I watched a video of a chap with an air quality meter, it was reading 11 somethings at a busy road junction and 50+ wotsits down in the tube.
(I cannot remember what the unit was nor can I provide a URL to the video).
But you can't remember where you found out this information?IT was compared to what TFL were saying is bad, it was (according to tfl/whoever sets the std) way lower at the junction than TFL claims it was.
So it could be an anecdote that you make up as you go along?