A retrospective cohort mortality study of males mining and milling attapulgite clay

Our plain-English summary of A retrospective cohort mortality study of males mining and milling attapulgite clay, by R.J. Waxweiler, R.D. Zumwalde, G.O. Ness, et al (American Journal of Industrial Medicine, 1988).

The study

• Authors: R.J. Waxweiler, R.D. Zumwalde, G.O. Ness, D.P. Brown
• Journal: American Journal of Industrial Medicine
• Published: 1988;13(3):305-15
• Cited by: 8 articles, as counted by NIH iCite
• Read it: PubMed · Publisher
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 000286).

Abstract

To evaluate the possible health effects of occupational exposure to a nonasbestos mineral fiber, a cohort of 2,302 males employed for at least 1 month between 1940 and 1975 at an attapulgite (clay fiber) mining and milling facility was followed through 1975. A significant deficit of mortality (SMR = 43, 90% CI 23-76) from nonmalignant respiratory disease (NMRD) was observed for the cohort based on age-, calendar year-, and race-specific rates for U.S. males. A marked deficit of NMRD was seen regardless of presumed dust exposure level, induction-latency period, or duration employed. A statistically significant excess of mortality from lung cancer was observed among whites (SMR = 193, 90% CI 121-293), but a deficit occurred among nonwhites (SMR = 53, 90% CI 21-112). Lung cancer risk in either race was not altered substantially with presumed dust exposure level, induction-latency period, or duration employed with one exception-those employed for at least 5 years in high-exposure-level jobs.

In plain English

This retrospective cohort mortality study followed 2,302 men who worked at least 1 month between 1940 and 1975 in the mine or mill of a U.S. facility producing attapulgite, a nonasbestos clay fiber. Deaths through 1975 were compared with U.S. male rates. Airborne fibers measured in 1976 were 0.1 to 2.5 micrometers long. There were 315 deaths, 80% of those expected, and no cause showed a statistically significant excess overall. Deaths from nonmalignant respiratory disease were significantly below expected (standardized mortality ratio, SMR, 43, where 100 equals expected; 90% confidence interval, CI, 23 to 76). Lung cancer was raised among whites (SMR 193, CI 121 to 293) but below expected among nonwhites (SMR 53, CI 21 to 112). Workers of both races with at least 5 years in high-exposure jobs had 5 lung cancer deaths versus 1.62 expected (SMR 309, CI 122 to 649). Smoking data were not available for the cohort, though smoking rates for the current workforce were reported. The authors concluded that, given the limitations, the findings neither strongly supported nor contradicted a link between U.S. attapulgite and human lung cancer.

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: Retrospective cohort mortality study comparing observed deaths with expected deaths based on U.S. male mortality rates (standardized mortality ratios with 90% confidence intervals)
• Who was studied: 2,302 males (1,235 white, 1,067 nonwhite) who worked in the mine or mill of one major U.S. attapulgite company's facility for at least 1 month between January 1, 1940 and December 31, 1975. Vital status was followed to December 31, 1975, giving 38,723 person-years at risk. U.S. attapulgite is mined and milled only in the Georgia-Florida area.
• Exposure: Attapulgite clay dust, a nonasbestos mineral fiber described as asbestiform and platy palygorskite. Airborne fibers measured by electron microscopy were 0.1 to 2.5 micrometers long (geometric mean 0.52) and 0.02 to 0.1 micrometers in diameter. No other fibrous minerals were found. In a 1976 survey, all respirable dust averages were below the MSHA nuisance dust standard of 5 mg/m3. Mill jobs in groups B and C were classed as high exposure, maintenance-type mill jobs as intermittent exposure, and mining and group A jobs as low exposure. There were no historical dust measurements.
• Main results:
  • Whole cohort: 315 deaths were observed, 80% of the number expected. No cause of death showed a statistically significant excess.
  • Whole cohort: liver cancer was raised but not quite statistically significant (SMR = 250, 90% CI 99-526). Stomach cancer showed a slight excess that was not statistically significant (SMR = 120, 90% CI 52-237).
  • Whole cohort: deaths from nonmalignant respiratory disease were significantly below expected (SMR = 43, 90% CI 23-76).
  • Whole cohort, 10 or more years since first employment: nonmalignant respiratory disease remained significantly below expected (SMR = 41, 90% CI 19-77), and the deficit did not depend on duration of employment.
  • Lung cancer differed by race. White workers had a statistically significant excess (SMR = 193, 90% CI 121-293). Nonwhite workers had a deficit (SMR = 53, 90% CI 21-112).
  • Among white workers after 10 years since first employment, lung cancer remained raised, with no trend by duration of employment.
  • Workers of both races employed at least 5 years in high-exposure jobs had 5 observed lung cancer deaths versus 1.62 expected (SMR = 309, 90% CI 122-649). This comprised 2 observed versus 0.45 expected among whites and 3 observed versus 1.18 expected among nonwhites.
  • White workers in intermittent- or high-exposure jobs, 20 or more years after starting such work: 6 lung cancer deaths observed versus 2 expected (SMR = 300, 90% CI 131-592). The comparable group of nonwhite workers had a decreased lung cancer risk.
  • Smoking data were not available for the cohort. The authors cited smoking rates for the current workforce: 44% of white and 46% of nonwhite males were current smokers, 21% and 11% ex-smokers, and 35% and 44% nonsmokers.
• The authors' conclusion: The authors concluded that, because of the study's limitations, their findings were neither strongly supportive nor contradictive of an association between U.S. attapulgite, or short thin fibers in general, and lung cancer in humans. They considered it doubtful that smoking explains SMRs above 200. They called for smoking data to be gathered and for mortality follow-up to be extended. They also stated that the absence of excess nonmalignant respiratory disease appeared consistent with the hypothesis that short fibers are less fibrogenic.
• Limitations the authors state:
  • No data on cigarette smoking were available for the cohort. Only current-workforce smoking rates were cited.
  • The completeness of the cohort could not be confirmed with an independent data source.
  • Some workers were lost to follow-up (140, 6%), some death certificates were missing (19 of 315 deaths), and race was missing for 50 persons, who were assumed to be white.
  • There was no individual exposure measurement over time and no historical dust data. Exposure categories were assumed to stay the same over time even though dust levels varied.
  • Few deaths were expected beyond 20 years after first exposure, and few people worked at the facility for long periods.
  • Expected deaths in the exposure subgroups were usually quite small.
  • The general U.S. population was used as the comparison group.
  • Follow-up for people lost to tracing was counted to the end of 1975, so risk might have been slightly underestimated.

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: Palygorskite (Attapulgite), Occupational Exposure, Mineral Fiber, Latency, Lung Cancer.

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• The lung biological activity of American attapulgite — R. Bégin, S. Massé, et al. (1987)
• Characterisation of palygorskite specimens from different geological locales for health hazard evaluation — R.P. Nolan, A.M. Langer, et al. (1991)
• Cytotoxicity of respirable dusts from industrial minerals: comparison of two naturally occurring and two man-made silicates — D. Nadeau, L. Fouquette-Couture, et al. (1987)
• Rat lung reactivity to natural and man-made fibrous silicates following short-term exposure — I. Lemaire, P.G. Dionne, et al. (1989)
• Exposure to chrysotile mining dust and digestive cancer mortality in a Chinese miner/miller cohort — S. Lin, X. Wang, et al. (2014)

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