Excessive Time Spent in Outdoor Light Increases Risk of Rheumatoid Arthritis: Results from a Prospective Cohort

Background: Sun exposure has been demonstrated to be a risk factor to some autoimmune diseases. However, the evidence of the correlation between sun exposure time and rheumatoid arthritis (RA) remains unclear. The objective is to investigate the association between time spent in outdoor light and RA risk.Methods: A prospective cohort study with a total of 358,450 participants. Restrictive cubic spline (RCS) was employed to investigate whether nonlinear associations existed between sunlight exposure and RA outcomes. Cox proportional hazards model was used to explore the association between sun exposure and RA risk. Sensitivity analyses, interaction analyses and subgroup analyses with a view to assessing stability and heterogeneity of outcomes were carried out.Findings: After a median follow-up of 12.34 years, 4,302 RA cases were documented. The basic model showed a dose-dependent linear relationship between outdoor sunshine duration and RA risk, and the incidence of RA was significantly increased when the average sun exposure time exceeded 2.5 hours/day [hazard ratio (HR):1.175, 95% confidence interval (95%CI): 1.074-1.286]. Over 2 hours/day in summer and over 3 hours/day in winter appears to have a marked impact on RA incidence.Interpretation: Prolonged exposure to outdoor light presents linear correlations with RA risk. Excessive exposure to natural light can be considered a suspected environmental risk factor for RA.Funding: This study was funded by grants from the National Natural Science Foundation of China (81872687, 82103932). The Open Project of Jiangsu Provincial Science and Technology Resources (Clinical Resources) Coordination Service Platform (No.TC2022B016).Declaration of Interest: None declared.Ethical Approval: A prospective cohort study was conducted using longitudinal data from the UK Biobank, which comprising an unprecedented amount of disease and lifestyle information data from approximately half a million people (aged 40-69 years, roughly averages for males and females) across the UK from 2006 to 2010. 22 assessment centers were established in England, Scotland and Wales and has been approved by the North West Multi-center Research Ethics Committee (MREC) as a Research Tissue Bank (RTB) approval.