Extreme, possibly fatal, mixtures of heat and humidity are emerging more rapidly across the globe, a comprehensive evaluation of local weather station data throughout the world suggested.

These potentially dangerous events, assessed by wet-bulb temperature (a measure combining air temperature and humidity) approaching or beyond 30°C, have more than doubled since 1979, according to Colin Raymond, PhD, of NASA's Jet Propulsion Laboratory in Pasadena, California, and colleagues.

In some places, wet-bulb temperature already has exceeded 35°C (95°F), the temperature at which humans can no longer regulate body heat, Raymond and colleagues reported in Science Advances. A wet-bulb temperature of 35°C translates roughly to a heat index of 160°F, and wet-bulb temperatures above 30°C are considered severe.

"Extreme heat is already a major public health challenge worldwide," said co-author Tom Matthews, PhD, of Loughborough University in England. "Our research also adds to the growing body of work suggesting that humid heat beyond prolonged human physiological tolerance -- a wet-bulb temperature around 35°C -- may emerge at regional scales with relatively modest further increments in global mean temperature."

"At local scales, around individual weather stations, our research suggests that this threshold may have been passed already," Matthews told MedPage Today.

The research contributes to the "growing evidence and chorus of scientific voices warning that global warming is pushing humanity towards a world that is simply too hot to live in," observed Liz Hanna, PhD, MPH, of the Australian National University Climate Change Institute, who wasn't involved with the study.

"Climate and health researchers have long argued that we are moving closer to a health emergency," Hanna told MedPage Today. "Currently, more than 30% of the global population experience at least 20 days per year above a lethal temperature threshold of 35°C with high humidity," she added. "This figure increases every year, and the geographical range of the planet that is too hot to sustain normal human life is expanding."

Earlier climate models have projected that combined heat and humidity could reach 35°C by mid-century. This study is the first to use observational data to suggest it already has done so.

Read the full article on the MedPage Today website, featuring commentary by Dr Elizabeth Hanna