Study analyzes impact of summer heat wave on hospitalizations

A team from the Barcelona Institute for Global Health and the French National Institute of Health and Medical Research (Inserm) analysed hospitalisations linked to high summer temperatures in Spain over more than a decade. The study concludes that the reasons for hospitalisation that are most affected by heat include:

• Metabolic disorders and obesity.
• Kidney failure.
• Urinary tract infection.
• Sepsis.
• Urolithiasis.
• Poisoning with medicinal and other non-medicinal substances.

The study, published in the journal Environmental Health Perspectives, included data from more than 11.2 million hospitalizations from 2006 to 2019. The data was limited to emergency hospitalizations from 48 provinces in mainland Spain and the Balearic Islands and was provided by the Spanish National Statistics Institute.

The team also calculated average daily temperature, average daily relative humidity, and concentrations of various air pollutants (PM2.5, PM10, NO2, and O3). Using different models, they assessed the relationships between temperature and various causes of hospitalization during the summer season (June to September) and by province.

As expected, statistical analysis showed that high temperatures had an "overall effect on hospitalizations for specific causes." Although heat increased the risk of hospitalization in all age groups, children under 1 year and adults over 85 years were the most vulnerable groups, with a higher risk of hospitalization. Gender differences were also found, with men having a higher risk of hospitalization for injuries than women on hotter days, while women had a higher risk of hospitalization for parasitic, endocrine and metabolic, respiratory, or urinary diseases.

"The mechanisms by which heat causes adverse health effects remain unclear, but they appear to be linked to the way our bodies regulate their own temperature," says Hicham Achebak, a researcher at INSERM and ISGlobal and a recipient of a Marie Skłodowska-Curie fellowship from the European Commission.

"Under conditions of heat stress, the body activates cutaneous vasodilation and sweating to lose heat. The subsequent responses can affect people differently depending on a number of factors, such as age, gender or existing health conditions. We know, for example, that women have a higher temperature threshold above which sweating mechanisms are activated and are more susceptible to the effects of heat," he adds.

Obesity and metabolic disorders The group of diseases most affected by heat included metabolic disorders and obesity. The risk of hospitalization for these diseases was almost double on the hottest days compared to days with optimal or comfortable temperatures.

"There are several reasons for this. For example, in obese people, heat loss processes work less efficiently because fat tissue acts as an insulator, making them more susceptible to heat illness," says Hicham Achebak.

Relative Humidity, Air Pollution, and Heat Waves Of the other variables included in the study, relative humidity did not play a significant role in the relationship between heat and emergency hospitalizations, except for the risk of acute bronchitis and bronchiolitis, which was higher on days with low relative humidity.

Additionally, days with high air pollution seemed to increase the risk of heat-related hospitalization for metabolic disorders and obesity, as well as diabetes, but not for other diseases.

"We observed that the additional effects of heat waves - or extremely high temperatures over several days in a row - were small and specific to a subset of diseases, mainly non-respiratory infectious diseases, endocrine and metabolic disorders, or diseases of the nervous system, among others. For this reason, we believe that current thermal health early warning systems should be activated not only during heat waves, but also during non-sustained extreme temperatures," says Joan Ballester Claramunt, a researcher at ISGlobal and senior author of the study.