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A lot of the air quality problem isn't cars though - it's also things like construction equipment and just sheer number of people passing through, especially in enclosed spaces. The London underground has some of the worst pollution in the capital, particularly in terms of particulates and bacteria. Anecdotally you might notice eg after holding the handrails on some of the escalators that your hand comes back black. There's a famous statistic quoted that a 20 minute journey on the Northern Line is as bad for you as smoking a cigarette.

There's an interesting article on this effect here: https://www.railway-technology.com/features/featureis-the-ai...

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