Extreme heat is a killer. It’s responsible for thousands of deaths around the world every year, with a huge proportion of those already attributed to rising global temperatures.

Studies have long shown that people over the age of 65 face an increased risk of hospital admission and death during extreme heat days, but US and Canadian researchers have now found that young and middle-aged people are at risk, too.

“By looking at emergency department visits for different causes and for several age groups, we were able to characterise with accuracy the varying impact on health on different populations,” says study co-author Francesca Dominici, from the Harvard T.H. Chan School of Public Health.

“An important goal of this study is to provide actionable information to clinicians and public health experts regarding how to prevent these emergency department visits, also considering that we can anticipate when these extreme heat events are likely to occur.”

Published in the BMJ, the study found that extremely hot days – with an average temperature of 34.4°C – are associated with a higher risk of emergency department (ED) presentation for adults of all ages.

The strongest association was for adults between the ages of 18 and 64.

The study was large, spanning more than 74 million adults and 22 million ED visits across 2939 US counties during the months of May to September from 2010 to 2019. It used medical insurance claims data to investigate links between hot days and rates of ED visits for any cause as well as specific causes (namely heat-related illness, kidney disease, cardiovascular disease, respiratory disease and mental disorders).

Extreme heat days increased people’s risk of an ED visit by 66% for a heat-related illness, as well as by 30% for renal disease.

But the risk varied with age – there was a 10.3% higher risk of ED visits in people ages 45 to 54 years old, compared to a 3.6% higher risk in those older than 75.

“Younger adults may be at greater risk of exposure to extreme heat, particularly among workers that spend substantial time outdoors,” says lead author Shengzhi Sun, from the Department of Environmental Health at Boston University School of Public Health (BUSPH). “Younger adults may also not realise that they too can be at risk on days of extreme heat.”

Gregory Wellenius, professor of environmental health at BUSPH, says the researchers chose to look at ED visits instead of hospital admissions for a reason.

“Many illnesses that lead to utilisation of the emergency department do not lead to hospitalisation because they can be treated in a short amount of time, particularly among the younger adult population,” he explains.

“By looking at emergency room visits, we aimed to obtain a more comprehensive picture of the true burden of disease that might be attributed to the days of high heat.”

The study also found differences in risk across regions – there was a higher risk of ED visits on extreme heat days in the US’s northwest, midwest and northwest, as opposed to the hotter southeast. The researchers say this shows heat is especially dangerous in cooler climates, where people may be less adapted to, or less aware of, heat.

This is crucial to recognise as global temperatures rise, particularly as countries, states and regions are deciding how to adapt.

“Although climate change is a global problem and heat threatens the health of everybody across the world, the impacts are felt locally, and the solutions have to be tailored to local needs,” says Wellenius.

“What works for heatwave preparedness in the Pacific Northwest is really different from what works in the southeastern US, so the solutions have to be localised to accommodate the needs of the local community.” Extreme heat is a particular problem in cities. Exposure to deadly urban heat has tripled since the 1980s, and with more than 50% of the world’s population currently living in urban areas, this signals an urgent need to redesign our future cities to keep us healthy as the world warms.

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