Researchers Identify Temperature, Humidity Combinations That Stress the Heart


The findings could inform revisions to safety guidelines and policies that help protect people with cardiovascular issues during heat waves.

New research findings show that cardiovascular strain begins at lower temperature and humidity levels than those that increase the body’s core temperature, according to findings presented at the American Physiology Summit.

Credit: New Africa -

Credit: New Africa -

Researchers have long known that more heart attacks occur during extreme heat events because of cardiovascular strain. This will become an increasingly common issue as global climate change results in more extreme weather events, including heat.1

“We wanted to establish the critical combinations of temperature and humidity that cause a continuous rise in heart rate—indicating cardiovascular strain—in healthy adults,” said the study’s first author Rachel Cottle, a doctoral candidate at Pennsylvania State University (PSU), in a press release. “We plan to use this as a baseline before identifying the conditions that are dangerous for more vulnerable groups such as older men and women.”1

The study was part of the PSU Human Environmental Age Thresholds (HEAT) project led by W. Larry Kenney, PhD, FAPS. It included 45 participants in their 20s who slowly walked on a treadmill or performed activities simulating minimal activities of daily living, all while inside environmental chambers that produced either warm and humid or hot and dry environmental conditions.1

“Our findings can help determine when alerts are needed to advise people to limit activity, stay hydrated, and remain in the shade or air conditioning as much as possible,” Cottle said in the press release. “They could also help determine when organized sporting events should be postponed, cooling centers should be implemented, or outdoor workers should take extra breaks.”1

According to the study, the investigators found that during minimal activity in warm and humid environments, cardiovascular strain increased when temperatures reached 95 degrees Fahrenheit and relative humidity was 65%, whereas increases in core temperature occurred at 95 degrees and 73% relative humidity. During slow walking in warm and humid environments, cardiovascular strain increased at 95 degrees and 52% relative humidity, whereas core temperature increased at 95 degrees and 58% relative humidity.1

In hot and dry environments, cardiovascular strain rose at temperatures above 113 degrees and 19% relative humidity, whereas core temperature increased at 118 degrees and 17% relative humidity during minimal activity. During slow walking in the same conditions, increases in cardiovascular strain occurred at 104 degrees and 25% relative humidity, and increases in core temperature occurred at 109 degrees and 22% relative humidity.1

“If we know that there’s a heat wave coming and it’s going to be 100 degrees and 70% relative humidity, which is above those limits that we saw for cardiovascular strain, then we can say, ‘Ok, if you can stay in air conditioning as much as possible, then do that. Make sure you’re adequately hydrated,” Kenney explained in an interview with the American Physiological Society.2


  1. Staying Safe When It’s Hot: Study Identifies Temperature-humidity Combinations that Stress the Heart. News release. American Physiological Society. April 18, 2023. Accessed April 17, 2023.
  2. Study Identifies Temperature-humidity Combinations that Stress the Heart. YouTube. April 12, 2023. Accessed April 17, 2023.
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