If you think more people are hospitalized for respiratory illnesses in the winter months, you’d be right. But according to new research from Spain, those illnesses are actually more lethal in the summer heat and they’re potentially more likely due to rising temperatures in a changing climate.

The study, published Tuesday in The Lancet Regional Health – Europe, was based on 1.7 million unplanned hospital admissions in Madrid and Barcelona. The respiratory cases represented about one in six of all hospitalizations during the study period, which stretched from 2006 to 2019. Patients died in 103,845 cases, with a peak of fatalities in August even though the lowest number of admissions occurred in the same month.

The study evaluated weather, facility staffing and other conditions, along with outcomes of various illnesses, such a bronchitis, chronic obstructive pulmonary disease, and pneumonia. Relative humidity didn’t appear to be a significant factor, nor did air pollution. When all the factors were evaluated, it was heat that correlated with higher rates of death. The effect appears to be a greater risk to women, too.

“It is also important to note that the impact of heat was immediate, in the first 3 days since the exposure,” said the research collaborators from Spain and France. What this suggests is that “the increase in acute respiratory outcomes during heat is more related to the aggravation of chronic and infectious respiratory diseases than the spread of new respiratory infections through indoor crowding to avoid heat, as respiratory infections usually take several days to cause symptoms.”

The researchers aren’t entirely clear on why heat appears to contribute to more in-hospital fatalities. The body manages heat by vasodilation (blood vessels widen) and sweat, and the changes can affect people differently on the basis of a range of factors, including age, health history, and the effects of certain medications. Dehydration, too, is common with heat and exertion; it can create extra work for the heart and respiratory system.

Heat can drive chemical changes at the cellular level that affect kidneys and other organs. Or, it may be as simple as breathing hot air: the authors note that previous research has found that inhaling hot, humid air caused bronchoconstriction (narrowing of the airways) in people diagnosed with asthma.

The authors caution that there are limitations to their work, such as its narrow focus on Madrid and Barcelona. And while they factored for patient age when assessing fatalities related to a respiratory illness, they did not include other characteristics like lifestyle factors or co-occurring diseases.

Still, the work suggests that preparedness could save lives, especially since the deaths are occurring in people who are hospitalized and have access to immediate care.

“These results have important implications for climate change health adaptation policies, and for the projections of climate change impacts on human health,” the study authors note.

“Unless effective adaptation measures are taken in hospital facilities, climate warming might exacerbate the burden of inpatient mortality from respiratory diseases during the warm season.”

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