Background: Extreme temperatures pose a substantial respiratory health threat, yet their impact on severe clinical outcomes in temperate settings remains unclear. We examined associations between heat and cold exposure and respiratory mortality and hospitalizations in Copenhagen, Denmark.

Methods: We conducted a time-stratified case-crossover study among adults aged ≥30 years from 2002 to 2018. Daily temperature was related to mortality, hospitalizations, and fatal hospitalizations for all respiratory diseases, acute lower respiratory infection (ALRI), and chronic obstructive pulmonary disease (COPD). Conditional logistic regression estimated relative risks (RRs) with 95% confidence intervals (CIs) across lags up to 21 days. Stratified analyses evaluated susceptibility by sex, age, and income.

Results: We identified 162,665 respiratory hospitalizations, 23,600 fatal respiratory hospitalizations, and 16,610 respiratory deaths. Extreme heat (99th percentile, lag 0-7 days) was associated with respiratory hospitalizations (RR=1.07; 95% CI: 1.01, 1.14), with stronger associations for fatal respiratory hospitalizations (RR=1.25; 95% CI: 1.08, 1.43) and fatal ALRI (RR=1.34; 95% CI: 1.09, 1.65). Heat effects on mortality exceeded those on hospitalizations for all respiratory diseases (RR=1.35; 95% CI: 1.14, 1.59) and COPD (RR=1.48; 95% CI: 1.17, 1.86). Risks were most pronounced among males and adults ≥75 years. Prolonged cold increased respiratory hospitalizations but showed a non-significant association with fatal hospitalizations.

Conclusion: Extreme heat and prolonged cold both increased respiratory morbidity, while extreme heat additionally elevated the risk of fatal respiratory hospitalization and mortality, particularly among males and older adults, suggesting a substantial preventable burden.