EPA highlights the many significant reductions in concentrations of air pollutants since the implementation of the Clean Air Act. Some of the most harmful pollutants to our physical health have been reduced to levels we may not have thought possible "back then." Sometimes in order to think about what is possible in the future, you need to look back at the past, and I'd say this is a pretty compelling case for us to make more great strides in air quality both domestically and globally over the next few decades.