To the Editor, In 2023, a systematic review and meta-analysis concluded that outdoor pollen may affect asthma exacerbations in adults [1]. However, the authors cautioned that the literature is heteroge- nous and most studies are ecological and have not considered individual-level confounders or effect modifiers. We here examined associations between daily concentrations of grass pollen, three tree pollens, and nettle pollen (represents weeds) assigned to home addresses using a deterministic model developed by the UK Met Office [2], and asthma hospital ad- missions among UK Biobank adult participants [3] from 2011 to 2022. Effect modification by demographic and environmental factors was examined. Bidirectional adjusted time-stratified case-crossover models as- sessed associations between emergency asthma hospital admis- sions (493 ICD9 or J45/J46 ICD10 codes) and same-day pollen levels (high vs. low) [2] assigned to home addresses. Further information on data sources and the statistical analysis is in the Supporting Information. Effect modification by daily mean NO2, PM2.5, and ozone was examined (using tertiles), and models were stratified by annual average NO2 and PM2.5 levels, % greenspace, sex, age, ethnic- ity, BMI, smoking, education, income, deprivation (as defined in Table S1), and genetic risk for atopy, the latter as a marker for sensitization (Supporting Information). There were 1893 asthma emergency hospital admissions (pri- mary diagnosis, > 14 days apart) among 1489 participants from January to September 2011–2022. Descriptive statistics for the pollen variables, study population, and confounders (i.e., me- teorological and pollution variables) are provided in Table 1; Tables S1 and S2, respectively. Asthma hospital admissions were higher on days of high alder and grass concentrations and when considering all pollen types together (Table 1). These findings remained fairly consistent when adjusting for air pollutants (Table S3), examining differ- ent lags (Table S4) and restricting to groups and time periods of interest (Table S5). Effect estimates for alder (but not birch) were also elevated when using a second pollen dataset for ex- ternal validation (European pollen reanalysis, not available for the other pollen types, Table 1) [4]. The adverse association ob- served with high grass concentrations is in line with most exist- ing literature, whereas that with alder has been reported in some but not all studies [1].
Publications · October 28, 2026