The report characterizes air emissions from commercial/institutional external combustion sources and reciprocating engines and is the fourth of a series of five project reports characterizing emissions from conventional combustion sources. This characterization was based on a critical examination of existing data, followed by a modified Level 1 sampling and analysis approach to resolve data gaps. The major deviation from Level 1 procedures was the additional of GC/MS analysis for polycyclic organic matter (POM). Tests were conducted at 22 external and six internal combustion sites. Results of the environmental assessment indicate that air emissions from these sources represent a potential environmental hazard. Emissions of criteria pollutants, except for CO, from most of the source categories tested are environmentally significant. Particulate, SO4, and SO3 emissions from the coal- and wood-fired sources are also significant. In addition, emissions of several trace elements are of concern: Al, Ba, Be, Ca, Cl, Co, Cr, Cu, F, Fe, K, Li, Na, Ni, P, Pb, Si, and V from coal-fired external combustion sources; Ni from distillate oil sources; and Ni, Cl, Cr, and V from residual oil sources. Several potentially hazardous POM compounds were tentatively identified in the emissions from solid-fuel-fired sources, particularly from one wood-fired stoker tested.