Joint effects of temperature and relative humidity on COPD hospital admissions in England, 2003-2021: A nationwide case-crossover study
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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 99 th 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.
Key Message
What is already known on this topic
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Higher ambient temperatures are consistently associated with an increased risk of COPD hospital admission, but little is known about the role of relative humidity.
What this study adds
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Using 19 years of nationwide individual-level data on 473,494 COPD hospital admissions in England, we identified a J-shaped association with temperature and a U-shaped association with relative humidity. Around 1,000 summer COPD admissions per year were attributable to non-optimal temperature and around 900 to non-optimal relative humidity. Evidence of interaction was weak.
How this study might affect research, practice and/or policy
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Relative humidity should be considered alongside temperature in environmental risk assessments, early warning systems, and heat-health action plans for people with COPD. Adaptation strategies, including housing improvements, indoor cooling, and tailored guidance, should be developed and evaluated to reduce temperature- and humidity-related COPD hospitalisations.
