How hot is too hot? This is how our bodies react to extreme heat

Talking about the weather may be a great way to break the ice, but when your country is going through a scorching heat wave and Lewis Halsey appears on the other side of the screen, small talk quickly acquires a scientific tone. This professor of health sciences at the University of Roehampton, in London, has just published a study in which he reveals the critical temperature for humans: between 104 and 122 degrees Fahrenheit (40 and 50 degrees Celsius). His experiment compared the resting metabolic rate of 13 participants at room temperature and at 122 degrees Fahrenheit (with 25% humidity). Skin and rectal temperature were also recorded, as well as heart rate. The aim was to understand the temperatures at which human metabolism begins to increase, and how it varies between different people. Their study was presented last Wednesday at a conference at the Society for Experimental Biology.

At the time of the interview, the scientist is enjoying a mild spring weather of 73.5 degrees, while the interviewer is enduring a suffocating 97 in Madrid, Spain. “Yes, it’s pretty high. But the heat in Madrid is dry. If you were in the street, naked and at rest, you would be in the thermoneutral zone. I would not,” he explains. “You would probably get arrested, though,” he adds with a laugh. Halsey defines the thermoneutral zone as the temperature range within which the metabolic rate is at resting levels. The body is rested, but with everything running and ready to go, like an idling engine.

The thermoneutral zone is somewhere between 82.5 and 90-odd degrees – as long as the person is naked or semi-naked, because “clothes create a microclimate that changes everything,” explains Halsey. The vagueness when setting the upper range is normal; it is precisely what the expert is trying to determine. However, defining a concrete point is not easy. “We found considerable changes in the responses of cardiac function to heat between categories of people, the most remarkable being between sexes,” he says. “Men and women generally show some key differences in their cardiovascular responses to heat.”

When the interviewer points out that 97 degrees is too much to be considered thermoneutral, the scientist replies that this range “does not necessarily correlate perfectly with the feeling of comfort.” Neither does it match perfectly the temperatures that are problematic for health due to the possibility of heat stroke or dehydration. “The study was carried out on a population in an ideal baseline situation, which hardly occurs in real life,” explains Alberto Cecconi, a cardiologist at La Princesa University Hospital in Madrid and adviser to the climate change research group of the Spanish Society of Cardiology. The World Health Organization states that the optimal room temperature for the body is between 64.4 and 75.2 degrees Fahrenheit. When the room temperature exceeds 95 degrees and is accompanied by high levels of humidity, it can put health at risk. If it reaches 104, it can be dangerous even with low levels of humidity, according to this organization.

In this context, the body has to take measures to maintain its temperature, something that can prove especially difficult. “When we drop below 82.5 degrees, we generate a body movement to produce heat and counteract the outside temperature,” explains Cecconi. “Of course, generating cold is more difficult. The main way we do it is with evaporation.” Sweat, which comes out on our skin and evaporates, lowers body temperature. There are also other mechanisms, such as vasodilation. “Blood vessels around the edge of the body open up, so blood can flow more quickly,” adds Halsey. “That blood is warm, but as it approaches the edge of the body, that warmth is dissipated by the external environment. It’s as if our body was opening the windows to ventilate.” The first consequence, as our blood is red and concentrated on the periphery of the body, is that our skin turns red. The second is that our heart speeds up to pump blood to a wide network of capillaries.

These reactions to extreme heat do not fully explain why some people increase their metabolic rate, or the amount of energy per unit of time, upon reaching those temperatures. “Sweating has a negligible cost; vasodilation should reduce it,” says Halsey, who suspects that some other function that science has not been able to detect is taking place in our bodies. “Our physiology, our interior, is doing something in response to the heat,” he explains. Increasing the metabolic rate may make sense when it is cold, Halsey notes, as it causes the body’s temperature to rise; “but responding to heat by increasing its metabolic energy costs, by producing more heat, is counterintuitive.”

For his part, Cecconi understands that “every time we are subjected to a stressful situation, the body is activated, either by cold or heat, we enter a state of alert and there is a greater metabolic cost.” The recent study confirms it, but does not explain it. “For this, a subsequent study would have to be carried out at the cellular level.”

There is a lot of scientific literature on how heat affects athletes and workers; Halsey has devoted his efforts to analyzing how it affects people at rest, a field about which there is very little written. Due to the part about the scant clothing and the state of rest, the scientist jokes that his study reflects how the heat would affect “a couple of British tourists holidaying on the Costa del Sol.”

Cecconi adds that these hypothetical tourists are also in good health, are relatively young and have no baseline problems. But people do not live permanently on vacation: they work, they move, they argue and they exercise. “It’s interesting to see how the body behaves under certain circumstances, as this study does,” he says. But heat is associated with a series of problems “such as an increased risk of stroke, heart attack and heart failure,” that this study does not take into account.

For this reason, the cardiologist says he welcomes this study and any subsequent ones that add to the existing clinical knowledge on the subject. But he also mentions dehydration, sunstroke and the effects of excessive heat on pollution. “We cannot focus on a single aspect and expect it to explain everything,” he says. “In the past, people used to say that we are what we eat. Now we say, ‘we are what we eat, the exercise we do, the air we breathe; we are the environment that we move around in.’

Environmental factors

Medicine is not the only science that explains how heat affects us. Sociology, politics and city planning also help understand its effects. Cristina Linares and Julio Díaz study it at the Reference Unit on Climate Change, Health and Urban Environment, created at the National School of Health of the Carlos III Health Institute, in Madrid. “They are different approaches,” explains Linares. “This study reflects acclimatization; all species acclimatize,” she says. “But humans can supplement it with what is called adaptation; with other types of measures that are not physiological, but do have an influence.” In fact, in recent years they have been crucial for humans to tolerate an increasingly pronounced rise in temperatures. “Biological species acclimatize at a certain rate,” explains her colleague, Julio Díaz. “The thing is that the temperature is rising at a higher rate.”

Adaptation is causing a paradox: although the heat has increased in recent years, the associated deaths are decreasing even in countries with hot summer weather. In Spain, between 1983 and 2003, mortality increased by 14% for each degree above the temperature considered a heat wave; between 2004 and 2013, however, mortality rose less than 2% for each degree.

Age can make certain people more vulnerable to heat waves, but socioeconomic status, these experts point out, is just as important. “A 70-year-old man who has an air-conditioned villa with a pool will not experience a heat wave in the same way as five immigrants who are crammed into a small apartment in an old building with no air,” says Díaz. “Poverty is a risk factor,” explains Linares.

Homes are important, but also the neighborhoods. This is why these two experts highlight the importance of creating parks and keeping them open to the public during heat waves. “If we want to adapt, with an increasingly intense heat in the cities, we have to transform the cities,” says Linares. “And in this regard, the vegetation cover is basic, because it is one of the simplest solutions.”

Her colleague backs up this idea, pointing out that the case of each city should be analyzed individually instead of just implementing the same plans everywhere, as one place will not experience the heat in the same way as another. “Local studies and local responses to a global problem,” says Díaz. “We have to face global warming from a comprehensive point of view. We cannot only talk about a plan for high temperatures. We have to analyze what happens with the pollution, what happens with forest fires. In general we are getting better, but we must take into account all the associated risks that the heat entails.”

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