The San Francisco Bay epidemiology studies on asbestos in drinking water and cancer incidence: relationship to studies in other locations and pointers for further research.

I would like to give my views on why the results in the San Francisco Bay studies (1, 2) are generally more positive than the other ecological epidemiology studies that relate asbestos in drinking water to the incidence of cancer. Many other people have speculated, and I think it would be very appropriate here for me to do this also. I see four possible explanations for the generally larger number of positive results from the San Francisco Bay studies as compared with the studies in Puget Sound (3), Connecticut (4, 5), and other places. First, it is possible that the San Francisco Bay Area results are spurious or that the epidemiology studies done in other areas lack enough sensitivity to detect a real effect. This seems doubtful, however, considering how much effort has been made by various parties to negate the San Francisco positive results and to no avail. Also, the studies from Duluth (6), Connecticut (4, 5), Puget Sound (3), and San Francisco Bay (1, 2) are apparently so well and thoroughly done that it is hard to dismiss any of them as insensitive in design. The second explanation is one of simple statistical power considerations. The San Francisco Bay Area study used the largest population. Thus, for cancers of low incidence, it had enough power to detect an association, whereas the other studies had too few cases to merit a similar association. The third explanation concerns differences in fiber amounts, fiber size and the follow-up period.