Extreme temperature events and hospitalizations for 59 autoimmune diseases in Korea: A nationwide outcome-wide,...

Extreme temperature events are increasing in frequency and intensity, yet their effects on autoimmune diseases remain poorly understood. Using nationwide inpatient claims from the Korean National Health Insurance Service (2002 -2023), we assessed short-term associations between extreme temperatures and hospitalizations for 59 autoimmune conditions. A time-stratified case-crossover design with conditional logistic regression was applied. Heat waves and cold spells were defined using district-specific temperature percentiles (95th, 97.5th, and 99th for heat; 5th, 2.5th, and 1st for cold) and single-day and consecutive exposures (2-3 days). Multiple testing used the Benjamini-Hochberg false-discovery rate. For FDR-significant outcomes (inflammatory bowel disease and Crohn's disease), subgroup analyses evaluated heterogeneity by sex, income level, and urbanicity with Wald tests of effect modification. Among 536,485 autoimmune disease-related hospitalization events, heat waves were more consistently associated with an increased risk of hospitalization compared with cold spells, with stronger associations at higher percentiles and with 2-3 consecutive days of exposure. The strongest associations were observed for gastrointestinal autoimmune conditions, with effect estimates progressively strengthening across higher percentile thresholds and longer durations. Three consecutive days of heat-wave exposure at the 95th and 97.5th percentile were associated with increased hospitalizations for inflammatory bowel disease (OR = 1.13 at both thresholds; 95% CI, 1.06-1.21 and 1.05-1.22) and its Crohn's disease subtype (OR = 1.15 at 95th and 1.17 at 97.5th; 95% CI, 1.05-1.25 and 1.07-1.29), both retaining significance after FDR correction. For inflammatory bowel disease and Crohn's disease, heat-related associations appeared larger in men, those younger than 65, low- and middle-income groups, and non-metropolitan residents, although formal interaction tests did not reach statistical significance. These findings identify autoimmune diseases, particularly inflammatory bowel disease subtypes, as potentially temperature-sensitive outcomes warranting confirmation in independent cohorts and further mechanistic investigation.