By Gunnar W. Schade, Texas A&M University

Urban Air Pollution in the U.S. has been decreasing near continuously since the 1970s.

Federal regulations, notably the Clean Air Act passed by President Nixon, to reduce toxic Air Pollutants such as benzene, a hydrocarbon, and ozone, a strong oxidant, effectively lowered their abundance in ambient air with steady progress.

But about 10 years ago, the picture on air pollutants in the U.S. started to change. The “fracking boom” in several different parts of the nation led to a new source of hydrocarbons to the atmosphere, affecting abundances of both toxic benzene and ozone, including in areas that were not previously affected much by such air pollution.