Carbon monoxide pollution has decreased (or been stable) in the countries I've bothered to check over the last decades (while the Flynn effect was in full swing):

https://www.epa.gov/air-trends/carbon-monoxide-trends

https://www.epd.gov.hk/epd/english/environmentinhk/air/data/...

https://iir-de.wikidot.com/emission-trends-carbon-monoxide