Climate change could make moisture-induced heat waves more dangerous, study finds

Temperature alone may overlook the real threat: This study shows that humidity can rapidly increase health risks during both hot and cold seasons, with climate change likely to make these compounded threats more common.

Study: Humidity may increase temperature-related health risks in the context of climate change. Image Credit: Quality Stock Arts/Shutterstock

In a recent study published in the journal scientific reportA group of researchers evaluated how combined temperature and humidity exposures affect health risks and identified higher risk thresholds under climate change scenarios.

temperature-humidity exposure background

What if the real danger during extreme weather is not just the heat or cold, but how humidity exacerbates it? Thousands of people die every year from extreme temperatures, but temperature is not the only threat. Humidity affects how our body adapts to temperature changes. As climate change increases both extreme temperatures and humidity, it is essential to understand how these factors affect human health, and more research is needed to improve risk assessment processes.

China Ambulance Dispatch Climate Study Design

The researchers analyzed emergency ambulance dispatch data from 13 large cities across China during 2013-2019, using daily data collected from local Centers for Disease Control (CDC) and ambulance services. These data provide an index of acute health outcomes.

Daily average temperature and relative humidity were obtained from the National Meteorological Database. To use the average air pollution level, environmental factors were taken into account, including particles smaller than 2.5 micrometres (PM2.5), ozone (o 3), and sulfur dioxide (SO2). Climate projections were produced using 12 global climate models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6), with shared socioeconomic pathways (SSP) and representative concentration pathways (rcps).

A time-series quasi-Poisson regression model with a distributed lag nonlinear model (DLNM) was applied to assess delayed health effects over seven days. Temperature and humidity were divided into percentage-based categories, generating 400 compound exposure scenarios, and grouped into hot-wet, hot-dry, cool-wet, and cool-dry categories.

The researchers incorporated city-specific estimates into national-level risk estimates using traditional meta-analysis methods. Thresholds for high-risk events were identified based on relative risk (RR) Patterns and statistical inflection points.

Compound Temperature-Humidity Health Risk Conclusion

The analysis included approximately 2.46 million ambulance dispatch records, revealing a clear link between weather conditions and health risks. It also showed that the relationship between temperature and ambulance dispatch follows a U-shaped curve, with extreme heat and extreme cold both associated with a higher risk of dispatch.

Analysis of combined temperature and humidity effects showed that all four mixed event types were associated with increased risk, and mixed temperature-humidity events posed a greater risk than single temperature exposures under the same temperature range. The greatest risk was assessed for the cold-dry event type (RR = 1.102; 95% CI = 1.045 – 1.161), followed by hot-wet (RR = 1.093; 95% CI = 1.068 – 1.118). Risk increased with pools even in hot-dry and cold-wet events RRs of 1.091 and 1.081 respectively.

Importantly, the findings show that temperatures do not need to be at the extremes of the distribution to pose a higher risk when combined with adverse humidity. This finding suggests that relying solely on temperature limits may underestimate real-world health risks. The study also showed that compound events posed consistently higher risks than single temperature exposures under the same thresholds.

Age-specific vulnerability and climate projections

Older adults were more likely than younger adults to call an ambulance during hot-wet and hot-dry periods. Additionally, older people aged 80+ were most at risk during hot-wet events, while people aged 60–79 also had an increased risk. Young and middle-aged adults were more vulnerable during cold-wet events, although some increased susceptibility was also seen in people aged 80 and older and those younger than 17, suggesting that patterns of risk were different in different age groups.

Spatial analysis revealed a higher frequency of mixed events in the southeastern and central climate zones, where it is warm and humid. Historically, these areas experienced 20 to 40 mixed events each year. However, with projected future climate scenarios, the frequency and area of ​​mixed events will increase significantly, especially under high-emission scenarios like SSP585.

The number of ambulance dispatches due to mixed incidents is expected to increase in the coming decades, reaching its highest level around the middle of the 21st century, and then declining later in the century. Heat-related events, particularly hot-wet conditions, are projected to become the major driver of health risks. In contrast, cold-related risks are expected to decrease over time.

Early warning and public health implications

Studies show that humidity substantially increases the health risks associated with extreme temperatures, making mixed temperature-humidity events more dangerous than temperature alone. Both hot and cold conditions become dangerous when combined with adverse humidity levels, especially for vulnerable populations. As climate change continues, these combined risks will occur more regularly, shifting the burden toward heat-related risks. It is important to include multiple meteorological factors in health risk assessments to obtain more accurate forecasts. These results underline the importance of improved early warning systems and public health strategies based on cumulative environmental risks.