Calls to 9-1-1 for serious emergency medical assistance increase significantly on days of extreme heat, especially in poor, elderly and urban populations, according to a new study from the University of Washington School of Public Health.

The study analyzed the relationship between extreme heat and Emergency Medical Service (EMS) calls in Seattle and King County, Washington, between 2007 and 2012. Researchers found that daily EMS call volume for Advanced Life Support (ALS) was higher on days that reached extreme heat temperatures.

ALS call volume also increased in areas with a higher percentage of poverty, people 65 years or older, and impervious surface (such as roads, parking lots and buildings).

“This research has many practical implications for managing EMS calls during extreme heat in King County,” says lead author Aubrey DeVine, who conducted the research as a master’s student at Washington. “Based on our results, public health can better prepare for extreme heat events by focusing on vulnerable communities. With climate change, it is reasonably expected for extreme heat events to increase in the future, making emergency service planning more crucial than ever before.”

The study, published Aug. 20 in the International Journal of Environmental Research and Public Health, used data of 121,794 ALS calls made during the summer months (May – Sept.).

Meteorological data modeled on a block-grid map of King County was used to calculate daily humidex values, which describe how hot the weather feels to the average person. Researchers determined the extreme heat threshold for ALS data to be a local maximum humidex of 92 degrees Fahrenheit.

Each EMS call was assigned to a block on the county grid map based on the call location. Data from the U.S. Census Bureau and the Washington State Department of Ecology were used to define the social, economic and environmental landscape of each block.

Results showed that daily ALS call volume on a heat day (when it felt hotter than 92 degrees) was estimated to be 1.067 times higher than the daily ALS call volume on a non-heat day.

Furthermore, a one percent increase in poverty was associated with 1.041 times the daily ALS call volume. Similarly, a one percent increase in population 65 years or older was associated with 1.057 times increase in call volume, and a one percent increase in impervious surfaces translated to 1.015 times the call volume.

Researchers also analyzed 441,119 calls for Basic Life Support (BLS) and found that BLS call volume also increased on days of extreme heat, as well as in locations with a higher percentage of poverty.

Co-authors are Michael Yost, Edmund Seto and Tania Busch Isaksen, all from Washington’s department of environmental and occupational health sciences. Phuong Vu from the department of biostatistics also took part.