Background To communicate health risks associated with short-term changes in air pollution the US EPA reports air quality through the Air Quality Index (AQI). Based on National Ambient Air Quality Standards (NAAQS), the highest AQI value among the individual pollutants (driver pollutant) is reported daily to the general public. However, it remains unclear whether the single-pollutant AQI fully represents the actual risks of air pollution related illness. Methods Daily fine particulate matter (PM2.5) concentrations and AQI values from 2000-2010 in Bronx, Erie, Queens and Suffolk Counties of New York were used to estimate quarterly PM2.5 effective daily concentrations (EDC) at each AQI category (i.e. good, moderate, and sensitive). Estimates were made separately for days when PM2.5 was the driver pollutant vs. days when the AQI of other pollutants were reported. Daily counts of total cardiovascular hospital admissions (ICD-9 code 390-459), concentration-response functions for PM2.5, and the estimated quarte...