If I remember correctly Pittsburgh, PA was the absolute dirtiest city in the USA.

I was there in 1943 to pick up an LST (Landing Ship, Tank) and head for the war in the Pacific.

I never saw the sun, not because of cloud cover. We could not even see the clouds.

All we saw was smoke, smoke and more smoke.

I know that after the war Pittsburgh made a very successful effort to eliminate this condition and in the mid sixties I was there on business a couple of times and was amazed at how clear and clean the air had become.

I have lived on Martha's Vineyard since 1985 and of course our air is about the best - even in the fog.