Hay Fever Symptom Burden From Grass Pollen: Implications for Public Health Warning Thresholds

This study quantifies the lagged association between grass pollen exposure and the onset of allergic rhinitis (AR) symptoms, including differences between asthmatic and non-asthmatic subgroups. Data included 114,834 self-reported AR symptoms collected from 25,194 uniquely identified users of a smartphone application. Reports were gathered prospectively for seven consecutive grass pollen seasons (2017-2023) in Melbourne. We adjusted for meteorological, air quality, and other time-varying covariates. A quasi-Poisson regression model combined with a distributed lag non-linear model (DLNM) was used, testing 0-7 days lagged effects. An association between grass exposure and daily AR incidence of any severity was observed on "low" exposure days (19 grains/m³), when exposure was modeled cumulatively across a 3-day lag period (RR 1.31, 95% CI 1.16-1.47). Higher grass concentrations were associated with increased risk at lag-1, with risk rising from RR 1.08 (95% CI 0.99-1.17) at 19 grains/m³ to RR 1.19 (95% CI 1.09-1.29) at 150 grains/m³. By lag-2, risk returned to a non-significant baseline. Asthmatics, users reporting medication use, and users reporting more severe symptoms exhibited elevated risk at later lags. The magnitude and duration of elevated risk varied across stratified models by age and sex. We provide new population-level evidence that grass pollen triggers allergic rhinitis symptoms predominately on the day of exposure, even at concentrations traditionally classified as "low", with symptom risk persisting longer in asthmatics and other sensitive groups. This study highlights the value of refining communication and early-warning systems to better support individuals who are sensitive to even low levels of exposure.