Increased mortality risks of winter temperature flips: A growing concern in aging society of subtropical climate regions

Climate change has led to unpredictable, intense temperature fluctuations during warming winters, particularly in subtropical regions. Among that, abrupt shifts from warm to cold temperatures over short periods pose health challenges yet remain poorly understood. Historical data from Hong Kong revealed a rising trend in the frequency of temperature flip events, characterized by larger temperature drops and faster cooling rates. The temperature flip events were associated with increased risks of total non-external (RR: 1.08, 95% CI: 1.05-1.11) and cause-specific mortality, with a higher risk observed at or below the cold weather warning threshold (RR: 1.13, 95% CI: 1.09–1.17). Greater magnitude of temperature drops, shorter durations, and more rapid cooling during flip events intensified the mortality risk, while the starting temperature showed an inverse relationship. Events that began with warming temperature followed by rapid cooling were particularly associated with increased pneumonia mortality risk (RR: 3.17, 95% CI: 2.14–4.68), while cold-start flips with rapid (RR: 1.21, 95% CI: 1.14–1.29) or slow (RR: 1.08, 95% CI: 1.03–1.13) temperature declines were linked to increased total mortality. The health impacts varied across genders and causes of death, with the elderly being especially vulnerable under both cold-slow (RR: 1.11, 95% CI: 1.04–1.18) and cold-rapid (RR: 1.20, 95% CI: 1.11–1.30) flip scenarios. These findings highlight that the increased mortality risk among vulnerable populations caused by sudden temperature flips may be overlooked. The potential neglect underscores the urgent need to enhance cold weather warning systems and health interventions to better protect vulnerable populations from the adverse effects of temperature shifts in aging society of subtropical climate regions.