Nitrogen oxides and sulfur dioxides are the two principal air pollutants regulated by the Clean Air Act, and the emissions of both have been slashed since 2005. In fact, sulfur dioxide pollution is down 70%, or from 10 to 3 million tons per year, and nitrogen oxides are down 55% or from 4 to 1.8 million tons.
Why the massive decline in air pollutants? In the above link, EIA states two reasons explain the huge drop. First, modern pollution controls have been installed on 91,000 megawatts of coal fired power. Second, especially since 2008, cheap natural gas has displaced large amounts of coal.
The air we breathe today is much cleaner than just 5 years ago and is getting cleaner still, thanks significantly to pollution controls and more gas-fired generation. That's a wonderful fact!