On the hottest days, risk of hospitalisation for people with metabolic disorders such as those of sugar and blood pressure, and obesity, almost doubled as compared to days recording comfortable temperatures, a new study has found.

The research analysing hospital admissions related to high temperatures during summer over more than a decade in Spain found that extreme heat impacted people with these conditions the most.

“There are a number of reasons to explain this. For example, in people with obesity, heat loss responses work less efficiently, as body fat acts as an insulator, making them more susceptible to heat disorders,” said Hicham Achebak, a researcher at the Barcelona Institute for Global Health.

Higher levels of air pollution further appeared to worsen the risk of hospital admission for people with these conditions, including diabetes, the researchers said.

The study also found that on hotter days, men showed a higher risk of hospital admission due to injuries, while women showed a higher risk of hospitalisation from infectious, hormonal and metabolic, respiratory or urinary diseases.

“Under conditions of heat stress, the body activates cutaneous vasodilation (more blood flows to skin) and sweat production in order to lose heat. The subsequent reactions can affect people differently depending on a series of factors, such as age, sex or pre-existing health conditions,” explained Achebak, corresponding author of the study published in the journal Environmental Health Perspectives.

“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 said.

The researchers analysed data of more than 11.2 million emergency hospital admissions between 2006 and 2019 from 48 provinces in mainland Spain and the Balearic Islands, an archipelago off eastern Spain in the Mediterranean.

Using statistical methods of analysis, the team estimated how temperatures affected the different causes of hospitalisation for summer (June to September) and by province. They also considered daily average temperatures and relative humidity, along with air pollutant levels, including those of PM2.5.

High temperatures were found to have “a generalised impact on cause-specific hospitalisations.”

Children under a year and adults older than 85 years were the most vulnerable, even as heat heightened the risk of hospitalisation across all age groups, the researchers said.

“The underlying mechanisms by which heat triggers adverse health outcomes remain unclear, but they seem to be related to how our body regulates its own temperature,” said Achebak.

Other conditions that increased an individual’s risk of hospitalisation because of extreme heat were those of kidney, including failure and stones, and urinary tract infection, the researchers found.

Heat was also found to raise the risk of hospitalisation in people with sepsis, in which chemicals released in the blood to fight infections trigger inflammation throughout the body.