Pictures tell the story:

Earth Day 2017: Jarring Photos of Polluted Great Lakes | Time.com

This is what NYC looked like when I was a kid:

https://upload.wikimedia.org/wikipedia/en/0/02/1966_NYC_smog_by_Neal_Boenzi_NYT.jpg

http://www.popsci.com/sites/popsci.com/files/styles/325_1x_/public/images/2017/02/chester_higgins_-_the_george_washington_bridge_in_heavy_smog._view_toward_the_new_jersey_side_of_the_hudson_river1973 .jpg?itok=zcOdiDIc&fc=50,50

How many of us are old enough to remember the smell?