From
https://www.thetimes.co.uk/article/pollution-charge-for-diesels-has-cut-citys-toxic-fumes-by-a-third-7vmf0wwst?shareToken=c4cb13264b46e8e695faa7f1bcae0 0af

Quote:
NO2 has fallen from 85 to 57 micrograms per cubic metre at the roadside in
central London since October 2017, when Sadiq Khan, the mayor of London,
introduced a £10 daily toxicity charge. The charge increased to £12.50 in
April as part of the capital’s Ultra Low Emission Zone (Ulez).

There were 13,500 fewer polluting cars driven into central London last
month compared with March, a reduction of 38 per cent, according to the
report by the Greater London Authority.

However, the level of NO2 in central London remains well above the legal
limit of 40mcg/m3 and almost a quarter of vehicles in the charging zone
breach the Ulez emissions standard. This is partly because diesel black
cabs are exempt from the charge.

The amount of traffic in central London has fallen by up to 9 per cent
since last year, suggesting that the introduction of the charge has
prompted some people to give up driving into the centre.