[Reading Science] "Heatwave, Fatal to the Heart from 34℃"

Young People Also Begin to Experience Heart Strain When Humidity Is High

Due to global warming, heatwaves with temperatures exceeding 35 degrees Celsius are frequent during the summer. Amid this, a study has found that even healthy adults begin to experience strain on their hearts at around 34 degrees Celsius in humid weather. It is pointed out that preventive measures should be taken by those with heart conditions or the elderly.

A research team from Pennsylvania State University in the United States published a study with these findings on the 22nd of last month in the international journal Journal of Applied Physiology. The team recruited 51 young and healthy volunteers for the experiment. Participants performed light physical activities in a specially controlled environment where temperature and humidity increased by set increments every five minutes, while their bodily changes were measured. Each participant swallowed a capsule equipped with a temperature sensor to measure internal body temperature and heart rate. As the temperature rose, participants’ heart rates increased, then plateaued as they adapted, repeating this pattern. However, when the temperature was continuously raised, the volunteers’ heart rates kept accelerating and remained elevated even after the experiment ended. This indicates that the cardiovascular system experienced a consistent burden due to the temperature increase.

In particular, when participants walked slowly in a high-humidity environment, they began to feel cardiovascular strain starting at around 34 degrees Celsius. In contrast, this threshold rose to 41 degrees Celsius in dry conditions. Additionally, it was found that participants started feeling cardiovascular strain about 20 minutes before their internal body temperature rose. Rachel Cottle, a professor at Pennsylvania State University who participated in the study, explained in the international journal Nature, "A rapid increase in heart rate means that the internal body temperature has started to rise," adding, "That is precisely the time to take preventive measures."

Another study published in August 2021 showed that temperature increases put strain on heart function even during rest. A research team from the University of Roehampton in the UK confirmed that heart rates of people resting at about 50% humidity varied significantly depending on temperature. At 50 degrees Celsius, participants’ heart rates were on average 64% higher than at 28 degrees Celsius. Researcher Lewis Halsy explained, "If you are resting exposed to sunlight at the beach or elsewhere, your heart rate will continue to increase."

When the temperature rises, the human body initiates two activities to adapt and maintain internal body temperature: sweating and increasing blood flow from the internal organs to the skin. The heart plays a central role in this process, so the heart rate inevitably increases. Healthy and young adults generally tolerate this well. However, for the elderly or those with heart conditions, exposure to extreme heat can be fatal. A study published last year reported that for every 1 degree Celsius increase in temperature, the mortality rate related to cardiovascular diseases rises by 2.1%.