Anything you do to cut emissions costs more. My dad, who oversaw construction on coal powerplant emission systems among other things, says getting 99% of pollutants out is pretty easy, but chipping away at the 1% left gets progessively difficult - and expensive.
Here's the rub: you'll never get 100%. As I say, stopgap. Eventually the earth will be used up and worn out.
And, as long as the sign is cool
Pete