I never knew that! We always assumed that they slipped. Live and learn. Thanks.
Yep, it's true.
There's no need for any fancy "emissions control" to be placed on the chimney.
But keeping your furnace in good operating condition should help keep the most toxic emissions to a minimum, and save on your heating bill to boot. And a lot of people simply cover their chimneys with a chunk of coarse metal screen (¼~½" openings). The screen doesn't impede the exhaust, but it helps keep the critters out.