Relations between asbestos exposure and lung cancer SMRs in occupational cohort studies

Our plain-English summary of Relations between asbestos exposure and lung cancer SMRs in occupational cohort studies, by F.D. Liddell, J.A. Hanley (British Journal of Industrial Medicine, 1985).

The study

• Authors: F.D. Liddell, J.A. Hanley
• Journal: British Journal of Industrial Medicine
• Published: 1985;42(6):389-96
• Cited by: 19 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 000487).

Abstract

It has long been accepted that excessive exposure to asbestos may produce lung cancer but not that there is a consistent "biological gradient." This can only be evaluated reliably in studies where, for every individual, exposure has been measured in terms of both duration and intensity. Even now, there are only at most eight such cohort studies of asbestos workers, while femoral methods of analysis have been available only recently. These methods, applied in these studies, yield good evidence that the "exposure-response" relation between accumulated exposure to asbestos and standardised mortality ratios (SMRs) for lung cancer may be taken as linear, but that at zero exposure the lung cancer SMR is not always unity--not surprising, because of well known difficulties with the choice of reference population and selection problems. This leads to a concept of "relative slopes" that take account of the background mortality in the cohort and make what appears to be the best use of the available data. Other approaches to the same data, and indeed to all cohort data known, are also considered. Each study is examined as closely as is possible in a short review, and the concepts of linearity and relative slopes appear justified. The relative slopes (b/a) in the line SMR = a[1 + (b/a) . (exposure)] vary much more widely than can be accounted for by differences in epidemiological methodology; as discussed elsewhere, reasons for the variation seem to lie rather in type and dimensions of asbestos fibre, industrial process, etc. Slopes in the line SMR = 1 + b1 . (exposure) vary about twice as much as do the relative slopes.

In plain English

The authors reviewed and statistically reanalysed published cohort studies of asbestos workers. It covered seven investigations with individual exposure estimates by duration and concentration, plus one study (G) based only on duration. They fitted straight lines relating lung cancer standardised mortality ratios (SMRs) to accumulated exposure in millions of particles per cubic foot times years (mpcf.y). Only in study C (Quebec miners and millers) did the least exposed workers have an SMR near 1. Two-parameter lines fitted adequately overall. Point estimates of 'relative slopes', which allow for each cohort's background lung cancer rate, ranged from effectively zero to 6.9. The article gave these as the increase in excess lung cancer risk per mpcf.y, multiplied by 100. On the same scale, slopes from lines forced through an SMR of 1 ranged from very small to 13.5. The authors concluded that in most cohorts the relation was effectively linear (though linearity was not proved) but often did not pass through an SMR of 1 at zero exposure, and found no common slope. They said differences may have arisen from fibre type, fibre dimensions and process, and noted differing follow-up and exposure periods.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Review and statistical reanalysis (maximum likelihood line fitting) of published occupational cohort studies of asbestos workers
• Who was studied: Seven cohort investigations of asbestos workers with individual exposure estimates by duration and concentration, plus one duration-only study (G). They covered US asbestos company retirees, New Orleans asbestos-cement workers, Quebec miners and millers, a Pennsylvania mainly textile plant, a Charleston SC textile plant, a Connecticut friction products and packings plant, and men at an amosite insulation products facility in Paterson NJ. Six other studies were discussed briefly but did not meet the inclusion criteria.
• Exposure: Accumulated exposure to asbestos in mpcf.y, accumulated over different periods depending on the study. For study G, duration of employment was used instead.
• Main results:
  • Lung cancer SMRs in the least exposed subcohorts were raised for studies A, E(a), E(b), F and G, depressed for studies B and D, and close to 1 only for study C.
  • For studies D, F and G (both versions), the hypothesis that the line passes through an SMR of 1 at zero exposure could be rejected.
  • Summed over eight cohorts, the goodness of fit statistic for the two-parameter lines was 17.94 with 30 degrees of freedom, giving no indication of inadequate fit. Study B was the one doubtful case.
  • Point estimates of relative slope ranged from effectively zero to 6.9 across the cohorts. One-parameter slopes ranged from very small to 13.5. Both are expressed as the rate of increase in excess lung cancer risk per mpcf.y, multiplied by 100 for presentation.
  • In study C/2 (Quebec men with at least 20 years' gross service), the relative slope was 0.203 (90% confidence interval 0.064 to 0.493). The authors said this showed the importance of asbestos concentration in evaluating exposure.
  • In study F (friction products and packings plant), the two-parameter fit gave a negative slope, and the 90% confidence limits included zero.
• The authors' conclusion: The authors concluded that in most cohort studies of asbestos workers the relation between lung cancer SMR and accumulated exposure is effectively linear, but the line often does not pass through an SMR of 1 at zero exposure. They considered the relative slope the more meaningful measure, and found it varied much less between cohorts than the one-parameter slopes. They stated there is no common slope, and that differences may have arisen from fibre type, fibre dimensions and process.
• Limitations the authors state:
  • The estimates are not strictly comparable because the studies used different follow-up periods and different periods over which exposure was accumulated.
  • Linearity was not proved; other possibilities exist.
  • The relative slope approach assumes that background mortality was the same in every subcohort.
  • Because there are usually few deaths, statistically significant test results are uncommon and some judgment is needed in interpretation.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Lung cancer.

Medical terms in this study, explained in our Lexicon: Asbestos Exposure, Lung Cancer.

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