I’ve lived in New York City for nearly four years now, and while I may be a few years shy of true “New Yorker” status, I know as well as anyone that the worst time to ride the subway is in peak summer. Temperatures in the underground stations can easily top 100 degrees Fahrenheit (38 degrees Celsius), and let me tell you—the resulting stench is enough to make even the staunchest urbanite consider migrating north to a little house upstate.
Of course, New York isn’t the only major city plagued by this problem. To investigate thermal discomfort across major subway systems, Northwestern University researchers Giorgia Chinazzo and Alessandro Rotta Loria crowdsourced and analyzed social media complaints from passengers in New York City, Boston, and London. Their findings, published today in Nature Cities, underscore the urgent need for expanded, publicly accessible data on environmental conditions in underground metro systems—especially as climate change raises the risk of extreme heat events.
“While extreme heat is typically discussed in relation to above-ground environments, there is substantial anecdotal evidence that extreme heat also occurs below ground and affects people, despite the limited availability of quantitative data,” Chinazzo, an assistant professor of civil and environmental engineering at Northwestern, told Gizmodo in an email.
Crowdsourcing complaints
Instead of relying on traditional surveys, Chinazzo and Rotta Loria gathered crowdsourced information about New York City, Boston, and London by searching for online comments about thermal discomfort in subway systems. They sifted through thousands of X posts and Google Maps reviews published between 2008 and 2024.
On Google Maps, they gathered all posts related to underground stations, but on X, they looked specifically for posts that mentioned both city transportation systems and heat-related keywords such as “hot,” “boiling,” “scorching,” and others. Notably, they filtered out posts that contained these keywords in a manner unrelated to thermal discomfort. “Hot dog” wouldn’t make the cut, for example.
The researchers compiled a dataset of more than 85,000 heat-related comments and their geolocations, timestamps, and the usernames of the people that posted them. This is a much larger dataset compared to those produced by traditional surveys, according to Chinazzo. She also noted that their data spans space and timescales in a way traditional surveys do not.
With that said, this crowdsourcing approach does come with limitations. For one, they were not able to ask passengers about the specific heat-related issues they face in the subway, Chinazzo said. She added that their study population was also limited to people actively posting on social media.
“And while we do acknowledge that thermal comfort is subjective, rather than exploring the severity of the discomfort we simply limited ourselves to quantify when thermal complaints were made (during the day, week, season, etc.), and specifically at which temperatures,” she explained.
A widespread public health threat
The analysis revealed trends that could help cities better plan for periods of extreme heat underground. For example, passengers most frequently complained about thermal discomfort during summer, when above-ground temperatures are at their peak.
“When it is hot above ground, it is even warmer below,” Chinazzo said.
That’s because the soil and rock surrounding subway systems act as thermal insulators, preventing heat from escaping. In some places, below-ground temperatures have even exceeded surface heat records. In 2008, 26 temperatures recorded in the London Underground reached 116 degrees F (47 degrees C), surpassing the city’s highest recorded surface air temperature of roughly 104 degrees F (40 degrees C), according to the study.
When Chinazzo and Rotta Loria examined the distribution of complaints by day of the week and hour of the day, they found that most occurred during weekdays and during typical commuting hours.
While these trends aren’t altogether surprising, they provide empirical evidence to support what anecdotal evidence has suggested for years: Underground temperatures pose a serious, widespread health threat to commuters during the hottest months of the year. Not only that, but extreme subsurface heat can also deform train rails, accelerate aging mechanical components, affect groundwater quality, disturb foundations, and disrupt underground ecosystems, according to the study.
This type of data can drive city officials to invest in protecting public health and mitigating the worst impacts of extreme heat underground. Chinazzo and Rotta Loria hope decision-makers will use their publicly available dataset to inform near-real-time control strategies for energy systems in underground metro environments and support live monitoring of subsurface environmental conditions. Such efforts could prove to be life-saving as climate change pushes extreme temperatures higher.
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