Episodes of strong heat trigger hospitalizations. The most affected are those over 85 years of age and those under one. This reality, which was taken for granted due to what is known about the consequences of high temperatures on health and its proven effect on increased mortality, had not been studied in detail until now. An article published this Wednesday in Environmental Health Perspectives has investigated more than 11.2 million emergency room admissions in 48 Spanish provinces between 2006 and 2019 to find out what the main causes of heat admissions are.
Metabolic and obesity-related disorders are the most affected by the heat: hospitalizations for these reasons almost double (an increase of 97.8%) when temperatures exceed the thresholds considered thermal comfort. This is followed by kidney failure (77.7%), urinary tract infection (74.6%), sepsis (54.3%), urolithiasis or kidney stones (49%) and poisoning by drugs and other substances. non-medicinal (47%).
To reach these conclusions, a team from the Barcelona Institute for Global Health – promoted by the “la Caixa” Foundation -, together with the National Institute of Health and Medical Research of France (Inserm), calculated the values of the average temperatures daily, relative humidity and concentrations of the main atmospheric pollutants. With the help of statistical models, they estimated the relationships between heat and the causes of hospitalization during the summer (from June to September) in all the peninsular provinces of Spain and in the Balearic Islands.
They found that relative humidity does not seem to influence urgent hospital admissions during hot days, except in the case of acute bronchitis and bronchiolitis, the risk of which increases on dry days. Those with high air pollution increase the chances of admissions in cases of metabolic disorders and obesity, as well as diabetes, but not for other conditions. Particles smaller than 2.5 microns (emitted, for example, by diesel engines and which have documented negative effects on health) are those that most correlate with income.
The effect of heat on health varies depending on sex: on warmer days, men showed a higher risk of hospitalization due to injuries. The authors study the hypothesis that they tend to carry out outdoor tasks and more risky behaviors more frequently.
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Women are more likely to have parasitic, endocrine and metabolic, respiratory or urinary diseases. In a meeting with journalists prior to publication organized by Science Media Center Spain, Hicham Achebak, one of those responsible for the study, explained that this may be related to physiological differences in thermoregulation: “Women have a higher temperature threshold to activate the sweating mechanism and they sweat less, which results in less heat loss through evaporation.” These are, however, speculations that require more research.
The study has also observed that the added effects of heat waves are small in terms of emergency room admissions. That is, the accumulation of several days of high temperatures does not seem to have more effect than if the same days of heat occurred separately. “For this reason, we believe that current heat-health early warning systems should be activated not only during heat waves, but also during non-persistent extreme temperatures,” says Joan Ballester Claramunt, another of the authors of the study.
Precisely, last week, the Ministry of Health activated the national plan for preventive actions for the effects of excess temperatures on health, which launches alerts when thermal conditions are reached (or predicted) from which the mortality of statistically significant in an area and depending on the forecast duration of heat waves.
Although a greater impact on hospitalizations was not observed when days with extreme temperatures are followed, the study does not rule out that there are negative health effects due to heat waves. It is striking that high temperatures hardly represent an increase in admissions for heart conditions, when they are supposedly disorders that heat can trigger. The hypothesis that the researchers consider is that many of these failures are so devastating that they do not even pass through the hospital, and directly cause death.
The heat, as was known and confirmed by these new data, affects the very elderly, the most vulnerable. And prolonged exposure to high temperatures can trigger what doctors call decompensations in the body (through different pathways) that can lead to both hospitalization and death (or both). Two different studies have estimated that in the hot summers of 2022 and 2023, more than 11,000 deaths each occurred as a result of the heat.
This is how heat deteriorates health
“The underlying mechanisms by which heat triggers adverse health outcomes remain unclear, but appear to be related to the way our body regulates its own temperature,” says Achebak. “In conditions of thermal stress, the body activates cutaneous vasodilation and sweat production to lose heat. Subsequent reactions can affect people differently depending on a number of factors, such as age, gender or pre-existing health conditions,” he adds.
The main causes of hospitalization have not been a surprise. In the case of obesity and metabolic disorders there are several reasons. “For example, in people with obesity, heat loss responses work less effectively as body fat acts as an insulator, making them more susceptible to heat disorders.”
The increase in hospital admissions for various kidney problems also makes a lot of sense. Older people do not usually feel thirsty until they are very close to dehydration. Those in weaker health are more likely to suffer kidney failure at these times. For this reason, doctors and authorities insist on drinking water when temperatures rise, even if there is no thirst.
The study “is of very good quality,” according to Dominic Royé, a climate health researcher at the Climate Research Foundation, who was not involved in it. In his opinion, it is important to continue knowing the effects of heat on health due to the increasingly frequent episodes of extreme high temperatures that are going to occur. “We are not aware of what is to come. In a world that warms by an average of two degrees, we will have extreme summers every two to five years, compared to the recent past of one in a hundred,” he says, citing research published in Nature in which he participated last year.
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