As part of a multi-year study of air pollution and respiratory hospital admissions in the Buffalo, Albany, and New York City, New York, metropolitan areas, filter samples were collected daily at suburban air monitoring sites and analyzed for their content of particulate phase aerosol strong acidity (i.e., hydrogen ion, H+) and sulfate (SO4 = ). In addition, daily hospital admissions for respiratory causes, other community air pollutant measurements (e.g., ozone, O3), and meteorological data (e.g., temperature) were also obtained for these metropolitan areas. The summer months (June-August) were selected for analysis because that is when the highest H+ (and O3) are usually experienced at these sites, and because these months are rarely complicated by other major influences (e.g., high pollen counts). Thus, any pollution-admissions relationships were expected to be most clearly discernible in this season. Prior to the health effects analysis, the summer admissions and environmental data were first detrended to eliminate long-wave autocorrelations, and day-of-week effects were removed via regression. Cross-correlations of the filtered 1988 and 1989 admissions and environmental data revealed strong associations between elevated summer haze pollution (i.e., H+, SO4 =, and O3) and increased total respiratory and asthma admissions on the same day and/or on subsequent days in Buffalo and New York City, especially during the summer of 1988 (when pollution levels were more extreme). Regression analyses indicated that the pollution-admissions associations remained significant (p < 0.05) even after the simultaneous inclusion of lagged daily maximum temperature. Mean effects calculations for these cities indicated that summertime haze can play a significant role in the occurrence of respiratory admissions in that season: accounting for an average 6 to 24% of 1988 Buffalo and NYC asthma admissions (depending on the pollutant index employed). O3 consistently had the highest mean effects estimates. Relative risk (RR) calculations indicated that the risk of admission for asthma was increased by a factor of 1.19 to 1.43 in these cities on maximum 1988 summertime pollution days, with H+ consistently having the highest RR estimates. These results are consistent with the hypothesis that ambient acid aerosol peaks (e.g., H+ > or = 100 nmol/m3) can potentiate the respiratory disease effects of O3. Associations were weaker in the less urbanized Albany metropolitan area and in the New York City (NYC) suburbs, even though the NYC suburban O3 exposures were similar to (and the H+ concentrations may even be somewhat higher than) those in the center city.(ABSTRACT TRUNCATED AT 400 WORDS)