated mortality study of the U.S. coal miners study.[1] However, we disagree with his assertion that the excess of lung cancer we observed must be attributed to smoking alone. Firstly, despite the smoking prevalence being higher in our cohort than in the U.S. population in 1970, smokers in our population were significantly less likely to be heavy smokers (> 24 cigarettes daily) than men in the general US population (12.4% vs. 28.0%). Secondly, greater weight should be given to the findings from the internal analysis, which controlled for smoking, than to the SMR analysis. Here, as Dr. Morfeld acknowledges, there was a clear dose-response relationship between coal- mine dust exposure and lung cancer. Of note, there was an inverse relationship between radiographic CWP status and lung cancer mortality, implying that CWP and lung cancer were competing causes of death, and leading to weakening of the relationship between dust exposure and lung cancer for the older miners (i.e., those exposed to the high levels of dust existing prior to the 1969 Federal dust regulations).

  Table 1 shows the mean cumulative coal dust exposures by year, as requested by Dr. Morfeld. These levels were lower in the last eight years of follow-up and were higher among lung cancer cases than among all subjects in that decade. This is probably explained by the fact that subjects with higher cumulative exposures were more likely to die during the earlier years of follow-up due to the effects of exposure and their age.

  Dr. Morfeld correctly stated that we lacked work histories on our miners after enrolment (1969 to 1971), but incorrectly asserted that work histories were absent before enrolment. The lack of work histories in the post-enrolment period may have introduced some misclassification of exposures. However, as we and others have emphasized,[2,3] and as demonstrated by our sensitivity analysis, any such misclassification is likely minimal as most participants accumulated the bulk of their exposure before enrolment.

  We agree with Dr. Morfeld, and stated in our article, that the British study of coal miners [4] had better exposure data than our study. While both studies show an excess of lung cancer in their most recent period of follow-up, [1,4] the British findings suggest that the excess is most strongly associated with silica, rather than with coal dust exposure as seen in our study. However, from both studies it is clear that there is an excess of lung cancer among coal miners which is unlikely explained by smoking alone.

  Finally, we are confused by Dr. Morfeld's reference to the healthy worker survivor effect as a reason for dismissing our findings. As has been shown [3] the study was subject to a strong healthy worker survivor effect, causing a reduction in mortality in the early years of follow-up.

  We remain firm in our conclusion that "Our findings and those from the British coal-miners cohort strongly suggest the need for continued investigation of lung cancer mortality and incidence among coal miners."[1]

  Table I: Coal mine dust by year of death among all cohort members and among those for whom the underlying cause of death was lung cancer

  Mean Cumulative Exposure

  Coal Mine Dust

  (mg/m3-yrs)

  All Lung Cancer

  (n= 8,829) (n= 568) Calendar year of death 1970-1989 89.7 86.9 1980-1999 82.0 75.5 2000-2007 42.8 51.5

  Literature

  1. Graber, J.M., Stayner, L.T., Cohen,R.A., Conroy, L.M., Attfield, M. D., Respiratory disease mortality among US coal miners; results after 37 years of follow-up. Occup Environ Med, 2014. 71: p. 30-39. 2. Kuempel, E.D., et al., Exposure-response analysis of mortality among coal miners in the United States. Am J Ind Med, 1995. 28(2): p. 167-84. 3. Attfield, M.D. and E.D. Kuempel, Mortality among U.S. underground coal miners: a 23-year follow-up. Am J Ind Med, 2008. 51(4): p. 231-45 4. Miller, B.G. and L. MacCalman, Cause-specific mortality in British coal workers and exposure to respirable dust and quartz. Occup Environ Med, 2010. 67(4): p. 270-6

  Conflict of Interest:

  None declared

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  Conflict of Interest:
  None declared.

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• Published on: 27 April 2016

  Lung cancer excess risks after coal mine dust exposure?

  • Peter Morfeld

  Dear Editor,

  I read with interest about the updated US coalminer mortality study[1]. The lung cancer SMR was slightly elevated (SMR=1.08, 95% CI: 1.00-1.18). This excess is unexceptionable because of a higher proportion of smokers at the start of follow-up in 1969/1971 (current smokers: 54%) in comparison to the US male population in 1970 (44.1%). Internal analyses showed an association of lung cancer mortality...

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  Dear Editor,

  I read with interest about the updated US coalminer mortality study[1]. The lung cancer SMR was slightly elevated (SMR=1.08, 95% CI: 1.00-1.18). This excess is unexceptionable because of a higher proportion of smokers at the start of follow-up in 1969/1971 (current smokers: 54%) in comparison to the US male population in 1970 (44.1%). Internal analyses showed an association of lung cancer mortality with coalmine dust exposure but only during the last follow-up interval from 2000 to 2007. Thus, it is of interest to see the distribution of the cumulative exposures across the different calendar time periods. It remains unclear why this information was not given in Table 2 (why does Table 2 report on 568 lung cancer cases but Table 4 on 583 cases?)

  This US study suffers from an incomplete assessment of occupational histories in coalminers: no start and end date of jobs held before 1969/1971 available, no information on jobs held after 1969/1971, and no end date of working as a coalminer for 16.2% of cohort members. Thus, only a crude assessment of exposure to coalmine dust up to the start of follow- up was possible: no time-dependent exposure analysis or lagging of exposures could be done. Crystalline silica concentration data suffered from additional limitations because measurements were available only after 1981 but had to be allocated to jobs held before 1969/1971.

  The largest study to date with better assessment of exposures in a time-dependent manner was performed in the UK[2]: the overall evidence does not support a lung cancer excess risk due to coalmine dust exposure. The findings of the US study do not seem to change this view despite obvious limitations due to the Healthy Worker Survivor Effect (models that adjust for time since last employment do not solve this problem)[3].

  References

  1 Graber JM, Stayner LT, Cohen RA, et al. Respiratory disease mortality among US coal miners; results after 37 years of follow-up. Occup Environ Med 2014;71:30-39.

  2 Miller BG, MacCalman L. Cause-specific mortality in British coal workers and exposure to respirable dust and quartz. Occup Environ Med 2010; 67:270-276.

  3 Naimi AI, Richardson DB, Cole SR. Causal Inference in Occupational Epidemiology: Accounting for the Healthy Worker Effect by Using Structural Nested Models. Am J Epidemiol. 2013;178:1681-1686.

  Conflict of Interest:

  Yes, I have a competing interest. The author performed epidemiological studies on German coalminers and gives scientific advice to the German coalmining industry.

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  Conflict of Interest:
  None declared.

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