How hot is too much? This is how our body reacts to extreme temperatures | Health & Wellness

Talking about the weather can be a great way to break the ice with an interviewee, but when Spain is going through a searing heat wave and Lewis Halsey appears on the other side of the screen, small talk takes on a scientific patina. This professor of Health Sciences at the University of Roehampton (London) has just published a study in which he points out the critical temperature for humans, between 40 and 50 ° C. His experiment compared the resting metabolic rate of 13 participants at room temperature and at 50ºC (with 25% humidity). Skin and rectal temperature, as well as heart rate, were also recorded. The aim was to understand the temperatures at which human metabolism begins to increase and how it varies between different people. His study was presented Wednesday at a conference at the Society for Experimental Biology.

At the time of starting the interview, the scientist enjoys spring weather, with 23 degrees, while his interviewer endures a suffocating 36. “Yes, it’s quite high. But the one in Madrid is a dry heat. If you were in the street, naked and at rest, you would be in the thermoneutral zone. I don’t,” he explains. “Although of course, they would probably arrest you,” he adds with a laugh. Halsey defines the thermoneutral zone as the temperature range within which the metabolic rate is at resting levels. When the body is rested, but with everything running and ready to go, like an idling engine.

The thermoneutral zone is between 28 and thirty-something 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 tries to determine. But it is difficult to fix a concrete point. “We found considerable changes in cardiac function responses to heat between categories of people, the most novel being between sexes,” he says. “On average, men and women show some key differences in their cardiovascular responses to heat.”

When the interviewer points out that 36 degrees is too much to be considered thermoneutral, the doctor replies that this range “does not necessarily correlate perfectly with feelings of comfort.” He also does not match perfectly with temperatures that are problematic for health due to heat stroke, nor with dehydration. “The study is carried out on a population in an ideal baseline situation, which hardly occurs in real life,” says Alberto Cecconi, a cardiologist at the La Princesa University Hospital and adviser to the climate change research group at the Spanish Society of Cardiology. . The World Health Organization affirms that the optimal ambient temperature for the body is between 18 and 24º C. When the ambient temperature exceeds 35º C, and is accompanied by high levels of humidity, it can put health at risk. If it reaches 40º C it can be dangerous even with low levels of humidity according to this organism.

In this context, the body has to take measures to maintain its temperature, something that is particularly difficult. “When we drop below 28 degrees, the body generates a body movement to generate heat and counteract the outside temperature,” explains Cecconi. “But of course, it is more difficult to generate cold. 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 are opened up so blood can flow more quickly,” adds Professor Halsey. “That blood is warm, but as it approaches the edge of the body, it is dissipated by the external environment. It is as if our body opened the windows to ventilate”. The first consequence, as our blood is red and concentrated on the periphery of the body, is that our skin becomes red. The second, 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 upon reaching these temperatures. “Sweating has a negligible cost, vasodilation should reduce it,” says Halsey, who suspects that some other function is happening in our bodies without science having been able to detect it. “Our physiology, our interior, is doing something in response to heat,” explains the professor. Increasing your metabolic rate may make sense when it’s cold, Halsey says, because it causes your body’s temperature to rise. “But responding to heat, by increasing your metabolic energy costs, producing more heat, is counterintuitive.”

For his part, Dr. Cecconi understands that “every time we are subjected to a stressful situation, the body is activated, either by cold or heat, we enter a situation of alert and there is a greater metabolic cost”. The present 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. But Halsey has devoted himself to analyzing how it affects people at rest, a field, he explains, about which there is “strangely, very little written.” Because of the scant clothing and the state of rest, the scientist sardonically says that his study reflects how the heat would affect “a couple of British tourists on vacation on the Costa del Sol.”

Cecconi, for his part, points out that these hypothetical tourists are in good health, are relatively young and have no basal problems. And that people do not live on vacation, that they work, move, discuss and exercise. “It’s interesting to see how the body acts under certain circumstances, as this study does,” he admits. But the heat is associated with a series of problems “such as an increased risk of stroke, heart attack, heart failure” that this analysis does not point out.

For this reason, the cardiologist celebrates this study, awaits the publication of subsequent ones, and adds it to all the clinical knowledge on the subject. He also points to dehydration, heat stroke and the effects of excessive heat on pollution.”We cannot focus on a single aspect and expect it to explain everything,” he points out. “Formerly, it was said that we are what we eat. Now it is said, `´we are what we eat, the sport we do, the air we breathe, we are the environment we move around in”.

environmental factors

Medicine is not the only science that explains how heat affects us. Sociology, politics and urbanism help to understand its effects. Cristina Linares and Julio Diaz study them from the Reference Unit on Climate Change, Health and the Urban Environment, created at the National School of Health of the Carlos III Health Institute, in Madrid. “These are different approaches,” explains Linares. “This study reflects acclimatization, all species acclimatize,” she says. “But humans can supplement them with what is called adaptation; with other types of measures that are not physiological, but that 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 rate, between 0’1 or 0’2 degrees per decade,” explains his partner, Julio Diaz. “What happens is that the temperature is rising at a higher rate. In Spain, according to studies, it increases at a rate of 0.47 degrees per decade”.

In this way, thanks to adaptation, the paradox arises that, although the heat has increased in recent years, the associated deaths are decreasing. 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 grade. The main responsibility is the national plan that has been activated since then —and in which both experts collaborated— every summer, with recommendations and awareness.

Age can make certain people more vulnerable to heat waves, but class, these experts say, is just as important. “A 70-year-old man who has his villa with air conditioning and a pool does not experience a heat wave in the same way as five immigrants, crammed into a small apartment, in an old building with no air,” says Diaz. . This would explain why, according to his studies, the Madrid districts of Puente de Vallecas and Carabanchel are the poorest and the ones that suffer the most from heat waves in the capital. “Poverty is a risk factor,” explains Linares. And this segregation by neighborhoods also influences whether or not the buildings are rehabilitated or that there are accesses to green areas.

The houses are important, but the neighborhoods too. For this reason, these two experts highlight the importance of creating parks (and keeping them open to the public during heat waves) in the city. “If we want to adapt, with an increasingly intense heat inside the cities, we have to transform the cities,” says Linares. “And in this the vegetal cover is basic, because it is one of the simplest solutions.”

His colleague endorses this idea, qualifying that the case of each city should be analyzed individually, and not transfer the plans from one to another, since the population of A Coruna, its buildings, its streets; they do not face the heat in the same way that they do in Jaen. “Local studies and local responses to a global problem”, says Diaz. “You have to face the warm-up from an integral point of view. We cannot only talk about a plan for high temperatures. You have to analyze what happens with 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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