Asbestos related diseases from environmental exposure to crocidolite in Da-yao, China. I. Review of exposure and epidemiological data

Our plain-English summary of Asbestos related diseases from environmental exposure to crocidolite in Da-yao, China. I. Review of exposure and epidemiological data, by S. Luo, X. Liu, S. Mu, et al (Occupational and Environmental Medicine, 2003).

The study

• Authors: S. Luo, X. Liu, S. Mu, S.P. Tsai, C.P. Wen
• Journal: Occupational and Environmental Medicine
• Published: 2003;60(1):35-41; discussion 41-2
• Type of article: Review
• Cited by: 62 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: Scattered patches of crocidolite, one form of asbestos, were found in the surface soil in the rural county of Da-yao in southwestern China. In 1983, researchers from the West China University of Medical Sciences (WCUMS) discovered that residents of two villages in Da-yao had hyperendemic pleural plaques and excessive numbers of pleural mesotheliomas.

Aims: To review and summarise epidemiological studies, along with other relevant data, and to discuss the potential contribution to environmental risk assessment.

Methods: This report is based on a review of several clinical/epidemiological studies conducted by WCUMS researchers since 1984, which included one cross sectional medical examination survey, one clinical/pathological analysis of 46 cases of mesothelioma, and three retrospective cohort mortality studies. Additional information acquired from reviewing original data first hand during a personal visit along with an interview of medical specialists from Da-yao County Hospital was also incorporated.

Results: The prevalence of pleural plaque was 20% among peasants in Da-yao over 40 years of age in the cross sectional survey. The average number of mesothelioma cases was 6.6 per year in the 1984-95 period and 22 per year in the 1996-99 period, in a population of 68 000. For those mesothelioma cases that were histology confirmed, there were 3.8 cases/year in the first period and 9 cases/year in the second. Of the 2175 peasants in this survey, 16 had asbestosis. Lung cancer deaths were significantly increased in all three cohort studies. The annual mortality rate for mesothelioma was 85 per million, 178 per million, and 365 per million for the three cohort studies, respectively. The higher exposed peasants had a fivefold increased mesothelioma mortality compared to their lower exposed counterparts. There were no cases of mesothelioma in the comparison groups where no crocidolite was known to exist in the environment. In the third cohort study, almost one of five cancer deaths (22%) was from mesothelioma. The ratio of lung cancer to mesothelioma deaths was low for all three studies (1.3, 3.0, and 1.2, respectively).

Conclusions: The observation of numerous mesothelioma cases at Da-yao was a unique finding, due mainly to their lifetime exposure to crocidolite asbestos. The finding of cases dying at a younger age and the relatively high ratio of mesothelioma cases to lung cancer could also be another unique result of lifetime environmental exposure to crocidolite asbestos. Although the commercial use of crocidolite has been officially banned since 1984, the incidence of mesothelioma has continued to show a steady increase, particularly among peasants. Since the latency of mesothelioma is approximately 30-40 years, the ban had little effect in the 1990s. The increased awareness and changes in diagnosis over time may also contribute to the increase. Furthermore, exposure to asbestos stoves and walls continued. The government implemented reduction of these exposures. However, from a public health standpoint, the most important issue is the complete avoidance of further exposure to asbestos.

In plain English

This article was the authors' review of studies done since 1984 by West China University of Medical Sciences researchers in Da-yao county, southwestern China. Surface soil there contains crocidolite ("blue clay"), which residents also used on walls and roads and to make asbestos stoves. In a 1984 survey of 2175 adults from two villages, 232 (11%) had pleural plaques, rising to 20% among those over 40, and 16 had asbestosis. None of 1338 comparison peasants had either. County hospital records listed 79 mesothelioma cases in 1984–95 (46 confirmed, 36 of them pleural) and 87 in 1996–99. Three cohort studies found mesothelioma mortality rates of 85, 177.5 and 365 per million per year. The second and third found no mesothelioma in their comparison groups. In the second cohort, the lung cancer death rate was 2.14 times the comparison group's (95% confidence interval 1.07 to 4.28). The authors concluded the many mesothelioma cases were due mainly to lifetime crocidolite exposure. They reported that vital status was unknown for about 10% of the first cohort and that 33 of the 79 hospital cases were unconfirmed.

