The air in the London Underground system, "the Tube", can get quite
stale and nasty, especially in our present heat wave (33 deg. Celsius
yesterday).

The main ventilation method is the air rammed through the tunnels by
the moving trains.  The sparking from the current collectors generates
ozone; the highly reactive nascent oxygen must help to scavenge some of
the dirt in the air.  Occasionally you can smell ozone on the Tube but
I reckon most of it reacts with the muck in the air before it reaches
the people.

Jeffrey Borinsky, London