Air quality in the US?!

Hej,

saa jeg har den her engelsk/amerikansk historie klasse, hvor vi skal til at have en debat! Vi skal debattere omkring "environment" og jeg skal agumentere at USA har gjort en masse godt, saa der kunne vaere hojere luft kvalitet. Nogen der ved noget om US historie omkring 1900-tallet, som kan hjaelpe mig lidt (meget)??

(Statement) Environment: A dominant theme throughout America's history is the importance of nature to our economic, cultural, and political development; in the 20th ccentury we have acknowledged this importance and are increasingly effective at protecting and ensuring the future of our natural resources.

(Question) What laws and/or economic factors have affected the quality of the air we breathe?