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 of earlier studies by WCUMS researchers (one cross sectional medical survey, one clinical/pathological review of hospital mesothelioma cases, three retrospective cohort mortality studies), plus the authors' own site visit
• Who was studied: Residents of Jin-bi and Huang-hai-tun villages, Da-yao county, Yunnan province, China. Cross sectional survey in 1984: 2175 adults aged 20 and over, with 1338 comparison peasants. Cohort I: 5603 adults born 1915–1955, followed 1977–83. Cohort II: 4598 residents aged 30 or older, followed 1987–95, with 5641 comparison residents. Cohort III: 1610 peasants from cohort II, with 2481 comparison peasants.
• Exposure: Lifelong environmental exposure to crocidolite in surface soil ("blue clay"). Residents used it as wall stucco, to fill road potholes, and in hand and machine production of asbestos stoves.
• Main results:
  • Cross sectional survey: pleural plaques in 232 of 2175 adults examined (11%), and 20% among those over 40
  • Cross sectional survey: 16 peasants had asbestosis, a prevalence of 1.4% among those over 40
  • No pleural plaques or asbestosis were found in the comparison group
  • Hospital records 1984–95: 79 mesothelioma cases, 46 confirmed. Of the confirmed cases, 36 were pleural, 8 peritoneal, 1 mixed pleural/peritoneal and 1 benign peritoneal.
  • Hospital records 1996–99: 87 more mesothelioma cases, 37 histologically confirmed
  • Cohort I: lung cancer deaths (excluding mesothelioma) numbered 4, SMR 6.67. There were 3 mesothelioma deaths, an average annual mortality rate of 85 per million.
  • Cohort II: 21 lung cancer deaths. The lung cancer death rate was 53.3 versus 24.9 per 100 000 in the comparison group, relative risk 2.14 (95% confidence interval 1.07 to 4.28).
  • Cohort II: 7 mesothelioma cases (177.5 per million/year) in the exposed group, none in the comparison group
  • Cohort III (peasants): 6 lung cancer deaths. The crude lung cancer death rate was 43.8 versus 14.5 per 100 000 in the comparison group, relative risk 3.02.
  • Cohort III: 5 mesothelioma cases in the study group and none in the comparison group. This accounted for 22% of the 23 cancer deaths, a rate of 365 per million per year.
  • Crude mesothelioma rate ratio of peasants (higher exposure) to non-peasants (lower exposure) was 4.7 (365 versus 78 per million/year). The authors read this as indicating a positive dose response relation.
  • Ratio of lung cancer to mesothelioma deaths: 4 v 3 in cohort I, 21 v 7 (3.0) in cohort II, 6 v 5 in cohort III
  • None of the mesothelioma cases had a history of making asbestos stoves
• The authors' conclusion: The authors concluded that the numerous mesothelioma cases in Da-yao were due mainly to residents' lifetime exposure to crocidolite asbestos. They considered the lung cancer excess probably caused by crocidolite fibres, since smoking rates were similar between study and comparison groups.
• Limitations the authors state:
  • The false negative rate of chest fluoroscopy screening was unknown
  • In cohort I, vital status could not be identified for about 10% of subjects
  • In cohort II, unknown causes made up about 15% of deaths in both groups
  • 33 of the 79 hospital mesothelioma cases from 1984–95 were unconfirmed, diagnosed only by x ray and clinical observation
  • The hospital-based incidence was uncertain. Some local cases were treated elsewhere, so true incidence should have been higher, while some patients came from other villages.
  • A synergistic effect of smoking and asbestos could not be discounted as a contributor to the lung cancer excess
  • Asbestosis was not evaluated in the cohort studies, because of lack of funding
  • The rise in mesothelioma mortality between cohorts may partly reflect the second cohort being about 10 years older

Topics

The National Library of Medicine indexes this study under subjects our site covers: Crocidolite, Mesothelioma, Pleural plaques and thickening.

Medical terms in this study, explained in our Lexicon: Environmental Exposure, Crocidolite, Pleural Plaques, Pleural Mesothelioma, Mesothelioma, Histology, Lung Cancer, Cohort Study.

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