The most widespread technological inventions to reduce emissions have been electrostatic precipitators (electrical cleaning systems) and filters designed to control emissions from power plants. These reduce particulate emissions from smokestacks by 99.5 percent but do nothing about gaseous emissions. The primary way to reduce sulfur dioxide (SO2) has been the use of scrubbers (an air pollution device that uses a spray of water or reactant to trap pollutants), which remove 95 percent of the SO2 residue. The available technologies have limitations—they create scrubber ash, a hazardous waste, and do nothing to control carbon dioxide (CO2) emissions.
With the aid of pollution-control equipment and improvements in energy efficiency, many industrial countries have reduced emissions. Since the passage of the CAA there has been a 90 percent reduction in emissions of hydrocarbons and CO from the average car in the United States and a 75 percent reduction in NO5.
Other pollutants, however, have gone mostly unregulated, notably CO2, the inevitable by-product of burning fossil fuels.