Hospitals in Baltimore see an uptick in asthma-related emergency room cases in the weeks following nighttime heat waves in their neighboring communities, according to new research from Johns Hopkins University.
The findings, published today in the journal GeoHealth, suggest Baltimore's Code Red Extreme Heat alert system, which relies on daytime temperature forecasts, may need to be updated to include nighttime temperature readings to be more helpful for people with asthma or other heat-sensitive health conditions.
"We need to understand at what point—at what temperature, scale, geography—does the heat become 'extreme' enough to pose a health risk to people with asthma," said Benjamin Zaitchik , a professor of earth and planetary science who studies how urban heat island effects differ between Baltimore neighborhoods and senior PI on the project. "I don't think it's overly dramatic to say there are life or death consequences, because people can have fatal asthma attacks, but it also affects quality of life, happiness, and a kid's ability to enjoy childhood."
Using machine learning and epidemiological models, first author Bianca Corpuz, Zaitchik, and their colleagues compared the number, location, and timing of asthma-related ER cases to temperature readings from the National Weather Service station at Thurgood Marshal Baltimore-Washington International Airport as well as sensors placed in Baltimore neighborhoods. The researchers looked at the relationship between asthma and temperature during the summer months from 2017-2022.
The research, which was funded by a five-year Department of Energy grant that has since been defunded, found that neighborhoods with the biggest swings in nighttime temperatures were the most likely to see increases in asthma-related ER cases at their local hospitals.
Neighborhoods in Baltimore's "Black butterfly," which includes areas where temperatures are often higher than the city average and fewer people have air conditioning units, tended to get hit the hardest. Following nighttime heat waves, hospitals in these neighborhoods saw even more asthma-related cases than other neighborhoods.
"When it's hot outside at night, it's stifling in an upstairs bedroom of a row home without air conditioning," Zaitchik said, adding that night is when most people are exposed to extreme heat. "During the daytime, people are less likely to be at home. Unless you work outdoors, most people have access to cool spaces. Adults work in offices or stores and kids go to daycare or camp."
Prolonged exposure—during a heat wave rather than a single day of extreme heat, for example—can lead to worsening symptoms, the researchers said.
"Triggers of an asthma exacerbation/asthma attack may begin with an increase in symptoms, such as cough, chest tightness and shortness of breath," said co-author Meredith McCormack , director of the Asthma Precision Medicine Center of Excellence at Johns Hopkins School of Medicine. "Initially, an individual may try using their inhalers more often and other measures before coming to the emergency room. So, there may be a lag of several days before symptoms are severe enough to prompt an emergency department visit."
Nighttime heat is particularly taxing, McCormack said, because the body doesn't get a chance to rest or recover. Some of the body's defenses are also lowered at night. Adrenaline, a hormone that helps regulate the immune system, has a cycle that naturally dips, usually around 3 a.m. Asthma symptoms may be heightened if a person is exposed to excessive heat during this time.
The researchers found that relying on the daytime maximum heat temperatures alone may miss some of the most important signals associated with asthma risk. Similarly, temperature readings at the neighborhood-level were better predictors than temperature recordings from the National Weather Service station, which provides one temperature reading for the entire city.
In 2024, Corpuz and a group of researchers showed that Baltimore is a mosaic of heat, with some neighborhoods experiencing more intense conditions than others.
"Heat exposure can vary drastically from one neighborhood to the next, and those differences matter for health," said Corpuz, a visiting researcher at Johns Hopkins who was a graduate student in the Department of Earth and Planetary Sciences at the time of the research. "When we measure temperature at the neighborhood scale, we can see patterns that aren't captured by regional weather data. That kind of local information is critical if cities want to better protect vulnerable communities."
Extreme heat events tend to exacerbate cardiac, pulmonary, and mental health issues. Next, the researchers plan to map out incidents of violence and track mental health conditions in Baltimore's neighborhoods during heat waves.
"Different kinds of heat affect different people in various ways," Zaitchik said. "And what we really need is a more nuanced and sophisticated heat risk communication and preparedness."
