Effect of population density on the results of the study of water supplies in five California counties

Our plain-English summary of Effect of population density on the results of the study of water supplies in five California counties, by P.M. Conforti (Environmental Health Perspectives, 1983).

The study

• Author: P.M. Conforti
• Journal: Environmental Health Perspectives
• Published: 1983;53:69-78
• Cited by: 1 article, 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. 000599).

Abstract

Two previous studies (1969-1971 and 1969-1974) examined the association between cancer incidence and chrysotile asbestos ingested through drinking water in the San Francisco-Oakland Standard Metropolitan Statistical Area (SMSA). Population density, an important covariable in the association between cancer and environmental agents, was not included in the analyses of these studies. The present work determines the effect of this covariable on the results of the second San Francisco-Oakland SMSA study. The original and reanalyzed results are compared to reassess the association between cancer and asbestos. The only change in the regression procedures of the original studies was the addition of population density as an independent variable in the reanalysis. The results of the reanalysis showed that population density had little effect on the results of the second study. Slightly more significance was found for asbestos regression coefficients in the reanalysis, including population density, than in the original analysis. These regression coefficients for asbestos indicated a positive association between ingested chrysotile asbestos and some cancer body sites. The conclusion of the reanalysis was that population density was distributed across the San Francisco-Oakland SMSA in such a way that it had little effect on the observation of an association between ingested asbestos and cancer.

In plain English

This study reanalyzed data from an earlier study of cancer incidence from 1969 to 1974 and chrysotile asbestos in drinking water in the San Francisco-Oakland area. That earlier analysis had not accounted for population density. The author grouped 722 census tracts into 427 'super tracts', of which 410 were analyzed. The regressions for 35 cancer sites among white males and females were repeated with population density added. Asbestos was measured as chrysotile fibers per liter of drinking water. The cancer sites with statistically significant positive associations with asbestos were essentially the same as in the original study. For males these included all sites combined, most digestive cancers, prostate, and lung small cell carcinoma. For females they included all sites, most digestive cancers, respiratory and breast cancers. Digestive tract and rectum were significant for females only in the reanalysis. The author concluded that population density had little effect on the association. The author cautioned that the study compared groups rather than individuals, that definitive conclusions were untenable, and that the findings did not lend themselves to judging regulation of asbestos in drinking water.

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: Reanalysis of an ecologic (group-level) study: regression of cancer standard incidence ratios by census tract grouping, adding population density as a variable
• Who was studied: Census tract groupings (super tracts) of the San Francisco-Oakland SMSA, with cancer incidence for 1969-1974 in white males and females. The 722 1970 census tracts were grouped into 427 super tracts; 410 were analyzed after 17 with incomplete information were removed.
• Exposure: Chrysotile asbestos in drinking water, measured as fibers per liter. Super tract groupings ranged from 25,000 to 36,000,000 fibers/L. The analysis also included population density, socioeconomic status, marital status, and the proportion of workers in asbestos-related industries.
• Main results:
  • Adding population density left the cancer sites with significant positive asbestos coefficients essentially unchanged from the original study.
  • White males: significant (p less than 0.05) positive asbestos coefficients for all sites combined, most digestive and digestive-related cancers, prostate, and lung small cell carcinoma. None of the population density coefficients in these equations was significant.
  • White females: significant positive asbestos coefficients for all sites combined, most digestive and digestive-related cancers, respiratory cancers, and breast cancer.
  • White females: two sites, digestive tract and rectum, were significant in the reanalysis but not in the original study.
  • For males, including population density slightly decreased the significance of the asbestos coefficients. For females, the coefficients were generally more significant, especially all sites and all digestive.
  • The correlation between asbestos and population density was 0.486.
• The authors' conclusion: The author concluded that population density was distributed across the area in a way that had little effect on the observed association between ingested asbestos and cancer at certain sites. The author recommended further direct-design studies of digestive cancers and ingested asbestos.
• Limitations the authors state:
  • The associations were indirect: groups of people in super tracts were analyzed, not individuals.
  • The assumptions of this indirect method make definitive conclusions untenable.
  • Case numbers were small for some cancer sites (for example, fewer than 0.5 small intestine cases per super tract), which reduced the power to detect an association.
  • The 35 cancer sites analyzed were not independent of one another. Grouped sites had more cases but could not identify the exact location in the body.
  • The author stated that the findings do not support conclusions about regulating asbestos in drinking water.

Topics

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

Medical terms in this study, explained in our Lexicon: Chrysotile.

Related studies

• Additional comments on the study of the effect of population density on the results of the California Bay area study — P.M. Conforti (1983)
• A graphical analysis of the interrelationships among waterborne asbestos, digestive system cancer and population density — M.E. Tarter, R.C. Cooper, et al. (1983)
• Answers to some questions raised by the presentation on the graphical analysis of the interrelationships between waterborne asbestos, digestive system cancer and population density — M.E. Tarter (1983)
• Asbestos in public water supplies: discussion of future problems — J.P. Hills (1979)
• Chemical contamination of California drinking water — H.H. Russell, R.J. Jackson, et al. (1987)
• Asbestos in drinking water and cancer incidence in the San Francisco Bay area — M.S. Kanarek, P.M. Conforti, et al. (1980)

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