The risk of hospitalization associated with hot nights and excess nighttime heat in a subtropical metropolis: a time-series study in Hong Kong, 2000-2019

Background: Recent studies showed increased mortality risks after hot nights, but their effect on hospitalizations, especially in vulnerable populations, remains under-studied. Methods: Daily hospitalization, meteorological (including hourly), and air pollution data were collected for the hot seasons (May-Oct) of 2000-19 in Hong Kong. We derived three hot-night metrics: HNday28°C, daily minimum temperature ≥28°C, the governmental definition of hot nights; HNe, hot night excess calculated by summing heat excess of hourly temperatures above 28°C at night; and HNday90th, hot nights classified using the 90th percentile HNe (17·7°C⋅ h) as a cutoff. We fitted time series regression with distributed lag nonlinear models to examine the associations of hot-night metrics with various hospitalizations. Findings: During the 3,680 study days, 5,002,114 non-cancer non-external (NCNE) hospitalizations were recorded. Half (1,874) of the days experienced excess nighttime heat (HNe>0) with a mean (SD) of 8·0 (6·8) °C⋅ h; 499 and 187 hot nights were identified by HNday28°C and HNday90th, respectively. Extreme HNe (99th percentile vs 0°C⋅ h) was significantly associated with increased NCNE hospitalizations over lag 0-4 days by 3·1% [95% confidence interval: 1·5%, 4·8%] overall, with enhanced effects in elderly (5·3% [3·2%, 7·4%]), low-SES individuals (5·3% [2·8%, 8·0%]), and circulatory admissions (3·4% [0·2%, 6·8%]). HNday90th, reflecting extreme HNe, better identified hazardous hot nights than the official HNday28°C. Interpretation: Excessive nighttime heat is significantly associated with increased hospitalizations, particularly affecting the elderly and socioeconomically disadvantaged individuals. Nighttime heat intensity should be incorporated in defining hot nights with public health relevance. Funding: British Heart Foundation.