Systems and apparatus for increasing combustion efficiency during combustion of a carbon-containing fuel such as a fossil fuel. The systems and apparatus utilize catalytically reactive particles within a reaction chamber, typically silica or alumina, that interact with waste exhaust gases produced during combustion of the fuel in order to produce a degrading atmosphere of hydroxy radicals or other reactive species. The degrading atmosphere apparently migrates to the source of combustion and increases the efficiency of combustion as evidenced by the reduction or elimination of soot and other pollutants normally produced. Typically, the reaction chamber is maintained at a temperature in a range from about 30 C. to about 600 C. Moisture is provided by the waste exhaust gases in order to catalyze formation of hydroxyl radicals by the catalytically reactive particles. The systems and apparatus can be adapted to be used in combination with diesel engines or other internal combustion engines and industrial burners. Such systems also reduce nitrogen oxides (NO_x), SO₂ and CO₂ typically found in exhaust gases.