Was curious myself, based in on his other comments, it's Poland.

This site disagrees with your opinion http://airindex.eea.europa.eu

Does it? I see a large concentration of red dots over Poland, and some over Belgium-France border, all due to coal plants. Then there's UK.

    Poland takes second place with an overall score of
    5.5/10. The carbon dioxide emissions in poland are 
    7.63 tonnes per capita per year, which is higher than
    the winner Turkey. The concentrations of PM2.5 are 22
    µg/m3 which is almost half of Turkey’s concentrations.
    There are 69 deaths attributable to air pollution per
    100,000 capita per year. Poland consists of 30.8% 
    forest area and 38.10% protected terrestrial and 
    marine area. Each year, the citizens of Poland discard 
    304.9 kg of waste per capita.

Yes.

From your own quote: Poland, second place (over all)

World Atlas has them at #3 https://www.worldatlas.com/articles/cities-with-the-worst-ai...

So...not "dirtiest"

As a third party, I appreciate the links you've provided but you're being pedantic about Poland having to be strictly 'the dirtiest'.

Second after Turkey, which is not in the EU.