Instead, many operators upgraded the plants so that they could keep the exemption and would not have to deal with clean air regulations. (realclimate.org)
BC and OC amounts decreased strongly in the early 1970s, probably because of clean air regulations in Europe. (giss.nasa.gov)
Over the next decade a host of federal air regulations will update limits on the emissions of multiple pollutants from stationary sources such as... (aceee.org)