well the squares look orange and ugly, and they don't produce food/prod/trade anymore.
move your worker on the square and press shift-c or click the button at the bottom that says "clear pollution". then wait quite a few turns (my usualy strategy is to put 3-4 workers on the same square to clean it up.. and this on a few squares. It goes much faster).
of course it's worthwhile, unless you want all your tiles to stop producing food/trade/prod and you want global warming...