claimed water (exposed areas) and control areas, which were known to use water supplies that did not receive any reclaimed water but possessed demographic characteristics similar to the reclaimed water areas. In the first study, the population of the exposed areas of the Montebello Forebay region was between 478,000 and 486,000, according to the 1970 and 1980 censuses. The population in the control areas was between 677,000 and 576,000.

In the second study, the size of the population exposed to reclaimed water had almost doubled since the time of the first study, to around 900,000 people in the 1990 census. This represents about 10 percent of the total population of Los Angeles County. The control areas were in three parts of the county (parts of Montebello Forebay, Pomona, and northeastern San Fernando Valley) and included about 700,000 people.

Study Design

Both studies used an ecologic study design in which the unit of analysis was the census tract. Each census tract was assigned a categorical exposure variable derived from estimates of the percentage of reclaimed water in the water supply, as determined by the water suppliers serving the area. Information on health outcomes came from existing morbidity and mortality data collected by state and county surveillance programs, death certificates, cancer registries, and the University of Southern California Cancer Surveillance Program.

The first study also included a household survey in 1981 of randomly selected women in the two study areas. Telephone interviews were conducted with approximately 1200 adult females in "high" reclaimed water areas and 1300 women in the control areas to investigate possible differences in spontaneous abortions and other adverse reproductive outcomes, as well as other measures of general health (bed days, disability days, perception of well-being) and specific diseases. Data were collected on demographic and socioeconomic characteristics and possible confounding factors such as smoking and alcohol consumption, level of education, consumption of bottled water, and length of residence in the study area.

Exposure Assessment

The proportion of the water supply that originated from reclaimed water varied over time and geographic location. Each water supplier in the study area (32 in the first study, 39 in the second study) was queried about its water sources, delivery practices, service areas, and production levels during the study periods.

The actual amount of reclaimed water in the water supplies is un-

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