Cold dominates the temperature–mortality burden and defines a longer window for protection: a seven-year time-series study of cold spells and heatwaves in Changsha, China

Extreme temperatures raise mortality, but two questions remain open: how much of the burden falls on moderate rather than extreme temperatures, and whether cold spells and heatwaves add risk beyond daily temperature. We addressed both in a subtropical Chinese city. Daily meteorological and cause-of-death data for Changsha, China (2015–2021; 2,557 days) were analysed with a quasi-Poisson distributed lag non-linear model of daily mean temperature (maximum lag 21 days), adjusted for long-term and seasonal trend, day of week and holidays. Cumulative relative risks (RRs) were referenced to the minimum-mortality temperature (MMT), attributable fractions estimated for moderate and extreme cold and heat, and event indicators then added to test for effects beyond temperature. Subgroup differences were tested by the Altman-Bland method, and estimates re-examined under alternative specifications. Among 344,379 non-accidental deaths, mortality was lowest at 28.0 °C (85.7th percentile) and rose at both ends of the range. Relative to this optimum the cumulative 21-day RR was 1.355 (95% CI 1.198–1.532) at extreme cold (2.2 °C, 2.5th percentile) and 1.139 (1.043–1.244) at extreme heat (31.1 °C, 97.5th percentile). Lag-specific cold effects ran from day 1.4 to day 16.4, heat from day 0 to day 4. Cold accounted for 7.70% of deaths (1.31–13.36) against 0.85% (0.24–1.42) for heat, roughly 9:1; 86% of the cold and 54% of the heat burden arose at moderate rather than extreme temperatures. Once temperature was modelled, neither event type added risk. Adults aged 65 years and over were most vulnerable, though of four pre-specified comparisons only the age contrast for extreme heat was significant ( P = 0.011). Cumulative RRs were robust; attributable fractions were sensitive to knot placement. Daily temperature, moderate cold in particular, drove temperature-related mortality in this subtropical city. Cold risk built up over about two and a half weeks, heat risk within days. Protection for older adults should start before winter and be sustained; heat response should be fast. Both burdens fell mainly on moderate days, so a home left too cold in winter or too hot in summer can shorten life when no warning is in force.