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

The California Bay Area studies concerning ingested asbestos and cancer have been the subject of much discussion. A number of very good questions were brought up after my presentation (1) and I would like to address a number of the points. One questioner asked whether the interconnections of the water supply systems were considered as a factor in my study. I think that the interconnection of water supply systems is an important point regarding the application of asbestos fiber counts from water samples to the levels of asbestos in drinking water of census tracts. For census tracts with more than one source of water, weighted averages of asbestos fiber counts were computed. These averages were weighted for source, treatment process, pressure zone, and/or time. The question, does the method of calculating population density take into account areas within the census tracts that were unavailable for residential purposes, such as bodies of water and public park lands, is an interesting one. The method for calculating the areas of census tracts did not take into account the portions of census tracts that were unavailable for residential purposes. The boundaries and/or areas of these portions of the census tracts were not accessible. Ultimately the areas within census tracts that were unavailable for residential purposes as well as the areas that were available for residential purposes but were uninhabited should be identified and considered in any analysis including population density. One questioner asked what the population density criteria were for aggregating census tracts