Days of extreme heat associated with increased risk of emergency department visits

Reviewed by Emily Henderson, B.Sc.Nov 25 2021

Extremely hot days with an average temperature of 34.4°C (93.9°F) are associated with a higher risk of emergency department visits among adults of all ages, finds a large study from the United States published by The BMJ today.

The results show that the adverse health effects of extreme heat are not limited to older adults and that some individuals and communities seem to be at greater risk than others.

This information "might be useful to clinicians, public health officials, and the public considering the potential for more frequent and severe extreme heat events attributable to the rapidly changing climate," say the researchers.

It is well known that extreme heat is associated with an increased risk of deaths and hospital admissions among adults aged 65 years and over, but less is known about the adverse health impacts of heat among young and middle aged adults.

So researchers based in the US and Canada set out to measure the association between ambient heat and visits to the emergency department among more than 74 million adults living in 2,939 US counties during the warm season (May to September) from 2010 to 2019.

Using medical insurance claims data, they examined associations between heat and rates of emergency department visits for any cause, heat related illness, kidney disease, cardiovascular disease, respiratory disease, and mental disorders.

Then they investigated whether these associations differed by age, sex, low income status, climate zone, and geographic region.

Overall, 21,996,670 emergency department visits were recorded during the study period.

Days of extreme heat were defined as those in the highest (95th centile) of the local temperature distribution during the warm season. On average across the country, extreme heat was 34.4°C.

The results show that days of extreme heat were associated with a 7.8% higher risk of emergency department visits for any cause compared to days in the lowest (1st centile) of the local temperature distribution during the warm season, with no clear threshold.

Days of extreme heat were associated with a 66.3% higher relative risk of emergency department visits for heat related illness, equivalent to an absolute risk of 24.3 per 100,000 people at risk per day.

Days of extreme heat were also associated with a 30.4% higher relative risk of emergency department visits for kidney disease and a 7.9% higher relative risk for mental disorders.

Days of moderate heat (maximum temperature 32.6°C) were also associated with a higher risk of emergency department visits for any cause and for heat related illnesses, kidney disease, and mental disorders.

Heat was not associated with a higher risk of emergency department visits for cardiovascular or respiratory diseases.

When the researchers examined these associations by geographic region, climate zone, age, sex, and low income status, they were more pronounced in the north east of the US, in counties with a continental climate, and among men, young and middle aged adults, and those receiving financial assistance for prescription drugs.

The researchers point to some study limitations, such as the difficulty of accurately measuring personal heat exposure and a lack of information on factors such as race, occupation, health behaviours, and socioeconomic means, that may have affected their results.

However, the large sample size meant they were able to comprehensively assess risk across different geographic regions and climate zones, suggesting that the results are robust.

As such, they say their study adds to the existing literature on the health effects of heat by showing that adults of all ages are at increased risk of heat related health effects rather than just elderly people, providing estimates of the potential impact of heat, and demonstrating that the risk of heat associated illness is apparent across every region of the conterminous US, particularly for regions with cooler climates.