Air temperatures as low as 34 °C can lead to a steady increase in heart rate under humid conditions, a study1 says. This rise, also known as cardiovascular strain, occurs even before a person’s internal temperature starts to increase, the study found.

The findings are among a slew of recent results about the heart’s struggles when exposed to heat. Scientists say this work is becoming ever more relevant as extreme heat events become more frequent. Just this week, the average temperature worldwide hit a record high two days in a row.

“More people are going to be exposed to heatwaves and potentially be at risk,” says Rachel Cottle, a researcher in exercise physiology at the Pennsylvania State University in State College and a co-author of the paper, which was published in the Journal of Applied Physiology. But work to identify the combination of temperature and humidity that endangers the heart could inform strategies to protect human health, she says.

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