Humidity can make extreme temperatures more dangerous for human health, according to a study published in Scientific Reports and announced on Apr. 19. The research found that both hot and cold weather combined with high or low humidity sharply increase the risk of emergency medical incidents, with these threats likely to become more common as climate change progresses.
The study matters because it suggests that temperature alone does not capture the full risk posed by extreme weather events. As global temperatures rise and humidity levels shift due to climate change, understanding how these factors interact is crucial for protecting public health.
Researchers analyzed about 2.46 million ambulance dispatch records from 13 large cities across China between 2013 and 2019. They matched this data with daily temperature and relative humidity readings from national meteorological databases, while accounting for air pollution levels such as particulate matter (PM2.5), ozone (O3), and sulfur dioxide (SO2). Climate projections were based on models from the Coupled Model Intercomparison Project Phase 6.
The analysis revealed a U-shaped relationship between temperature and ambulance dispatches: both heat waves and cold spells increased emergency calls. When researchers looked at combined temperature-humidity scenarios—categorized as warm-wet, warm-dry, cold-wet, or cold-dry—they found all types raised health risks beyond what would be expected from temperature alone. Cold-dry conditions posed the highest risk (relative risk of 1.102), followed closely by warm-wet events.
Importantly, even moderate temperatures could pose significant danger when paired with unfavorable humidity levels—meaning existing warning systems based solely on temperature may underestimate real-world risks. Older adults were especially vulnerable during hot periods with high or low humidity; younger people faced higher risks during cold-wet conditions.
Looking ahead, the frequency of compound heat-humidity events is projected to increase in southeastern and central China under high-emission scenarios like SSP585. While heat-related emergencies are expected to rise mid-century before declining later on, improved early warning systems that consider both temperature and humidity will be important for public safety going forward.
