Most people today are aware of the health hazards associated with poor air quality. But for a long time, little attention was paid to the consequences of industrial activity, to smoke and pollutants from coal-burning factories and to cars and trucks crowding city streets and highways, releasing an uncontrolled amount of pollutants.?
Until two major events made it very clear that things had to change to save people’s lives.
The Great Smog of 1952 covered London with a thick blanket of airborne pollutants. Its choking effects led to the deaths of 4,000 to 6,000 people over the course of five days.,  The year after, New York City disappeared under a toxic blanket of sulfur dioxide and carbon monoxide. Reports connected the week of smog to 260 deaths.
The danger coming from air pollutants could no longer be denied. It took the U.S. government several attempts, but the Clean Air Act of 1970 marked a major shift in controlling sources of air pollution across several sectors.