Increasing humid conditions in temperatures as low as 34 degrees Celsius can lead to a progressive rise in heart rate, also known as cardiovascular strain, warns a new study.

While previous studies have established heat-related morbidity and mortality in humans, the new study published in the Journal of Applied Physiology, showed that even moderate heat can affect the human heart.

The study aimed to identify combinations of ambient temperature and water vapour pressure at which a continuous rise in heart rate begins to occur and also compare those environments to the environmental limits for the maintenance of heat balance.

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Even as extreme heat events are becoming more frequent, with last week, the global average temperature hitting a record high two days in a row, the study adds much relevance.

Researchers from the Pennsylvania State University (Penn State) asked 51 young, healthy participants to engage in light physical activity inside an environmental chamber, where temperature or humidity rose every 5 minutes.

These participants were given a capsule each, with sensors that helped the researchers to monitor their core temperature — the temperature of internal organs — as well as their heart rates.

Whether systematically increasing ambient temperature or humidity, the onset of cardiovascular strain occurred at lower environmental conditions compared with elevated core temperature.

In other words, the findings revealed that the increase in heart rate occurs even before a person’s internal temperature starts to rise, Nature reported.

“More people are going to be exposed to heatwaves and potentially be at risk,” Rachel Cottle, a researcher in exercise physiology at Penn State, was quoted as saying.

The study which identifies the combination of temperature and humidity that endangers the heart could inform strategies to protect human health, she said.

Further, it also showed that in humid conditions, that is around 34 degrees Celsius, participants who were walking slowly too experienced cardiovascular strain.

The team found that the participants’ heart rates always rose about 20 minutes before their core temperatures began to increase. Because heart rate is so easy to measure, it could be a useful warning sign, the researchers said.

“If all of a sudden you notice your heart rate going up quickly and progressively, then that might mean that your core temperature will start to rise. That’s when you need to take precautionary measures,” Cottle said.