Synergistic impacts of heat, pollen, and air pollution on allergic rhinitis and asthma under climate change: A 20-year time-series study

Abstract

Background: Climate changes are increasing the frequency of concurrent extremes in temperature, air pollution, and aeroallergens, yet evidence on their joint and synergistic health impacts remains limited. We aimed to quantify the independent, joint, and interactive short-term effects of temperature, air pollutants, and airborne pollen on allergic rhinitis and asthma using long-term general practitioner (GP) data. Methods: We conducted a population-based time-series study using 20 years of GP data. Daily maximum temperature, PM2.5, ozone, and pollen concentrations were linked to allergic rhinitis and asthma outcomes. We estimated cumulative relative risks (RR) over lag 0-14 days using distributed lag non-linear models, comparing high (95th percentile) versus median exposure levels. We evaluated effect modification through stratified analyses and quantified additive interaction for joint exposures at extreme levels (90th and 95th percentile) using relative excess risk due to interaction (RERI) and attributable proportion (AP). Findings: Pollen exposure was strongly associated with allergic rhinitis (RR=2.54, 95% CI: 2.40-2.69) and with asthma (RR=1.49, 95% CI: 1.38-1.61). In joint-effects analyses, co-exposure to extreme heat and high pollen concentrations was associated with an increased risk of allergic rhinitis (RR= 2.07, 95% CI: 1.77-2.41), with clear evidence of synergistic interaction on the additive scale (RERI=0.48, 95% CI: 0.32-0.64, AP=0.23, 95% CI: 0.17-0.30). Similarly, co-exposure to high pollen and ozone was associated with elevated allergic rhinitis risk (RR = 2.03, 95% CI: 1.76-2.34), with positive additive interaction (RERI = 0.40, 95% CI: 0.25-0.54; AP = 0.20, 95% CI: 0.13-0.26). The same exposure combinations, heat-pollen and pollen-ozone, also exhibited positive synergistic interactions for asthma. Conclusion: Our findings identify pollen as a central driver of climate-sensitive allergic morbidity, with heat and ozone acting as key amplifiers through synergistic interactions. Our findings highlight the need for integrated early-warning systems, and risk assessments that account for joint environmental exposures.