Experts had hoped that the Copenhagen Metro’s two most modern lines would be an example for future subway engineers to follow. The researchers were therefore surprised to find that the newest modern metro lines appear to be even more polluted than the older ones. The good news is that this is easy to fix. Simply running already installed fans continuously could significantly improve air quality.
Public transport is undeniably better for the planet and better for the air we breathe outside. But recent research suggests that the air inside metro systems is dirtier than we suspected.
A new study, published in January in Environment International, examined air quality on metro trains and underground stations in Copenhagen, Denmark. Experts had hoped that the Copenhagen Metro’s two most modern lines, opened in 2019 and 2020, would be an example for future subway engineers to follow. Instead, they got a cautionary tale – in the metro air, the researchers found worrisomely high levels of fine particle pollution, which is associated with cancer, respiratory disease and other health problems.
“It is easy to calculate that this could be your largest single exposure to air pollution, just by spending half an hour a day taking the metro,” co-author Matthew Stanley Johnson says. The study found that pollution inside metro trains was 10–20 times higher than in above-ground Copenhagen.
Comparing the Copenhagen metro system with others from major cities around the world, Johnson and his co-authors identify clear directives for designers of new metro systems and an easy way to improve air quality for commuters in Denmark right away.
Making air quality personal
From his office window at the University of Copenhagen, environmental chemist Matthew Stanley Johnson can spy one of Denmark’s 15 monitoring stations mounted on the roof of the Niels Bohr Institute. These city monitoring stations are important for weather forecasting and serve as the main source of information for regional air quality information.
But crucially, “it does not tell you the air quality where people spend their time,” Johnson says. “Nobody’s up on the roof of that building. They are down by the street riding their bike or inside the metro or inside an office.”
With only this bird’s-eye view from regional monitors, “Historically, we have not had very good data connecting air pollution exposure with health outcomes.” So hoping to make air quality monitoring personal, Johnson and his team fashioned wearable air quality monitors for riders in an important shared space for city dwellers – the public transport system.
Hit the brakes
The first underground metro system opened in London in 1863, with steam-powered locomotives chugging beneath the city in shallow tunnels. Today, major cities across the world rely on metro systems as people-movers – the New York City Subway system has an annual ridership of 1.3 billion, the Shanghai Metro 2.8 billion and the Delhi Metro 1.8 billion.
Metro systems reduce overall emissions and bring local pollution down by keeping cars off the road. But counterintuitively, the trains themselves create air pollution. Metal grinding against metal as trains brake or chatter along rails creates a superfine dust of iron-rich nanoparticles that is small enough to work its way into the deepest nooks and crannies of our lungs.
Previous research has established that breathing in PM2.5 – airborne particles with an aerodynamic diameter smaller than 2.5 microns, about a third of the size of a red blood cell – is associated with developing cancer, cardiovascular disease and respiratory disease. There is also a link with airborne disease.
“In addition to suppressing the immune system, these particles damage the lining of your respiratory tract, enabling airborne pathogens to lodge more easily”, Johnson explains.
Duct tape and wires
Researchers have found that exposure to air pollution is a co-factor in COVID mortality. And there is reason to suspect that PM2.5 from metros may be even more dangerous than pollution from other sources. In a different study, researchers found that iron-rich particles from the Stockholm metro were more damaging to DNA than particles from burning wood or pellets, or from brake and tire wear from above-ground traffic.
Johnson hoped that the Copenhagen Metro system – especially the M3 and M4 lines, which were opened in 2019 and 2020, respectively – would have lower PM2.5 levels than older systems since the health risks were already well documented by the time construction began.
To gather the data, the researchers sampled air from every metro station in Copenhagen and from inside the trains during non-peak hours. (The test subjects wore portable air quality monitors strapped to their arm – “the very first ones had duct tape and wires,” Johnson explains.)
The results show that mitigation strategies the engineers used did not help as much as was hoped – and some might even be harmful.
The ineffective piston effect
The researchers were surprised to find that Copenhagen’s newest modern metro lines are even more polluted than its older ones, the M1 and M2, which opened in 2002.
To understand why, consider the central challenge of metro air control. The many people gathered in an enclosed space like a metro car are quickly awash with exhaled CO2. The air must be refreshed from outside the train car, and in the Copenhagen Metro that air comes from inside the system – the underground tunnels the trains run through.
The problem, Johnson explains, is that the tunnels in Copenhagen are ventilated passively through a principle called the piston effect. When a train whizzes past an opening like a ventilation shaft or a station, it creates a positive pressure zone ahead of it and a negative pressure zone behind it, forcing old, stale air out before its arrival and sucking new air in as it leaves.
However, based on the researchers’ data, the piston effect only seems to boost air quality for stations within 3 kilometres of a full exit, where the train fully emerges from underground. Both the M1 and M2 lines have above-ground stations at either end of their lines, but the limited benefit of the piston effect leaves about 4 kilometres entirely unventilated. And worse, the M3 runs in an entirely underground closed loop.
The portable air quality monitors clocked an average PM2.5 concentration of about 219 μg/m3 inside trains on the M3 line versus an average 88 μg/m3 on the M1/M2 lines and about 11 μg/m3 on H.C. Andersens Boulevard, one of Copenhagen’s busiest streets.
But how does the Copenhagen Metro stack up internationally? “We are bad, but we are not the worst,” Johnson says. That dubious honour goes to London and Paris, “because they have the oldest metro systems in the world and deep tunnels.”
Nevertheless, boarding the train from the metro station takes you to cleaner air in the Paris or Naples metro systems, but not in Copenhagen, the researchers found – and a feature designed to protect passengers may be to blame.
Generally, the air in metro trains is cleaner than in stations. But in Copenhagen, the PM2.5 concentrations on trains were just as high or higher than at the stations. Unlike Paris or Naples, the Copenhagen Metro has sliding doors that isolate the station from the tunnels when a train is not present. This keeps dirty air trapped in train cars, the authors say.
Train hoppers in the tropics can breathe a little easier – in metro systems in warmer places like Valencia, Spain, where summer temperatures regularly soar to 40°C, active ventilation in the form of air conditioning is non-negotiable. “They end up with better air quality,” Johnson explains.
It’s a breeze
Johnson and his team identified some easy fixes for the Copenhagen system – one using existing infrastructure.
The Copenhagen Metro already has fans installed in its passive ventilation shafts, but they are only activated during emergencies. Simply running the fans continuously would funnel clean air from above ground into the system and could significantly improve air quality.
“The fans use a little electricity, but if you compare the cost of running the fans with the health costs, I think the choice is clear,” Johnson says.
In a way, the good news is that the metro may be Copenhagen’s biggest exposure to air pollution, Johnson says. “It is just one single source that we actually control, and something can be done about it,” he adds.
“This is a sign that the Copenhagen Metro needs to clean up its act,” Johnson says, not that people should ditch their local metro system in favour of a single-occupancy car.
In the meantime, wearing a well-fitted N95 mask will reduce the PM2.5 you breathe in.