Air pollution science

It’s been 50 years since cleaning up the air in the United States began in earnest. Skies are much clearer now than in the mid-20th century. Leaded gasoline is gone, power plants have been abandoning coal and sulfur dioxide has dropped by 91%. Despite these growing improvements, why have epidemiologists been unable to show the demonstrable public health benefits that their computer models predict? 
The “pivotal regulatory science” used in setting air pollution standards are epidemiological studies measuring the effects of particulate matter on our health. The recently proposed changes to improve the transparency of regulatory science at the EPA have brought these studies to the fore.