Mortality of a cohort of men in a silicosis register: further evidence of an association with lung cancer

Our plain-English summary of Mortality of a cohort of men in a silicosis register: further evidence of an association with lung cancer, by T.P. Ng, S.L. Chan, J. Lee (American Journal of Industrial Medicine, 1990).

The study

• Authors: T.P. Ng, S.L. Chan, J. Lee
• Journal: American Journal of Industrial Medicine
• Published: 1990;17(2):163-71
• Cited by: 42 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. 003886).

Abstract

Lung cancer mortality from 1980 to 1986 was studied in a cohort of 1,419 men in a silicosis register who had no previous exposure to asbestos and polyaromatic hydrocarbons. The 28 deaths from lung cancer were statistically in excess of expected (SMR 2.03; 95% CI 1.35-2.93). Excess risks of lung cancer were found in both underground workers (SMR 3.41; 95% CI 1.10-7.97; based on 5 deaths) and surface workers (SMR 1.87, 95% CI 1.18-2.81; based on 23 deaths). All lung cancer deaths were smokers. There was an increase in SMRs with longer latency periods and years of exposure, with the greatest risk found in those who had worked for 30 or more years after more than 30 years since first exposed (SMR 3.07, based on 16 deaths). The risk for lung cancer was higher in those with tuberculosis (SMR 2.52; 95% CI 1.52-3.94) and showed an increasing trend with severity of silicosis, from category 1 to 3 and from category A to C, with highest risk in those with tuberculosis and category 3 (SMR 4.44 based on 3 deaths) or tuberculosis and category C (SMR 7.63 based on 7 deaths). Most of the excess lung cancer risk in silicotics is due to smoking, but a synergistic effect between smoking and silica/silicosis on the risk of lung cancer is also likely. In particular, a possible role of silicosis and tuberculosis as the fibrotic seedbed for malignant growth in the lung is strongly supported.

In plain English

This study concerned silica and silicosis, not asbestos. The authors followed 1,419 men on a Hong Kong silicosis compensation register from July 1980 through 1986, comparing deaths with those expected in Hong Kong's general male population. They had excluded 68 workers thought to have had past or concomitant exposure to asbestos or polyaromatic hydrocarbons. There were 356 deaths, about three times the expected number (SMR 3.02; 95% CI 2.71-3.35). The 28 lung cancer deaths were about twice the expected number (SMR 2.03; 95% CI 1.35-2.93), with excesses in both underground (5 deaths) and surface workers (23 deaths). All 28 had smoked, and 19 had tuberculosis. Men with tuberculosis had more lung cancer deaths than general-population rates predicted (SMR 2.52; 95% CI 1.52-3.94). Lung cancer risk showed an increasing trend with more severe silicosis on X-ray, reaching SMR 6.74 (95% CI 2.90-13.26) in large-opacity category C (8 deaths). The authors concluded that smoking accounts for most of the excess, but that a combined effect of smoking and silica/silicosis is also likely. They noted that lung cancer risk was probably underestimated.

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: Registry-based cohort mortality study comparing deaths with the general male population of Hong Kong, using standardized mortality ratios (SMRs)
• Who was studied: 1,419 men on Hong Kong Silicosis Register I (silicotics alive as of June 30, 1980, registered for compensation), followed from July 1, 1980 to the end of 1986; 235 underground workers and 1,184 surface workers
• Exposure: Silica exposure and silicosis in underground work and surface work (quarrying, rock excavation). Excluded were 68 workers thought to have had past or concomitant exposure to asbestos (35, from ship-building and repair and demolition) or polyaromatic hydrocarbons (33)
• Main results:
  • All causes of death in the whole cohort: 356 deaths, SMR 3.02 (95% CI 2.71-3.35)
  • Lung cancer in the whole cohort: 28 deaths, a statistically significant excess (SMR 2.03; 95% CI 1.35-2.93)
  • Underground workers: lung cancer SMR 3.41 (95% CI 1.10-7.97), based on 5 deaths. Surface workers: lung cancer SMR 1.87 (95% CI 1.18-2.81), based on 23 deaths
  • Lung cancer risk showed an increasing trend across radiological silicosis categories. The highest SMRs were 4.04 in small-opacity category 3 (7 deaths; Table III 95% CI 1.62-8.33) and 6.74 in large-opacity category C (8 deaths; Table III 95% CI 2.90-13.26)
  • The highest lung cancer risk by time and exposure was in men with more than 30 years since first exposure and more than 30 years of exposure: SMR 3.07, based on 16 deaths
  • Of the 28 men who died of lung cancer, all were ever-smokers, 19 had tuberculosis, and 5 had been exposed underground
  • Men with tuberculosis had an increased lung cancer risk compared with general-population expectations: SMR 2.52 (95% CI 1.52-3.94)
  • The greatest excesses were in men with both tuberculosis and category 3 silicosis (SMR 4.44, 3 deaths) and men with both tuberculosis and category C silicosis (SMR 7.63, 7 deaths)
  • 90% of the cohort were smokers, compared with 57% of Hong Kong men of the same age in 1982. Assuming a 20-fold lung cancer risk for ever-smokers, the authors calculated that a 53% excess in lung cancer deaths might be expected from this difference in smoking alone
  • The trends in lung cancer risk by silicosis category, latency, exposure years and tuberculosis were unchanged when underground workers were excluded
• The authors' conclusion: The authors concluded that smoking accounts for most of the excess lung cancer deaths among the silicotics in this cohort, but that a synergistic effect between smoking and silica/silicosis is also likely. They also concluded that the results strongly support a possible role of silicosis and tuberculosis as a fibrotic seedbed for lung cancer.
• Limitations the authors state:
  • A cohort of already-diseased men was compared with the general male population (the reverse of the healthy worker effect)
  • Much higher death rates from silicosis, tuberculosis and other chronic lung diseases compete with lung cancer, so lung cancer risk is likely underestimated
  • Lung cancer risk could not be assessed separately in non-smokers, because all lung cancer deaths were in smokers
  • Detailed data on the amount and duration of smoking were not available
  • Lung cancer on X-ray could be mistaken for massive fibrosis, which could have created an artificial trend with large opacities
  • Silicosis categories might have differed in exposure intensity, even though exposure duration was similar
  • There were no data on differences in lung cancer risk or smoking among ethnic subgroups of Chinese, although the authors considered the resulting bias remote

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: Silicosis, Lung Cancer, Latency, Synergistic Effect.

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