As temperatures rise due to global warming, scientists are worried about how extreme heat may impact heart health. Research published in the Journal of Applied Physiology warns that air temperatures as low as 93° F may lead to a steady increase in heart rate under humid conditions. This increase, referred to as cardiovascular heart strain, can occur even before a person’s internal temperature starts to rise.

The study involved healthy participants engaging in light physical activity inside an environmental chamber. As the chamber got hotter, participants’ heart rates increased and then plateaued. However, when the chamber continued to heat up, heart rates began to rise again and continued to rise even after the experiment. This rapidly increased heart rate indicates cardiovascular strain.

A related study published in Physiological Reports reported that even at rest, heat can affect the heart. Researchers found that at 50 percent humidity, the heart rate of participants at rest was, on average, 64 percent higher at 122° F than at 82° F. These findings are concerning, as extreme heat events are becoming more common around the world and could be lethal for vulnerable and older populations.