Humidity, heat, and harm: Global review exposes the rising health risks

Extreme heat events are intensifying around the world, and a new international review led by Monash University researchers is providing crucial insight into where these events are hitting hardest, how they’re impacting human health and infrastructure, and what needs to change in global responses.

Published in the Annual Review of Environment and Resources, the study brings together researchers from 18 global institutions and offers the first global comparison of two heat indicators – ambient temperature and wet-bulb globe temperature (WBGT) –  that incorporates humidity, solar radiation and wind. 

By analysing trends from 1990 to 2023, the review shows a steady rise in extreme heat days, with sharp increases seen in the past two decades, especially in Africa, the Middle East, and parts of Asia.

Between 1990 and 2023, the average annual number of heatwave days was 15.6 days per year, but for the first 17 years (1990-2006), the average annual number was fewer, at 12 days. Between 2007 and 2023, heatwave frequency increased, with the average rising to 19.3 days. 

Extreme heat events are characterised by both high temperatures and prolonged durations, and are typically classified as heatwaves if they last for at least two or three consecutive days.

Looking beyond the temperature

“We’ve gone beyond just looking at temperature,” says first author Dr Shuang Zhou, from Monash University’s School of Public Health and Preventive Medicine. “By including WBGT, we can better-capture the real health burden of heat, especially in humid regions where the body’s ability to cool down is severely compromised.”

This dual-metric approach is critical, because heat alone doesn’t tell the full story. While ambient temperature reflects the general warmth of an environment, WBGT captures the felt experience of heat – especially in conditions where high humidity hampers the body’s ability to regulate its core temperature. 

This is particularly dangerous for people without access to cooling, those working outdoors, and individuals with pre-existing health conditions.

The review maps the geographic and temporal trends of extreme heat, but also provides one of the most comprehensive assessments to date of its effects. The authors examine a wide range of health outcomes – from heat-related illness and cardiorespiratory illness to infectious diseases, mental health conditions, and adverse pregnancy and birth outcomes. 

The gaps in protection

In addition, extreme heat was found to strain energy systems, drive up cooling demands, and result in significant economic losses, particularly in resource-limited settings.

“Humidity significantly alters how heat affects the body, yet it’s often left out of current warning systems and policies,” says senior author Professor Yuming Guo. “Our review highlights not just the risks, but the gaps in protection – especially in low and middle-income countries where support systems are weakest.”

Despite increasing recognition of heat-related health risks, the review found that many Heat Health Action Plans (HHAPs) and Heat Health Warning Systems (HHWSs) remain inadequate, largely due to their failure to address social inequalities and their limited inclusion of humidity in risk models. As the climate warms and humidifies, these omissions could result in preventable harm.

The review spells out the direct and indirect effects of extreme heat to human health – including cardio-respiratory diseases, infectious diseases, kidney disorders, metabolic diseases, mental health disorders, and adverse pregnancy and birth outcomes. Extreme heat also strains energy production and consumption, resulting in economic loss and further health impacts.

The  research team has proposed a new multilevel framework for improving heat preparedness and resilience, which recommends targeted actions across five levels: 

• International coordination to support funding and data sharing 

• National policy reform to prioritise vulnerable groups

• Institutional collaboration to enable effective forecasting and response

• Community-led initiatives to educate and protect at-risk populations

• Individual behaviour change supported by accessible tools and infrastructure.

A framework for harm reduction

The study also outlines opportunities for future research, including better integration of heat and humidity metrics in predictive models, evaluation of HHAP effectiveness in real-world settings, and investment in monitoring technologies for early warnings – especially in under-resourced regions.

“We hope this framework can help guide governments and health systems towards more responsive, inclusive and effective heat-health strategies,” Dr Zhou said. “The window for action is narrowing, but with the right tools, policies and partnerships, we can reduce harm and build resilience.”