Carbon oxides make up the largest single group of pollutants. Carbon monoxide, a colorless and odorless poison, is a gas produced when fuel is incompletely burned in engines, furnaces, and space heaters. Automobiles and other vehicles are the major source of this pollutant. Another gas, carbon dioxide, is a product of all normal combustion (burning). Although carbon dioxide is not a serious pollutant in itself, some scientists believe that a long-term buildup of this gas in the atmosphere could cause what they call a “greenhouse effect” by reducing the flow of heat from the earth back into space, thus causing a potentially dangerous warming of the earth.