Extreme heat makes us sick. And, in many ways, it kills us.
The most striking – albeit infrequent – effect is heatstroke, a medical emergency characterized by a body temperature exceeding 40°C (104°F), which causes an altered mental status, along with a high risk of multiple organ failure and death. However, there are many more potentially serious physiological imbalances linked to high temperatures. Heat stress – which occurs when the body’s need to dissipate heat exceeds its capacity to do so – can also lead to cardiovascular problems, worsen sleep quality, increase irritability and exacerbate mental health disorders.
Experts warn that extreme heat events will become increasingly frequent. And they predict that changes will have to be made sooner, rather than later, to prevent greater harm.
“There will have to be a change in mindset. We have to learn to live with extreme heat,” warns Elisabeth Diago, a researcher at the Barcelona Institute for Global Health (ISGlobal), A simple example: “Always carry water with you and drink it, even if you’re not thirsty.” And, Diago exclaims, “You can’t go running at four in the afternoon… you could faint from the heat!”
Self-care, community support networks for vulnerable people, better-equipped housing, as well as changes in urban health (for instance, avoiding the urban heat island effect) will all be key.
When humans face episodes of intense heat, the body responds by deploying physiological strategies to maintain the internal body temperature at around 37 degrees Celsius, the optimal level for life. For instance, the body uses sweating, which is the main thermoregulation mechanism for dissipating heat. It also uses peripheral vasodilation, which increases blood flow to the skin to facilitate heat loss.
All of this usually helps maintain the appropriate body temperature, but these mechanisms have limits. And, when these limits are exceeded, what experts call “heat stress” appears, manifesting as a range of discomfort: from fatigue and cramps to irritability. Even more serious conditions can arise, such as syncope (fainting) or heat stroke.
The impact of high temperatures on health can be devastating. In the summer of 2022, one of the hottest on record in Europe, there were more than 61,000 heat-related deaths on the continent, according to a study. This year, just in June, more than a thousand deaths were recorded in Spain alone.
Deaths from heatstroke, however, are rare; most heat-related fatalities result from the exacerbation of pre-existing conditions.
A recent ISGlobal report for the DKV Health and Environment Observatory notes that, according to scientific literature, for every one-degree Celsius increase in ambient temperature, heat-related mortality rises by 35%, while deaths linked to cardiovascular disease increase by 2.1%. The number of people who fall ill also rises by 18% (25% in the case of those over 65). “The effects aren’t the same for everyone, because vulnerability is different,” explains Diago, who coordinated the report.
“It all depends” she continues, “on exposure; on factors that are intrinsic to the person, such as their underlying illnesses; and also on structural vulnerability, such as the neighborhood that they live in, or on their socioeconomic capacity to access certain resources.”
Children and the elderly bear the brunt of the heat. This is partly because neither a child nor an elderly person has their body’s thermostat functioning optimally: the elderly have a weakened system due to age, while children are still underdeveloped. Older people also tend to have underlying health conditions that can worsen more easily in the heat. This is according to Alfons Aguirre, head of the Emergency Department at Hospital del Mar, in Barcelona. “[Elderly people’s] heat adaptation mechanisms are often more compromised. Furthermore, if they have a neurological disorder, they lose their thirst reflex and are at greater risk of dehydration,” he describes. Unwanted isolation is also a contributing factor.
Outdoor workers make up another at-risk group, due to their direct exposure to high temperatures. Pregnant women are also vulnerable: being exposed to extreme heat, especially during the first trimester, is linked to a higher risk of premature birth, low birth weight and stillbirth.
Women also die more frequently during heat waves. According to ISGlobal, in the summer of 2022, mortality among women was 56% higher than among men. Experts attribute this gap to physiological differences in thermoregulation (especially during menopause), but also to the fact that women experience greater isolation and have less access to air-conditioned spaces.
The price of poor sleep
The human body uses nighttime hours to release the heat that has accumulated during the day. But when nights are very hot, this entire physiological recovery process is compromised. It’s harder to fall asleep, we wake up more often, and sleep tends to be less deep and restorative. This process is explained by Marta Garaulet, a professor of Physiology at the University of Murcia, in Spain. “The body has to dedicate more effort to regulating temperature: physiological activation increases and the normal architecture of sleep is disrupted. In general, heat is associated with more nighttime wakefulness and a reduction in deep sleep and REM sleep, which are important phases for physical, cognitive and emotional recovery.”
Garaulet clarifies that heat doesn’t alter our biological clock to the same extent as, for example, nighttime light does. Still, it can “disrupt the expression” of circadian rhythms. “We sleep worse, go to bed later, wake up more tired and, the next day, we tend to be less active. All of this can end up affecting our sleep-wake cycle, metabolism and overall well-being.”
