PolyU research unveils critical role of humidity and indoor conditions in heatwave severity assessments, introducing a new framework for standardised heatwave indices

Unprecedented extreme heat represents a critical climate challenge, resulting in approximately 490,000 heat-related deaths globally each year, with 45% of these fatalities occurring in Asia. A study by The Hong Kong Polytechnic University (PolyU) has revealed that some existing heatwave indices are unable to accurately capture heatwave severity in diverse geographical regions and climate conditions. It is suggested to include humidity and indoor environmental conditions in identifying a dangerous heatwave. The development of a global heat risk framework and early warning systems are also recommended to alleviate the impact of heatwaves on human health. The research findings have been published in the international journal Nexus.

Working outdoors in high temperatures for extended periods can lead to heat stress, dehydration and heat-related illnesses. Currently, there is no global standard for measuring the severity of heatwaves, and each index has different definition of  dangerous heat stress. A research team led by Prof. Qihao WENG, Chair Professor of Geomatics and Artificial Intelligence of the Department of Land Surveying and Geo-Informatics, Associate Director of the Research Institute for Land and Space, Director of the PolyU Research Centre for Artificial Intelligence in Geomatics, and Director of the Jockey Club STEM Lab of Earth Observations together with Postdoctoral Fellow Dr Pir MOHAMMAD, examined the effectiveness of six heatwave indices in identifying dangerous heatwave, including maximum daily air temperature, humidity index, humidex, wet bulb globe temperature, lethal heat stress index and universal thermal climate index.

The research team conducted case studies on recent heatwave events observed in Spain and the United States in 2022, as well as in India in 2023. The results revealed that the lethal heat stress index, which combines temperature and humidity, is more effective in identifying dangerous heatwave days in a low-humidity environment compared to other indices. It can also differentiate between areas that are affected by extreme heat stress and those that are not. The other five heatwave indices failed to effectively identify dangerous heatwave conditions in diverse climate, geographic and meteorological conditions.

Heatwave lethality depends not only on the maximum air temperature, but also on relative humidity, particularly in sub-tropical regions like Hong Kong. The study showed that, in humid conditions, even if the ambient temperature is only 28 °C, the saturated air moisture makes effective sweat evaporation and heat dissipation difficult and causes body temperature to rise faster, potentially leading to fatigue, dehydration or heat stroke. The team suggested that evaluating heat stress in humid conditions is crucial for better understanding the impact of heatwaves on human health, particularly for construction workers and agricultural labourers who spend substantial time outdoors.

In addition, the existing heatwave indices, such as the Hong Kong Heat Index, which is based on outdoor environment conditions, rely on outdoor heat mapping data provided by climate services. However, most heat-related mortalities occur indoors. In Hong Kong’s densely populated environment with many high-rise buildings, residents of rooftop houses and subdivided flats are particularly vulnerable to extreme heat. With rapid global warming, the researchers emphasised that building morphology and material, overheating conditions, and air-conditioning are essential in calculating indoor heat stress. They also suggested revisiting thresholds to establish new global criteria for defining dangerous heatwave limits, considering varied climatic conditions and human adaptability based on age, gender and household conditions.

Prof. Qihao Weng said, “Understanding the relations between climate, heat stress and health is crucial for formulating targeted adaptation and mitigation strategies. The findings underscore the need for a global heat risk framework and early warning systems, as well as the adoption of more sophisticated heat stress mitigation and adaptation measures by local authorities in response to rapid global warming.”

Dr Pir Mohammad said, “The study highlights the potential gap between reported outdoor heat data and often overlooked indoor heat impacts. Integrating these perspectives could provide a more comprehensive understanding of heat-related risks and their public health implications and thus help establish a more precise evaluation of dangerous heatwave conditions.”

The research team also discussed enhancing heatwave predictions to mitigate their impacts. They emphasised the importance of interdisciplinary collaboration among climate scientists, health professionals, policymakers and communities to safeguard public health in a changing climate.

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