I’ve lived in London for 15 years and I’ve never experienced that. I suspect it’s something of an urban myth.

Happens to me! It's mostly the tube I think though - airborne brake residue etc, is what I've been told.

Me too, in the tube – definitely not a myth. A friend even got his white dress shirt ruined by the stuff.

That’s credible - it could be quite localised on the tube to certain stations or lines if that is the case - I’ve noticed that I get black marks all over my hands occasionally from the escalator handrail in some stations but it’s only a small minority of the ones I’ve been to.

Damn, how much would it cost to just run some air filters down there?

'merican here with normal snot; when I'm doing stuff (house fixy stuff) that turns my mucous colors, it's my sign that I need to go grab a dust mask. Is this not really a concern down there in the underground because $REASONS, or one of those "umm, never thought about it, but now that you mention it..." things?

It's more: this is how we have to get to work so go back to your book :D

Beijing and Shenzhen both made my snot go black when I was there sourcing LED components.

Used to happen to me in New York 15 years ago. Hasn't happened since, though.

Happened to me systematically in Wuhan, a few times (but much less) in Shanghai