For many commuters, stepping into a subway station means stepping into a wave of heat.
A new study suggests that this uncomfortable experience is not just common—it is getting worse as cities grow warmer.
Researchers from Northwestern University conducted one of the largest studies ever on thermal comfort in underground transit systems.
Their findings show that subway riders frequently report feeling uncomfortably hot while traveling underground. As temperatures above ground rise, complaints about heat below ground increase as well.
The study, published in the journal Nature Cities, examined subway systems in Boston, London, and New York—three of the oldest and busiest metro networks in the world.
Instead of relying on traditional surveys, which can be costly and limited to short time periods, the researchers used crowdsourced data from social media and online reviews.
They collected and analyzed more than 85,000 public comments posted between 2008 and 2024. These comments came from platforms such as X (formerly Twitter) and Google Reviews, where commuters often share their frustrations about daily travel conditions.
Using natural language processing, the researchers filtered posts to identify complaints related specifically to heat.
Words such as “hot,” “boiling,” “roasting,” and “scorching” helped them pinpoint comments describing thermal discomfort, while unrelated phrases like “hot dog” or “warm smile” were excluded.
In the end, the team identified more than 22,000 clear complaints about heat in subway systems over the 16-year period.
When the researchers compared these complaints with recorded outdoor temperatures, they found a strong connection. For every increase of 1 degree Celsius above 10°C, complaints rose significantly. In Boston, complaints increased by about 10 percent. In New York, they rose by 12 percent. London showed the biggest jump, with complaints increasing by 27 percent for each degree of warming.
The highest number of complaints appeared during summer months, especially July, and during particularly hot years such as 2018 and 2019 when heat waves were widespread.
Underground environments tend to trap heat. Soil and rock surrounding subway tunnels act like insulation, preventing heat from escaping easily. In some cases, underground temperatures can become even hotter than those above ground. For example, temperatures inside the London Underground have reached 47°C (116°F), which is higher than the city’s highest recorded outdoor air temperature.
Interestingly, some commuters also reported feeling too warm during winter. Researchers suggest this may happen because people dress for cold outdoor weather but then enter relatively warm underground stations and trains.
The study also found that timing plays an important role in how people experience heat underground. In London, complaints peaked in the late afternoon, while in Boston and New York they were highest around midday and again at about 9 p.m. Complaints tended to drop on weekends, possibly because trains are less crowded or because riders feel less stressed than during weekday commutes.
The researchers say these insights could help transit agencies respond more efficiently to heat problems. Instead of running cooling systems or fans all day, operators could focus cooling efforts during specific times when riders are most uncomfortable.
Extreme heat underground is more than just unpleasant. It can also affect public health and damage infrastructure by warping train rails, aging equipment faster, and even disrupting underground ecosystems.
The researchers argue that better monitoring of underground temperatures is urgently needed. With cities warming due to climate change and human activity, understanding and managing heat in subway systems will be essential for making urban transportation safer and more comfortable in the future.
