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 <18 years) asthma exacerbation, with cases identified through electronic health records (EHR) of the Children’s Hospital of Philadelphia (CHOP) health system from March through October in the years 2011–2016. Daily pollen counts were obtained from the local National Allergy Bureau certified pollen counter. We applied conditional logistic regression to estimate odds ratios (OR) and 95% confidence intervals (CI) for the association between the pollen level (vs. none detected) and odds of asthma exacerbation, adjusting for temperature, relative humidity, and holidays. We estimated same-day exposure effects, as well as effects from exposure lagged by up to 5 days. There were 35,040 asthma exacerbation events during the study period, with the majority occurring among black, non-Hispanic children (81.8%) and boys (60.4%). We found increased odds of asthma exacerbation among Philadelphia children in association with tree pollen, both for total tree pollen and most individual tree types. Increased odds from total tree pollen were observed at the lowest levels studied (≤5 grains/m³, unlagged, OR = 1.06, 95% CI: 1.02, 1.10), and exhibited a positive exposure-response pattern of effect; tree pollen levels above 1000 grains/m³ (unlagged) were associated with 64% increased odds of asthma exacerbation (95% CI: 1.45, 1.84). Grass pollen was associated with asthma exacerbation only at levels above the 99^th percentile (52 grains/m³), which occurred, on average, two days per year during the study period (with 2-day lag, OR = 1.38, 95% CI: 1.19, 1.60). There was an inverse association (reduced asthma exacerbation) with ragweed pollen that was consistent across analyses. Pollen from other weeds was associated with increased odds of asthma exacerbation, without a clear exposure-response pattern (2-day lag, significant increases ranging from 8% to 19%). Increased odds from tree pollen and weeds (other than ragweed) were higher among children with allergic rhinitis. While there are known benefits from urban vegetation for human health, there are risks as well. It is important to note, however, that pollen is released during a limited time frame each year, and advisories informed by local data can enable susceptible individuals to avoid outdoor exposure on high-risk days.

树木，草和杂草中的花粉会引起敏感人群的哮喘加重。但是，关于效果的知识还存在差距，例如来自不同植物类群的相对风险和效果阈值水平。我们的目的是描述费城儿童花粉与哮喘恶化之间的局部联系，并评估其影响是否因儿童的特征和临床因素（例如，儿童的年龄，种族/民族，合并症）而改变。我们对儿童（年龄<18岁）哮喘急性发作进行了时间分层的病例交叉研究，并于2011年3月至10月通过费城儿童医院（CHOP）卫生系统的电子健康记录（EHR）确定了病例–2016年。每日花粉计数是从当地国家过敏局认证的花粉柜台获得的。我们应用条件逻辑回归来估计花粉水平（相对于未检测到）与哮喘加重几率之间的关联的比值比（OR）和95％置信区间（CI），并针对温度，相对湿度和假期进行调整。我们估算了当日暴露的影响，以及最长不超过5天的暴露带来的影响。在研究期间，发生了35,040起哮喘急性发作事件，其中大多数发生在黑人，非西班牙裔儿童（81.8％）和男孩（60.4％）中。我们发现，与树木花粉有关的费城儿童中，哮喘加重的几率增加，无论是树木总花粉还是大多数树木类型。³，未滞后，OR ＝ 1.06，95％CI：1.02，1.10），并表现出正的曝光-响应模式。超过1000粒/ m ³（未滞后）的树花粉水平与哮喘急性发作几率增加64％相关（95％CI：1.45，1.84）。草花粉仅在99以上的水平与哮喘恶化相关联的^第百分位数（52格令/米³），平均在研究期间每年发生两天（滞后2天，OR = 1.38，95％CI：1.19，1.60）。豚草花粉呈负相关（减少哮喘发作），在所有分析中均一致。来自其他杂草的花粉与哮喘发作的几率增加相关，而没有明确的暴露-应答模式（2天滞后，显着增加，范围为8％至19％）。过敏性鼻炎患儿中，树花粉和杂草（豚草除外）的几率增加。尽管已知城市植被对人类健康有好处，但也存在风险。但是，必须注意的是，花粉是每年在有限的时间范围内释放的，而根据本地数据提供的建议可以使易感人群避免在高风险日暴露在室外。