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.
A new study conducted by The Jockey Club School of Public Health and Primary Care (JCSPHPC) at The Chinese University of Hong Kong (CUHK) and Oxford Population Health has been published recently in The Lancet Regional Health ‒ Western Pacific. The study reveals an association between excessive nighttime heat and an increased risk of emergency hospital admissions in densely populated cities, even among residents with access to air-conditioning. It examined the health effects of nighttime heat in Hong Kong over a 20-year period from 2000 to 2019.
Urban heat island effect and the impact of nighttime heat on public health in Hong Kong
Given the constant rise of global temperatures due to climate change, urban populations face greater exposure to extreme nighttime heat. Unlike other adverse effects, nighttime heat can disrupt sleep quality and duration, both of which are important risk factors for various diseases.
Hong Kong, home to of 7.4 million people within just over 1,000 square kilometres, experiences a pronounced urban heat island effect, making urban areas significantly warmer than nearby rural regions. This context provides an ideal setting for investigating the impact of nighttime heat on public health, a factor often neglected in past studies that have primarily concentrated on average daily temperature exposure.
Nighttime heat intensity and its impact on health and adaptation strategies
The study also highlighted that atypical nighttime heat during the late hot season (September‒October) could exceed people’s physiological and infrastructural ability to cope, resulting in an increased hospitalisation risk of 7.2%. Additionally, the frequency of multiple consecutive hot nights, particularly since 2014, has intensified these risks.
Dr 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.”
Professor Kin-fai Ho, Associate Director (Graduate Studies and Research) and Professor at JCSPHPC, 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 underscore the need for policymakers in re-evaluating the criteria for defining hot nights and in adopting more precise and comprehensive measures like HNe. Such approaches could be implemented globally to guide nighttime heat adaptation strategies and public health responses to climate change.
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