Discovery of the effect of humidity on the human heart, study suggests

A new study warns that rising humidity conditions in temperatures as low as 34°C can lead to a progressive increase in heart rate, also known as cardiovascular strain.

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

The aim of the study is to identify the combination of ambient temperature and water vapor pressure at which a sustained increase in heart rate occurs and to compare those to the environmental limits for maintaining heat balance.

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Even as extreme heat events continue to rise, last week, the global average temperature reached record highs two days in a row, the study adds much relevance.

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

These participants were each given a capsule fitted with sensors that allowed researchers to monitor their core temperature – the temperature of internal organs – as well as their heart rate.

Whether systematically increased ambient temperature or humidity, the onset of cardiovascular stress occurred at lower environmental conditions than at elevated core temperature.

In other words, the findings showed that the increase in heart rate occurs before a person’s core temperature begins to rise, Nature reported.

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

He said studies that identify the combination of temperature and humidity that puts the heart at risk could inform strategies to protect human health.

Furthermore, it also showed that in humid conditions, which is around 34 °C, participants who were walking slowly experienced cardiovascular stress.

The team found that participants’ heart rates always increased approximately 20 minutes before their core temperature began to rise. 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 that your heart rate is increasing rapidly and progressively, it may mean that your core temperature will start to rise. That’s when you need to take precautionary measures,” Cottle said.