As described in the Berkeley News, the study, forthcoming in the American Economic Review, found that polluting emissions from U.S. manufacturing fell by 60 percent between 1990 and 2008—a period in which manufacturing output grew significantly—primarily because manufacturers adopted cleaner production methods in tandem with increasingly strict environmental regulation. “In the 1960s and 1970s, people worried that Los Angeles, New York and other U.S. cities would have unbearable air pollution levels by the end of the 20th century,” said Shapiro. “Instead, air pollution levels have plummeted, and the evidence shows that environmental regulation and the associated cleanup of production processes have played important roles in those steep declines.”

Also read The Daily Californian article regarding the study.