Unless you never lift your eyes up to the sky you know the air is cleaner now that it was 70 years ago when a red cloud was seen hovering over Philadelphia. You could easily see it crossing over from New Jersey on the Tacony bridge but it’s gone. It cost trillions to remove it. Exhaust gas was the cause of most of it so cars and trucks had expensive equipment installed to prevent, remove or change the nature of the exhaust which is now so clean that it’s just about invisible.
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