A new study by researchers at Oxford Population Health and the Chinese University of Hong Kong has shown that excess nighttime heat may be associated with an increased risk of emergency hospitalisation in densely populated cities, even though many residents have access to air-conditioning. The study, which examined the health effects of nighttime heat in Hong Kong over a 20-year period from 2000 to 2019, is published in The Lancet Regional Health ‒ Western Pacific.
As global temperatures continue to rise with climate change, urban populations are increasingly exposed to extreme heat during the night. Among other adverse effects, nighttime heat can impair sleep quality and quantity, which are important risk factors for disease risk.
Hong Kong has a population of 7.4 million people living in just over 1,000 square kilometres and a significant urban heat island effect (where urban areas are significantly warmer than surrounding rural areas). This made it an ideal setting to explore the impact of nighttime heat on public health, a factor seldom considered in previous studies that have typically focused on average daily temperature exposure.
The researchers compared the local government metric for defining hot nights ‒ a daily minimum temperature of 28°C (HNday28°C) or higher ‒ with two alternative measures: hot night excess (HNe; the sum of the extra degrees per hour above the 28°C threshold at night) and hot night day defined using the 90th percentile HNe as a cut-off (HNday90th).
Key findings:
- There was a 3.1% increase in hospitalisations associated with extreme HNe (equivalent to experiencing ~2°C above 28°C for 12 hours at night), compared to no HNe;
- Larger increases in hospitalisations (3.4‒5.3%) were observed for those with circulatory conditions, the elderly, and socioeconomically deprived individuals;
- There was evidence of a potential threshold effect for HNe exposure, with significant excess risks emerging at 15–20°C⋅h (equivalent to experiencing 2°C above 28°C for 10 hours);
- There was no excess risk of hospitalisations on days that used the government definition of a hot night (HNday28°C), but the new metrics (HNe and HNday90th) showed a clear association between hot nights and risk of increased hospitalisations.
These findings suggest that the current standard for defining hot nights may fail to capture critical periods of intense nighttime heat that pose the greatest risks to public health.
Peter Ka Hung Chan, Senior Environmental Epidemiologist at Oxford Population Health and co-lead author, emphasised the importance of considering heat intensity throughout the night ‘Nighttime temperature typically drops gradually until shortly after sunset. Like our study, previous research suggested that excess heat in the early night is more likely to disturb sleep onset and may be more harmful. So, we can’t simply have a single cut-off temperature and disregard the intensity of nighttime heat people experience, even if later in the same day temperatures fall below the standard.’
The study also highlighted that unusual nighttime heat during the late hot season (September‒October) could overwhelm people’s physiological and infrastructural capacity to adapt to the heat, leading to even higher risks of hospitalisation (7.2%). The occurrence of multiple consecutive hot nights, especially since 2014, has further exacerbated these risks.
Kin-fai Ho, Professor at the JC School of Public Health and Primary Care at The Chinese University of Hong Kong, added ‘By adjusting for multi-day temperature averages, this study isolated the effects of hot-night metrics independent of average temperatures. Although more extreme events were relatively rare in the late hot season, there is compelling evidence that they had a more severe impact on health.’
These findings could support policymakers in considering the criteria for defining hot nights and adopting more accurate and comprehensive measures like HNe, which could be applied globally to inform nighttime heat adaptation strategies and public health responses to climate change.