When scorching temperatures meet high humidity, the air can feel suffocating, much like being trapped in a sauna. “It’s no longer just hot—it’s dangerous,” says Prof. Miaoni Gao, a climate scientist from Nanjing University of Information Science and Technology, China.

Unlike dry heat, humid conditions limit the body’s ability to cool itself through sweating, increasing the risk of heat-related illnesses. In recent years, these compound heat–humidity extremes—also known as humid-heat waves—have become more frequent and intense across eastern China. Vulnerable groups such as the elderly and children are particularly affected, as their bodies are less capable of adapting to extreme conditions. As global warming accelerates, humid-heat stress is emerging as a major yet under-recognized public health challenge in densely populated regions.

In a new study published in Atmospheric and Oceanic Science Letters, Prof. Miaoni Gao’s team reports that compound heat–humidity extremes in eastern China are now lasting longer than ever before, significantly increasing health risks, particularly for the elderly and young children. Based on climate records dating back to 1961, the researchers found that the active season for compound heat–humidity extremes has expanded, especially in the Yangtze–Huaihe region and South China. Over the past decade, the season in South China alone has lengthened by about one month.

What’s driving this trend? “It’s the western expansion of the Western North Pacific Subtropical High,” explains Prof. Gao. This climatic shift intensifies solar radiation and drives persistent warm air and moisture into eastern China, amplifying and prolonging the active season of compound heat–humidity extremes.

But heatwaves aren’t just weather events—they’re also public health emergencies. In 2023 alone, over 37,000 heat-related deaths were recorded nationwide. The study found that physiologically vulnerable groups—primarily the elderly and young children—now make up 39% of the total population exposed to compound heat–humidity extremes. Notably, one-third of the increase in exposure can be attributed to the lengthening of the heatwave season. In South China, this proportion soars to 56%.

As China’s population continues to age, with over 200 million people now aged 65 or older, the convergence of demographic vulnerability and extended heatwave seasons could lead to a major public health crisis.

The researchers call for urgent action: “We can no longer rely on fixed-schedule summer warnings,” says co-author Ms. Xinyue Sun. “We need dynamic early warning systems that reflect the changing timing of heatwave seasons, along with targeted health interventions to protect the most vulnerable populations.”

IMAGE: The westward extension of the western North Pacific subtropical high leading to a prolonged season of compound humid–heat extremes (CREDIT: XINYUE SUN)

Citation:

Miaoni Gao, Xinyue Sun, Buda Su, Lu Tang, Xingyu Cheng, Tong Jiang, 2025. Prolonged seasons of compound heat–humidity extremes amplify vulnerable population exposure in eastern China. Atmospheric and Oceanic Science Letters, https://doi.org/10.1016/j.aosl.2025.100663.