how is 20 to 50 % reduction "very small effects"?
> In the graphs below, the blue bars show the average level of pollution from late February through to late March, taken from 2015 and 2019. The red bars show the average level of pollution between the same dates this year.
You can easily see that PM2.5 has roughly halved in London.
[1] https://ncas.ac.uk/en/18-news/3057-air-pollution-falling-acr...