Risk assessment using exposure intensity: an application to vermiculite mining

Our plain-English summary of Risk assessment using exposure intensity: an application to vermiculite mining, by P.M. Vacek, J.C. McDonald (British Journal of Industrial Medicine, 1991).

The study

• Authors: P.M. Vacek, J.C. McDonald
• Journal: British Journal of Industrial Medicine
• Published: 1991;48(8):543-7
• Cited by: 9 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Estimation of exposure-response relations from epidemiological data is complicated by the fact that exposures usually vary in intensity over time. Cumulative exposure indices, which do not separate the effects of intensity and duration, are commonly used to circumvent this problem. In this paper the estimation of relative risk for specific ranges of exposure intensity from such data is considered using existing statistical methods for fitting multivariate relative risk models. This has the advantage that it does not assume that exposure intensity and duration have equivalent effects on risk. It also throws light on the possible existence of a threshold. The procedure was applied to data from a cohort of 406 vermiculite miners to examine the lung cancer risk associated with exposure to fibrous tremolite, which contaminated the vermiculite. The pattern of exposure-response differed substantially from that obtained using a cumulative exposure index to assess risk.

In plain English

This paper described a statistical method for estimating lung cancer risk by intensity of exposure. The usual cumulative exposure index assumes that intensity and duration have equivalent effects on risk; this method does not. The authors applied it to an existing cohort of 406 men employed for at least one year before 1963 at a vermiculite mine and mill in Montana. The ore there was contaminated by an amphibole fibre in the tremolite series, and deaths were followed to July 1983. After 13 men were excluded, 393 miners contributed 8024 person-years and 21 lung cancer deaths. A second, case-control analysis used 182 controls. Years worked at four estimated fibre concentration levels were analysed, leaving out exposure in the nine years before each year at risk. In both analyses the lowest level (less than five fibres/ml) had negative, non-significant risk coefficients. The highest level had about the same coefficient as level 2, although its average concentration was 12 times higher. The authors stated that the results were consistent with no risk at low concentrations, a rapid rise, then a levelling off, a pattern not seen with cumulative exposure. They noted that the small data set gave imprecise estimates and that smoking data were unavailable.

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: Methodological paper presenting a statistical approach (multiplicative relative risk model), applied to existing cohort data by both a person-years analysis and a case-control (internal comparison) analysis
• Who was studied: 406 men employed at a vermiculite mine and mill in Montana, all employed before 1963 for at least one year, with mortality followed to July 1983. 13 who died within nine years of first employment were excluded. The 393 analysed contributed 8024 person-years and 21 lung cancer deaths. The case-control analysis used 182 controls (1 to 17 per case).
• Exposure: An amphibole fibre in the tremolite series contaminating the vermiculite ore. Exposure intensity was estimated as respirable fibres/ml for each year of employment from available air measurements and grouped into four levels, the lowest being less than five fibres/ml. Exposures within nine years were excluded as a lag period.
• Main results:
  • In both the person-years and case-control analyses, the risk coefficients at the lowest exposure level (less than five fibres/ml) were negative and not statistically significant.
  • In both analyses, the risk coefficient for the highest exposure level (level 4) was about the same as that for level 2, even though level 4's average fibre concentration was 12 times that of level 2.
  • The two approaches gave similar results except for level 3. Its coefficient was 0.1319 (p 0.028) in the person-years analysis and 0.2563 (p 0.006) in the case-control analysis, as read from scan-damaged table figures.
  • In neither analysis was the coefficient for years since first exposure statistically significant.
  • When risk coefficients were extrapolated from a cumulative exposure model, the directly estimated coefficients for levels 2 and 3 (case-control results) were well above the extrapolated range. The extrapolated coefficient for level 4 was high compared with the direct estimate.
  • An additive relative risk model, fitted as an alternative, gave very similar results.
• The authors' conclusion: The authors concluded that the results were consistent with a relation in which risk is absent at low concentrations, increases rapidly as concentrations increase and levels off at high concentrations, a pattern not evident when risk was modelled on cumulative exposure. They also concluded that extrapolations based on cumulative exposure may be misleading unless intensity and duration affect risk equally.
• Limitations the authors state:
  • The data set was small, so the estimates of the tremolite and lung cancer relation lack precision.
  • No information about smoking was available for the cohort.
  • Because exposures in the cohort were short, it was not possible to determine whether the multiplicative or additive model is more appropriate for longer durations.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Lung cancer, Talc and vermiculite, Tremolite.

Medical terms in this study, explained in our Lexicon: vermiculite, Cumulative Exposure, Relative Risk, Lung Cancer, Tremolite.

Related studies

• Cohort study of mortality of vermiculite miners exposed to tremolite — J.C. McDonald, A.D. McDonald, et al. (1986)
• Health of vermiculite miners exposed to trace amounts of fibrous tremolite — J.C. McDonald, A.D. McDonald, et al. (1988)
• Pulmonary changes after exposure to vermiculite contaminated with fibrous tremolite — J.E. Lockey, S.M. Brooks, et al. (1984)
• Mortality in a cohort of vermiculite miners exposed to fibrous amphibole in Libby, Montana — J.C. McDonald, J. Harris, et al. (2004)
• The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part II. Mortality — H.E. Amandus, R. Wheeler (1987)
• The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part I. Exposure estimates — H.E. Amandus, R. Wheeler, et al. (1987)

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