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RATIONALE Outdoor air pollution causes emergency department (ED) visits and hospitalizations for childhood asthma. In the U.S., the Air Quality Index (AQI) alerts the public to air quality and provides behavioral recommendations to reduce exposure and harm, yet little is known about the relationship between the AQI and childhood asthma exacerbations. OBJECTIVE To test for association between the AQI and childhood asthma exacerbations resulting in ED visits and hospitalizations. METHODS Retrospective time-stratified case-crossover study, conducted using medical records data from 2010 thru 2018 for children aged 6-17 years with a primary diagnosis of an asthma exacerbation (defined as an ED visit or hospitalization for asthma) at UPMC Children's Hospital of Pittsburgh. Daily AQI data was obtained for Allegheny County, PA from the Environmental Protection Agency (EPA). Conditional logistic regression was used for analyses of the AQI (as both a continuous and categorical variable) and asthma exacerbations. Stratified analyses were conducted to explore modification of the AQI effects on asthma exacerbations by race and other covariates. RESULTS There were 6,573 events. Particulate matter <2.5μm (PM2.5) was the primary pollutant responsible for the AQI, followed by ozone (62% and 29% of days with events respectively). The overall AQI was associated with asthma exacerbations (e.g., as continuous, per 10-unit increase= lag day 2: OR 1.014 95% CI 1.003-1.025, lag day 3: OR 1.012 95%CI 1.001-1.023). By pollutant specific-AQI, the association was strongest for PM2.5. In stratified analyses, the AQI was associated with exacerbations in Black and younger children (6-11 years) on lag day 4. CONCLUSIONS The AQI is associated with asthma exacerbations among children in Allegheny County. This is driven primarily by PM2.5, with Black and younger children particularly affected. Healthcare providers should discuss the AQI in asthma management.