Associations between conjunctivitis and ambient PM2.5 and physical activity: A nationwide prospective cohort study

Currently, there is no evidence of fine particulate matter pollution (PM2.5) altering the relationship between physical activity (PA) and the risk of conjunctivitis. Based on the UK Biobank study, we included 308,507 participants aged 40–69 years at baseline (2006 to 2010) and prospectively followed up for conjunctivitis diagnosis till 2020. Annual concentrations of PM2.5 in 2010 were estimated for each participant using Land Use Regression models. PA levels during work and leisure time were reported via the International Physical Activity Questionnaire at baseline. We used Cox proportional hazards models to examine the associations of PM2.5 and PA with incident conjunctivitis, as well as their interaction at both multiplicative and additive scales. During the 11.6 years of follow up, we identified 4002 incident conjunctivitis cases. High-PA (≥3000 metabolic equivalent of task [MET]-mins/week) was associated with lower risk of conjunctivitis (hazard ratio [HR]: 0.79, 95% confidence interval [CI]: 0.73–0.86) compared to low-PA (0 to <600 MET-mins/week), while every 1 μg/m3 increment in PM2.5 was associated with a 16% higher risk of conjunctivitis (HR = 1.16, 95% CI: 1.09–1.23). We did not observe statistically significant interactions between PM2.5 and PA on their associations with conjunctivitis. Habitual PA and PM2.5 exposure were oppositely related to incident conjunctivitis. The benefits of PA remain in people irrespective of exposure to air pollution.