For millions of commuters, the workday doesn’t just begin with a train ride. It also begins with a blast of heat.
In one of the largest studies ever conducted on thermal comfort in metro systems, Northwestern University scientists found that subway riders consistently report feeling uncomfortably hot while underground.
Rather than relying on traditional surveys — which are expensive and capture only brief snapshots of conditions in time and place — the team turned to real-world feedback. Searching for comments about thermal discomfort underground, the scientists scraped social posts and online reviews published between 2008 and 2024. Then, they analyzed more than 85,000 crowdsourced comments from across Boston, London and New York.
The findings show a clear pattern: As above-ground temperatures rise, below-ground thermal complaints increase. The scientists hope their work could help transit agencies anticipate extreme heat, implement targeted mitigation strategies and adapt underground infrastructure to an ever-warming climate.
The study will be published on Tuesday (March 10) in the journal Nature Cities.
“No one wants to feel uncomfortable,” said Northwestern’s Giorgia Chinazzo, who led the study. “But while discomfort might seem like a minor inconvenience, extreme heat also represents a serious threat to public health. Extreme heat causes more deaths per year than all other natural hazards combined. Having information about when and where people feel uncomfortable could help transit agencies and policymakers make targeted interventions, such as increasing the number of operating fans during specific times of day.”
An expert on how buildings influence people’s health and wellbeing, Chinazzo is an assistant professor of civil and environmental engineering at Northwestern’s McCormick School of Engineering, where she directs the Architectural Engineering and Design Program. Chinazzo authored the study with Alessandro Rotta Loria, the Louis Berger Associate Professor of Civil and Environmental Engineering at McCormick.
It’s getting hot in here
Most people have heard of urban heat islands — a phenomenon in which a metropolitan area is significantly warmer than surrounding rural areas. But many people might not realize that urban heat doesn’t stop at the sidewalk. It seeps into soil, tunnels and underground transit systems — where it lingers.
Unlike above-ground spaces, underground environments retain heat because soil and rocks can act as thermal insulators. In some areas, below-ground temperatures can even exceed surface records. Subsurface temperatures in the London Underground, for example, have reached 47 degrees Celsius (116 degrees Fahrenheit), surpassing London’s highest recorded air temperature.
Chinazzo and Rotta Loria wondered how this heat affects people’s daily lives. In a 2023 study, Rotta Loria quantified the rising temperatures in subterranean transportation systems, parking garages and basement facilities. But long-term data on how commuters actually experience underground temperatures are scarce.
Untapped data
To bridge that gap, the team turned to crowdsourcing. Commuters vent their frustrations online every day, including exasperation about sweltering subway stations. Yet no prior studies have used crowdsourced data to examine thermal discomfort in underground metro systems. Focusing on subway systems in London, Boston and New York, the team analyzed more than 85,000 posts from X and Google Reviews.
“We focused on these three cities because their transit systems are the oldest and biggest,” Rotta Loria said. “London has the oldest metro system in the world. In the U.S., Boston has the oldest system, and New York has the largest one. In principle, this technique could be applied to any place on Earth, but this was our starting point.”
After collecting posts, the team used natural language processing to extract comments with keywords like “hot,” “warm,” “uncomfortable,” “roasting,” “scorching” and “boiling,” while omitting those keywords when unrelated to thermal (dis)comfort. For example, the technique filtered out comments about “hot dogs,” “warm smiles” and “uncomfortable crowds.” In the end, the team confidently identified more than 22,000 thermal complaints, posted across a 16-year period. Those posts represented various times of day and all seasons.
When comparing the complaints to recorded temperatures, Chinazzo and Rotta Loria found that for each increase in 1 degree Celsius above 10 degrees Celsius, complaints increased by 10% in Boston, 12% in New York and 27% in London. Complaints peaked during summer months — especially July — and spiked during major heatwave years such as 2018 and 2019.
Although complaints spiked in summer months, commuters still expressed some discomfort in colder weather. As a possible justification, commuters may often be dressed warmly, prepared for freezing outdoor temperatures. But, after venturing underground, they may feel overdressed and uncomfortable in the warmer subterranean conditions.
“Even in winter, people may complain because they are wearing heavy clothing,” Rotta Loria said. “It’s another facet of the same problem.”
Energy-efficient interventions
Chinazzo and Rotta Loria also discovered that timing matters. In London, complaints tend to spike in the late afternoon. In Boston and New York, complaints peaked at noon and again at 9 p.m. In general, complaints decreased on the weekends. These findings suggest that social factors, not just temperature, shape how people experience heat.
“Thermal conditions are not necessarily better on the weekends,” Chinazzo said. “But the metro might be less packed, and people might be dressed more comfortably. State of mind also matters. For example, a tourist on vacation might be less inclined to complain compared to someone headed to work.”
By analyzing complaint trends — both in real time and retrospectively — could help inform targeted, energy-efficient heat-mitigation strategies. For example, running fans more throughout the whole day every day might help people feel more comfortable. But it also might waste energy when more fans are not needed.
“Depending on the metro system under consideration, it may not make sense to cool down the underground environment all day,” Rotta Loria said. “But we could enhance cooling during specific times when we know people are uncomfortable. That means spending less energy and less money.”
The consequences of extreme heat
Unfortunately, underground heat is not just uncomfortable. It’s also potentially consequential. In addition to affecting public health, extreme heat can deform train rails, accelerate the aging of mechanical components, affect groundwater quality, disturb civil infrastructure foundations and disrupt underground ecosystems.
Chinazzo and Rotta Loria emphasize the urgent need for expanded public datasets on underground environmental conditions, which remain sparse worldwide. Improved monitoring and transparency could help engineers, transit agencies, urban planners and policymakers prepare for a hotter future.
“Essentially, underground environments are hot, and it’s because of us,” Rotta Loria said. “When trains brake, they release heat. Most lighting systems release heat. Humans release heat. Then heat also comes from the surface. The primary driver is us. The heat underground is because of us. And it’s up to us to solve it.”
The study, “Thermal discomfort in urban underground metro systems,” was supported by the National Science Foundation (award number 2046586).
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