Tree pollen and asthma-related hospital admissions in England: a national case time series analysis

• National analysis of 437 543 asthma admissions in England, 2008-2022. • Small-area case time-series design linking hospital and pollen data. • Tree pollen exposure increased asthma admissions, mainly in children. • Associations appeared even at moderate pollen concentrations. • Findings support early warnings for pollen-sensitive populations. Evidence linking pollen exposure to asthma exacerbations is limited and inconsistent across regions, pollen types, and age groups. We assessed the association between pollen concentrations and asthma-related hospital admissions across England at high spatial resolution. We use unplanned asthma admissions records (2008–2022) from Hospital Episode Statistics, linked to 10-km gridded data on alder and birch pollen. Tree pollen exposure was categorised as low, medium, or high. Age-specific case time series analyses were conducted using conditional Poisson regression, controlling for temperature and air pollutants (PM 2.5 and NO 2 ). Analyses were restricted to January–August, when tree pollen is present. Elevated asthma admission risk was associated with both pollen types, with a non-linear exposure–response that increased sharply at low levels and attenuated at higher exposures. For alder pollen, relative risks (RRs) across all ages were 1.014 (95%CI: 0.998, 1.031) for low, 1.026 (1.007, 1.046) for medium, and 1.019 (0.995, 1.044) for high exposure. For birch, RRs were 1.016 (0.996, 1.037), 1.041 (1.019, 1.06), and 1.032 (1.005, 1.060), respectively. Risks were mostly limited to children, with medium alder pollen exposure associated with RRs of 1.047 (0.993, 1.105) and 1.112 (1.066, 1.159), and birch with RRs of 1.131 (1.066, 1.201) and 1.079 (1.029, 1.131) in 0–4 and 5–14-year-olds, respectively. No evidence of association was found in older groups. Moderate tree pollen levels are associated with increased asthma admissions in younger populations in England. Further work is needed to understand group and individual susceptibility.