As heat rises, subway riders’ heat complaints grow in big cities

Summary’s story in brief

When outdoor temperatures climb, subway riders also increasingly report uncomfortable heat belowground, according to a study published in Nature Cities. The findings suggest that the shift from warmer surface conditions to hotter underground spaces can show up in the way people complain online, even in subway systems where riders may expect cooler temperatures compared with outside.

The researchers looked across seasons and across times of day and day of week, using crowdsourced posts and reviews from three cities—New York, Boston and London—over a 16-year span. They analyzed more than 85,000 social media posts on X and Google Maps reviews recorded from 2008 to 2024, searching for keywords related to “thermal discomfort” while filtering out irrelevant results such as “hot dog.”

In the study’s analysis, outdoor temperature changes tracked with changes in the volume of complaints about heat underground. Researchers said a 1-degree Fahrenheit (0.56-degree Celsius) increase in outdoor temperature corresponded to a 10% increase in complaints in Boston, a 12% increase in New York and a 27% increase in London.

The study’s authors said riders may expect underground temperatures to be naturally cooler, but the patterns still showed complaints rising as outdoor temperatures rose. “Interestingly, over the weekend, people complained less,” Giorgia Chinazzo, an assistant professor in Northwestern’s Department of Civil and Environmental Engineering, said. Chinazzo co-authored the study with associate professor Alessandro Rotta Loria, and she speculated that one possible reason was that people dressed differently on weekends than on workdays.

The research also drew commentary from outside experts who reviewed the approach and its limits. Flavio Lehner, an assistant professor of Earth and atmospheric sciences at Cornell University who was not involved in the work, said the study “follows the template of previous studies that link environmental conditions to human behavior using social media data.” Lehner said limitations included that the research monitored only three city transit systems and that it can be difficult to control for other factors influencing social media behavior.

Kris Ebi, a University of Washington public health and climate professor who was not involved in the study, said the actual impact of subway heat is likely to be greater than what researchers found because vulnerable groups are underrepresented on social media. Ebi said the study’s results “provides compelling evidence that cities should be planning for measures to keep people safe during hot weather.”

The study’s findings point to how transit operators and policymakers could respond as heat becomes more frequent. Chinazzo said, “We’re all experiencing rising temperatures,” and that aboveground warming “will be reflected underground,” with people complaining more over time. She described mitigation and adaptation steps—such as installing fans or operating cooling at more specific times of day, and offering drinking water at certain times—as strategies that could be more widely implemented in the future, and she said “And it will be worse in the future,” in describing the trajectory of heat risks.

While the study focused on using online signals to measure discomfort, the researchers and outside experts framed the underlying message as a planning issue for public health. If social media complaints can reflect changes in heat exposure belowground, cities may use the same kind of evidence to prioritize cooling, water access and other safety measures as summer extremes intensify.