Where do you think are the most polluted places in a city? On the busiest streets, perhaps? Or near sites of heavy industry?
In Paris, at least, the answer is "neither". A study carried out by French health watchdog ANSES has found that the most clogged air in the city is actually under its streets: on the Paris Metro. The air underground, the paper finds, has the highest levels of PM-10 particulates (which means air particles of up to 10 micrograms in diameter) than anywhere at street level.
According to Pangea Today, which drew on information in French paper Le Monde, the particulate levels underground were at least double those on Paris's streets. Sometimes, they were up to 40 times as high:
While the average concentration rate of PM10 outside is of 25 to 30 µg/m3, the average rate at underground platforms is of 70 to 120 µg/m3, with the peak rate reaching as much as 1,000 µg/m3 between 2 and 5 a.m.
So why the underground fug? In both Paris and London, the causes are roughly the same. Particulate matter enters through station entrances and ventilation shafts, and then, since it's heavier than the air around it, is trapped underground. The friction caused by older braking system can also generate airborne particles.
All this is obviously worrying for metro passengers, but they're only exposed to the bad air for short stretches of time. Drivers and maintenance and station workers, meanwhile, spend a good portion of their day underground. For them, these pollutants represent a serious health hazard – and it's largely for their sake that metro networks should invest in modern braking systems and better air filtration.