Ambient daily pollen levels in association with asthma exacerbation among children in Philadelphia, Pennsylvania

•Further, strong evidence that tree pollen triggers asthma exacerbation.•Even low levels of tree pollen (≤5 grains/m3) associated with small risk.•Clear exposure-response trend with increasing tree pollen levels.•Asthma patients with allergic rhinitis at greater risk from tree and weed pollen.•Grass pollen risks only at high levels that occurred a couple days per year.•Pollen-related health advisories may not align with risks for asthma patients. Pollen from trees, grasses, and weeds can trigger asthma exacerbation in sensitized individuals. However, there are gaps in knowledge about the effects, such as the relative risks from different plant taxa and threshold levels of effect. We aimed to describe the local association between pollen and asthma exacerbation among children in the City of Philadelphia, and to evaluate whether effects are modified by children’s characteristics and clinical factors (e.g., child’s age, race/ethnicity, comorbidities). We conducted a time-stratified case-crossover study of pediatric (age