It is mainly a diesel / gasoline thing. There are often many more fine particulates in the air in EU cities, although with the newer Euro standards having ever

It is mainly a diesel / gasoline thing. There are often many more fine particulates in the air in EU cities, although with the newer Euro standards having ever larger effects, that gap is dropping rapidly. FWIW, I and many people I know experience the same thing whenever we return to Europe from longer periods away.