On top of the sweaty armpits in your face and packed carriages, it turns out that air pollution on subways is another factor determined to make your morning commute even worse. That's despite subway trains being powered by electricity.

A new study in the journal Environmental Science & Technology has looked into Canadian subway systems and found that the air pollution levels on the platforms of Toronto’s underground trains are “comparable with a typical day in Beijing.” While not all of Canada’s subways faired quite this poorly, the findings support plenty of other studies that highlight that subways in many cities worldwide are a wheezing pocket of air pollution.

The research by the University of Toronto compared the PM2.5 levels on Toronto's subway lines with Vancouver's SkyTrain and Montreal's Metro system. On a poor air quality day in Toronto, the outdoor PM2.5 value is about 30 micrograms per cubic meter. On the days they studied, the researchers found that Toronto subway platforms and trains had on average 100 micrograms per cubic meter.

“That’s comparable with a typical day in Beijing,” lead research Greg Evans said in a statement. Of course, it's important to remember that you’re only exposed to these high levels for the short length of your travels, perhaps 20 minutes to an hour, not a whole day.

Outside of Toronto, Vancouver’s Skytrain system – which is primarily above ground – had PM2.5 values of 17 micrograms per cubic meter and Montreal’s subway, entirely below ground, averaged 36 micrograms per cubic meter.

PM2.5 is particulate matter, both organic and inorganic, that's smaller than 400th of a millimeter. These particulates are linked to numerous deadly health risks, namely involving the heart and lungs. A major source of the harmful particulates comes from fossil fuel combustion and other industrial activity.

Subway trains are primarily electric, so the particulates are likely coming from a different source. Alternatively, the study found a high concentration of metal in the particulates, which suggests it could be from the train's wheels grinding against the track.

“We know from analyzing the composition of the particles that it’s not just everyday grime,” added Evans. “The metal concentrations are very high, and the ratios of manganese to iron are similar to what you see in steel.”

While the study only focuses on a few Canadian subways, the study authors say “the findings are similar to previously published studies conducted in metro systems across the world.” However, since the sampling was conducted during peak hours, the measurements may overestimate exposure outside of rush hour.

A separate study released earlier this year on London’s pollution levels found that cars produce the most amount of pollution per commuter, however the drivers are also the least exposed to it. Instead, those traveling on the London Underground are exposed to the highest levels of large-sized particles.