Back in 1970, Los Angeles was known as the smog capital of the world — a notorious example of industrialization largely unfettered by regard for health or the environment. Heavy pollution drove up respiratory and heart problems and shortened lives.

But 1970 was also the year the environmental movement held the first Earth Day and when, 45 years ago this month, Congress passed a powerful update of the Clean Air Act. (Soon after, it was signed by President Richard Nixon, and it was followed by the formation of the Environmental Protection Agency and passage of the Clean Water Act, making him one of the most important, though underappreciated, environmentalists in American history.)

Since that time, the Clean Air Act has repeatedly been challenged as costly and unnecessary. As a fight brews over President Obama’s new use of the law to address global warming, it’s worth re-examining the vast difference the law has already made in the quality of the air we breathe, and in the length of our lives.

Numerous studies have found that the Clean Air Act has substantially improved air quality and averted tens of thousands of premature deaths from heart and respiratory disease. Here, I offer new estimates of the gains in life expectancy due to the improvement in air quality since 1970 — based on observations from the current “smog capital” of the world, China. (To learn more about how this was calculated, click here.)