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Heat waves have hit much of the U.S. this summer. Scientists are learning how high temps affect the body. Aleksandar Nakic/Getty Images
  • Research indicates that the upper temperature limit for humans is probably between 40°C, or 104°F, and 50°C, or 122°F.
  • Extreme heat makes your body work harder to function and could lead to heat-related illness and even death.
  • Sensitive groups, such as older people and those with chronic illness, are at greater risk.
  • It is important to take steps to keep cool when temperatures are high.

July 4 was the hottest day ever recorded worldwide. In recent weeks, parts of the United States like Death Valley, CA, and several cities in Texas, had temperatures above 104 degrees Fahrenheit.

As humans face increasing temperatures due to climate change, scientists have explored what temperature limit humans can safely tolerate.

Now, researchers from the University of Roehampton in London say they may have honed in on a temperature range where the body starts to function less optimally.

According to Prof. Lewis Halsey and his research team, the upper critical temperature (UCT) is likely to be between 40°C and 50°C (104°F and 122°F).

According to the researchers, this is significant because understanding the temperatures that cause our metabolic rate to increase and how this temperature varies for different individuals can have large implications for workers, athletes, travelers, and medical practitioners.

Halsey presented the findings at the SEB Centenary Conference in Edinburgh, Scotland, in early July.

The temperature rise due to climate change is already having devastating effects on the Earth including diminished ice sheets and glaciers, altered geographic ranges for animals and plants, and shifted seasons.

Halsey said the findings that he will be presenting are a continuation of work previously published in the journal Physiological Reports.

Dr. Daniel Atkinson, GP Clinical Lead at Treated, who was not a part of the research, said the original study suggests that increased temperatures lead to an increase in metabolic rate.

Metabolic rate, per Atkinson, is the amount of energy your body uses to maintain its normal functions.

“At 40 degrees, metabolic rate increased by 35% compared to baseline (‘normal’) and by a further 13% at 50 degrees (so 48% compared to normal),” he stated.

“So the hotter the conditions, the harder your body has to work to keep your regular functions working.”

Atkinson further said that it’s similar to how a sauna works.

“People sit in saunas to ‘sweat off’ calories,” he explained, “but it’s not just sweating that causes calorie expenditure.

“Higher temperatures make your body work harder to keep your organs functioning, and this uses more calories.”

Halsey said that his presentation will delve into what has been learned since the 2021 study.

“We are finding a smaller mean increase in metabolic rate in response to high temperatures but still noteworthy on average, and in particular in some participants,” he said.

He added, “We are yet to find an association between a characteristic and whether metabolic rate (MR) went up much or not, e.g. not sex specific or age specific.”

Halsey said his team also has new data related to the specifics of how heart function changes in the heat, as well as sex differences between males and females.

One particular difference, he said, is that women have a more pronounced increase in heart rate than men.

Additionally, women show a reduction in the amount the heart muscle shortens when blood is pumped out of the heart.

Dr. Naheed Ali, a physician-writer at Healthcare Propulsion in Miami Beach, FL, who also did not take part in the study, said extreme heat can affect different people differently due to various reasons, including age, their overall health, and their individual susceptibility.

“Vulnerable populations, such as the elderly, children, pregnant women, individuals with chronic illnesses, and those with limited access to cooling resources, are often more susceptible to the negative impacts of high temperatures,” he stated.

Factors like socioeconomic status and geographic location can also play a role, Ali said.

Exposure to extreme heat can cause illnesses like heat cramps, exhaustion, and heat stroke, ranging in severity from mild to life-threatening, he added.

Additionally, they can make existing heart and respiratory conditions worse.

“In extreme cases, extreme heat events can result in excess mortality, particularly during heatwaves,” he said.

According to Ali, common symptoms that people might experience due to extreme heat may include:

Ali noted that heatstroke is the most severe form of heat-related illness. This condition can lead to a high body temperature greater than 103°F, an altered mental state, hot and dry skin, and a lack of sweating. It can also potentially cause death.

Atkinson and Ali said that some ways to protect yourself from extreme heat include the following:

  • Stay hydrated by drinking plenty of water. Additionally, Ali suggested that you’ll want to avoid caffeine and alcohol since they can be dehydrating.
  • Wear lighter-colored clothes that are loose-fitting and lightweight. This allows for sweat to evaporate and cool your body, according to Ali.
  • Try to stay indoors when it’s hot. Atkinson noted that between 11 a.m. to 3 p.m. is generally the hottest time of the day.
  • Keep your home or workplace well-ventilated. Use air conditioners or fans to keep cool, advised Ali.
  • Close your curtains to block out sunlight. You’ll especially want to do this for windows that face the sun, said Atkinson.
  • Avoid intense exercise on hotter days. According to the Academy of Nutrition and Dietetics, this can quickly raise your core temperature, putting you at even greater risk for heat exhaustion or heat stroke.
  • Be aware of local weather forecasts and heat advisories. The National Weather Service provides heat alerts when extreme weather events are anticipated.

Ali further noted that it’s important to keep an eye on those who are particularly vulnerable, such as older people and those with chronic illnesses, to make sure they are able to keep cool.

“If necessary, [seek] medical attention for severe symptoms or [seek] shelter in designated cooling centers during heatwaves,” he concluded.