Anecdotal evidence: I grew up in Australia and I had reasonably severe asthma when I was a kid, when I got older I only got wheezy when I got a cold. Moved to the UK for 8 years and immediately started getting regular asthma attacks after moving over and had to go on a preventer. Lived in London for the three quarters and Guildford for the last quarter. Moved home post COVID and a couple months after arriving home my asthma went back to only being an issue when sick. I'm confident it was pollution related.