Additional comments on the study of the effect of population density on the results of the California Bay area study

Our plain-English summary of Additional comments on the study of the effect of population density on the results of the California Bay area study, by P.M. Conforti (Environmental Health Perspectives, 1983).

The study

• Author: P.M. Conforti
• Journal: Environmental Health Perspectives
• Published: 1983;53:191-2
• Read it: PubMed · Publisher · Free full text

In plain English

This was a short commentary, not a new study. The author answered questions raised after presenting an earlier Bay Area analysis of asbestos in drinking water and cancer. The author explained that tracts served by more than one water source were given weighted average fiber counts. Tract areas used for population density did not exclude land unavailable for housing. The analyses also accounted for schooling, family income, the share of workers in construction, electrical and textile industries, and marital status. Citing the earlier results, the author said asbestos remained significantly associated with cancer despite San Francisco County's high population density and fiber counts. Of 722 census tracts in 1970, 212 were reported as having what the scanned text prints as '107 or more' asbestos fibers per liter of drinking water; 149 of these were in San Francisco County. Tests for linearity showed departures only for breast, female reproductive and urinary cancers in white females. The author concluded that no area dominated the results. The author also judged comparisons with industrial (inhaled) exposure, or with an Iowa population-density study, inappropriate.

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: Commentary in which the author replied to questions about the author's earlier census-tract regression study of ingested asbestos and cancer
• Who was studied: Census tracts of the San Francisco-Oakland Standard Metropolitan Statistical Area (five Bay Area counties). There were 722 census tracts in 1970, and 1960 and 1970 tracts were combined into 'super tracts'.
• Exposure: Asbestos fibers in drinking water (ingested asbestos), measured as fiber counts in water samples. Tracts with more than one water source were given weighted average counts. No fiber type is named.
• Main results:
  • The analyses included the proportion of workers in construction, electrical and textile industries as an indicator of possible occupational asbestos exposure. They also included schooling, income and marital status.
  • Citing the regression results of the earlier paper, the author reported that asbestos was significantly associated with cancer despite San Francisco County's high population density and high asbestos levels.
  • Of the 722 census tracts in 1970, 212 were reported as having high asbestos fiber concentrations in drinking water. The scanned text prints the cutoff as '107 or more' fibers per liter.
  • 149 of the 212 high fiber concentration census tracts were in San Francisco County.
  • The other 63 high fiber concentration tracts (30%) were in Alameda and San Mateo Counties.
  • The highest concentrations of asbestos in drinking water were found in San Mateo County.
  • Chi-square tests for linearity across four asbestos groupings showed no nonlinearity for white males. Only breast, female reproductive and urinary cancers in white females departed from linearity.
  • The author judged comparisons with industrial asbestos exposure inappropriate because the exposures differed (inhalation versus ingestion).
  • The author judged the results not comparable with an Iowa population-density study by Haenszel and colleagues, because of differences in study area and adjustment.
• The authors' conclusion: The author concluded that, given the linear associations and the many high fiber tracts outside San Francisco County, no area in the study dominated the results.
• Limitations the authors state:
  • Census tract areas used for population density did not exclude parts of tracts unavailable for housing, such as bodies of water and public parks.

Related studies

• Effect of population density on the results of the study of water supplies in five California counties — 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)
• Comments on the California studies — R.C. Cooper (1983)
• Asbestos in drinking water and cancer incidence in the San Francisco Bay area — M.S. Kanarek, P.M. Conforti, et al. (1980)
• Additional notes on the case-control study in western Washington on the cancer risk from asbestos in drinking water — L. Polissar, R.K. Severson, et al. (1983)

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