Joint effects of temperature and relative humidity on COPD hospital admissions in England, 2003-2021: A nationwide case crossover study

Background: Heat exposure has been linked to chronic obstructive pulmonary disease (COPD) outcomes, but evidence on relative humidity and humidity-temperature interaction remains limited and inconsistent. Methods: We conducted a nationwide time-stratified case-crossover study of 473,494 COPD hospital admissions in England during the summer months (June-August) from 2003 to 2021. Case-control pairs were linked to high-resolution daily maximum temperature and relative humidity data derived from HadUK-Grid and ERA5-Land, respectively. Exposure-response relationships were modelled using distributed lag non-linear models. Joint effects were examined using interaction models, including linear interaction, temperature spline by humidity category, and fully categorical specifications. Analyses were stratified by age group, sex, and socioeconomic deprivation. Results: COPD admission risk increases steeply at higher temperatures following a J-shaped curve. The exposure-response for relative humidity is U-shaped with a relative risk of 1.08 (95% Confidence Intervals: 1.00 - 1.15) at the 99th relative humidity percentile. We find limited evidence of effect modification by socioeconomic deprivation for both exposures. An estimated 1,040 (95%CI: 820 - 1,270) summer COPD admissions per year are attributable to non-optimal temperatures and 910 (95%CI: 680 - 1,160) to relative humidity. Higher relative risks are observed with concurrent high temperature and high humidity, but the evidence is weak. Conclusion: High temperature and high relative humidity are associated with increased risk of COPD admission, with limited evidence of an interaction. Relative humidity should be considered alongside temperature in environmental risk assessment and targeted adaptation strategies, such as proactive heat-health advice for people living with COPD.