In her research, the scientist also discovered the power of a nap in contexts of extreme heat: “We saw that, in 11% of people, the siesta doesn’t appear as a constant habit throughout the year, but rather as a seasonal behavior, occurring mainly in summer.” She discovered this in studies conducted in Murcia, where it’s common for summer afternoons to reach 40°C (104°F). “Under these conditions, the body naturally tends to reduce activity and favor rest. The siesta wouldn’t just be a cultural custom, but also a biological response to the environment. When the heat is extreme, resting during the middle of the day helps you avoid heat stress, conserve energy and reorganize activity for cooler times,” she explains.
According to Garaulet, “in warm regions, a short, well-placed nap can be an adaptive physiological response.” And she offers further advice: “It’s advisable for the bedroom to be cool, rather than hot, for a good night’s sleep. And, even though it’s sometimes thought otherwise, very hot showers right before bed can make it difficult to sleep if they raise body temperature too much, or prevent the natural heat loss we need to sleep.”
Increased irritation and stress
Like a domino effect, inadequate rest is one of the factors that also contributes to a decline in mental health. In primary care, persistent heat has been associated with an increase in consultations for anxiety (43%) and depression (23%), according to the ISGlobal report. For every degree the ambient temperature rises, the risk of suicide increases by up to 1.7%. Psychiatric hospitalizations rise by almost 10% during heat waves.
Experts attribute this impact to a confluence of factors. These range from psychological mechanisms – related to the perception of heat through feelings of stress and anxiety – to behavioral elements, with changes in behavior occurring due to the heat. Social mechanisms are also involved, linked to changes in interactions due to the hot weather (increased isolation, for example).
Heart exhaustion
In situations of heat stress, the risk of cardiovascular dysfunction also increases. In its attempt to dissipate heat, the heart overexerts itself. “And that can contribute to the decompensation of cardiovascular diseases,” Aguirre summarizes.
Christian Garriga, a pediatrician and the corporate medical director of DKV, emphasizes that some medications can actually worsen the situation. “Antihypertensives (to control high blood pressure), beta-blockers (which reduce heart rate and blood pressure) and diuretics (which help eliminate excess water and salt through urine) affect the vascular system and also [the body’s] fluid balance.” With heat, this mechanism of peripheral vasodilation can lower blood pressure, while sweating leads to the elimination of more electrolytes. The aforementioned medications further accentuate these symptoms and can cause dehydration, dizziness, fatigue, or even fainting.
The lungs work harder
Respiratory illnesses can also get out of control. In the heat, the body increases cardiac output and hyperventilation, which can worsen conditions such as asthma or chronic obstructive pulmonary disease (COPD). “With increased vasodilation, the body compensates with a higher heart rate and greater respiratory effort to oxygenate the blood,” explains Bernardino Alcázar-Navarrete, president-elect of the Spanish Society of Pulmonology.
This situation, in people with chronic illnesses, can complicate their clinical condition. Alcázar-Navarrete gives an example: “It has been described that the risk of hospitalization for COPD increases by 8% with the summer heat. There are exacerbations because, when the temperature changes, the air density also changes… and patients notice it more. [Heat waves] also tend to coincide with episodes of haze, when there is more ambient dust and suspended particles. And, furthermore, with the heat, there’s a greater risk of dehydration. This, in turn, affects secretions.” Mucus is thicker and more difficult to eliminate by coughing, which favors the obstruction of the airways.
Altered metabolism
Heat stress can also cause metabolic dysfunctions. For example, heat alters the way the body processes glucose. Rosa Corcoy, director of the Diabetes Unit at Sant Pau Hospital, in Barcelona, explains that “in people with diabetes being treated with insulin, absorption can increase in the heat, decreasing insulin requirements and leading to a tendency toward hypoglycemia. At the other extreme, people with diabetes and very high glucose levels may be more prone to hyperglycemic decompensation (a diabetic coma), because both hyperglycemia and heat promote dehydration.”
The endocrinologist also points out that the metabolism of nutrients generates heat. And that partly explains why, in times of extreme heat, we prefer fresh, light meals instead of high-calorie foods: “To begin with, if the food we eat is fresh, it already helps lower body temperature, which is what we want. But also, if we eat a large meal, the body’s effort to metabolize that food will generate more heat… and that will raise body temperature. And what the body seeks is to balance body temperature, not raise it.”
People with obesity are another vulnerable group. “They have more difficulty eliminating heat, because they have less body surface area relative to their mass. And adipose tissue also acts as thermal insulation,” Corcoy emphasizes.
The strain on the kidneys from dehydration
People with kidney disease make up another group that can suffer imbalances. The explanation is that, during heat waves, there’s a greater risk of dehydration. This subsequently leads to an electrolyte imbalance that forces the kidneys to work harder. As a result of this extra stress, pre-existing kidney diseases can be exacerbated.
Experts warn that we must prepare for a world with increasingly frequent periods of extreme heat. “We have to change our way of life,” Diago warns. This involves strengthening self-care, staying hydrated (with water, not soft drinks or alcohol), protecting yourself from the sun, and cooling down, however you can.
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