Combined and interaction effects of outdoor and reading time on myopia onset: evidence from over 210 000 schoolchildren in China

While numerous studies have examined the individual effects of outdoor and reading time on myopia, the interaction between these factors in large populations remains underexplored. We hypothesised that increased outdoor exposure may mitigate the risk of myopia associated with prolonged reading. We included 219 802 initially non-myopic primary school students in the Tianjin Child and Adolescent Research of Eye study. We collected the behaviour exposure data via a questionnaire. We used Cox proportional hazard models, with semesters since entrance as the time scale, to assess the associations of outdoor and reading time with the onset of myopia. We evaluated interaction effects on the additive scale using a generalised propensity score-weighted Cox model, and quantified them by excess relative risk due to interaction (RERI), attributable proportion (AP), and synergy index (SI). Additionally, we conducted subgroup analyses to assess the robustness and consistency of the results. Among the 219 802 children, 130 246 developed myopia, resulting in a cumulative incidence of 59.3% over three years. Increased outdoor time of 1-2 hours (hazard ratio (HR) = 0.97; 95% confidence interval (CI) = 0.95, 0.98), 2-3 hours (HR = 0.90; 95% CI = 0.88, 0.92), and >3 hours (HR = 0.86; 95% CI = 0.84, 0.89) were associated with a reduced risk of myopia onset, whereas prolonged reading time of 1-2 hours (HR = 1.02; 95% CI = 1.01, 1.03), >2 hours (HR = 1.03; 95% CI = 1.01, 1.05), and >3 hours (HR = 1.05; 95% CI = 1.03, 1.08) were linked to an increased risk. The interaction effects on the additive scale were significant between outdoor time and reading time with RERI of -0.04 (95% CI = -0.08, 0.00), AP of -0.04 (95% CI = -0.07, 0.00), and SI of 0.71 (95% CI = 0.50, -0.93), particularly in boys, those in grades from 1 to 3, and those with myopic parents. Outdoor time and reading time are not only independently related to the onset of myopia but also interact with each other. These findings highlight the importance of promoting outdoor time and managing reading time to inform targeted myopia prevention strategies.