Extreme heat events increasingly pose significant global health risks, especially in vulnerable and under-resourced regions. By investigating the differences in these events since 1990 for the first time, researchers have proposed targeted strategies and potential improvements to mitigate the impacts of extreme heat events and humidity.
The review, in collaboration with an international team of experts from 18 institutions across the globe, was led by researchers from Monash University's School of Public Health and Preventive Medicine, and published in Annual Review of Environment and Resources. It revealed that between 1990 and 2023, the average annual number of heat wave days was 15.6 days.
During the first 17 years (1990–2006), the average annual number of heat wave days averaged 12 days. However, between 2007 and 2023, heat wave frequency increased, with the average annual number of days rising to 19.3 days. A clear global increase in heat wave days was observed, especially in Africa, the Middle East and parts of Asia.
Extreme heat events are characterised by both high temperatures and prolonged durations, and are typically classified as heat waves if they last for at least two or three consecutive days.
Unlike other reviews, to measure the changes of extreme heat events over location and time, the researchers used ambient temperature and the wet-bulb globe temperature (WBGT), which accounts for humidity, solar radiation, and wind.
First author Dr Shuang Zhou said, for the first time, they'd mapped the global distribution of extreme heat events using both traditional temperature and wet-bulb globe temperature, which incorporates humidity.
"This matters because humidity dramatically amplifies heat stress and health risks, especially in vulnerable populations. Our review goes beyond describing impacts – we also propose a multilevel framework for improving heat preparedness at every level, from international policy to individual action," Dr Zhou said.
The review highlights the direct and indirect effects of extreme heat to human health, including heat-related illnesses, cardio respiratory diseases, infectious diseases, kidney disorders, metabolic diseases, mental health disorders, and adverse pregnancy and birth outcomes. In addition, extreme heat also strains energy production and consumption, resulting in economic loss and further health impacts.
Senior author Professor Yuming Guo said, extreme heat was no longer a problem of the future, it was a present and growing threat.
"Our findings highlight not only the severe and widespread health effects of heat and humidity, but also how uneven protection systems are distributed. Many low- and middle-income countries remain unprepared. We urgently need stronger, more equitable heat health systems that reflect the realities of a warming, humidifying world," Professor Guo said.
The review highlighted that extreme heat events are intensifying worldwide, with hotspots emerging in regions such as the Middle East, eastern South America, and northern Africa, where the intensity, frequency, and duration of heat events are increasing at the fastest rates.
These events have caused significant mortality and widespread health impacts. In response, governments and international organisations developed various heat health action plans (HHAPs), including heat health warning systems (HHWSs). However, the review found current risk-reduction strategies remain inadequate.
After identifying major limitations in HHAPs and HHWSs, especially their lack of equity and insufficient consideration of humidity, the authors proposed a new multilevel framework. This framework integrates international, national, community and individual strategies for improving heat resilience, offering actionable guidance to governments and health systems worldwide.
Recommendations to reduce extreme heat impacts
International
To play a key role in coordinating international collaboration, guiding policy development, promoting technical and financial support, facilitating knowledge dissemination, and supporting capacity building to strengthen global resilience to heat.
National
Setting clear targets, ensuring transparent decision-making processes, promoting inclusive and equitable policies and prioritising the needs of the most vulnerable.
Institutional (e.g, health sectors, meteorological agencies)
Must work together across sectors to monitor heat risks, develop accurate forecasts, conduct collaborative research, and prepare emergency responses, with accurate warning information promptly communicated to the public and communities to ensure timely and effective interventions.
Community
Coordinating local resources and tailoring local plans, managing emergency responses and community engagement, disseminating, educating, training, communicating, and supporting vulnerable groups and promoting sustainable practices.
Individual
Using cooling equipment, such as air conditioners.
Full list of the collaboration institution: EPhysLab,CIM-UVigo, Universidade de Vigo, the Spanish Council for Scientific Research, CSIC (Spain); University of Adelaide, Australian National University, University of Sydney(Australia); University of Washington, University at Albany, State University of New York (USA); London School of Hygiene & Tropical Medicine (UK); University of Tokyo and Nagasaki University (Japan); Seoul National University (South Korea); National University of Singapore (Singapore); Chinese Center for Disease Control and Prevention (China); and Technische Universität Berlin (Germany) (Spain).
Read the research paper: https://doi.org/10.1146/annurev-environ-111523-102139
