Field studies were conducted on two municipal incinerators with capacities of 50 tons and 250 tons per day in evaluating combustion air effects on atmospheric emissions from municipal incinerators. Separate tests were made at several different levels of underfire combustion air over as wide a range as operating conditions would permit. Solid particulate samples were collected under each test condition for comparison purposes. The results of these field tests show that 50 percent underfire and 50 percent overfire air approaches an ideal distribution, which keeps the particulate emissions well below the loadings which result when using a much higher percentage of underfire air. The following gaseous pollutants were measured for each furnace: Oxides of nitrogen, formaldehyde, carbon monoxide and hydrocarbons. Two schematic diagrams are included.