The report characterizes air emissions from industrial external combustion sources and is the last of a series of five reports characterizing emissions from conventional combustion sources. The emissions characterization of industrial combustion sources was based on a critical examination of existing data, followed by a modified Level 1 sampling and analysis approach to resolve data gaps. Tests were conducted at 32 external combustion sites. Emissions assessment results indicate that air emissions from these sources represent a potential environmental hazard. Criteria pollutant emissions, except for CO, from most of the source categories tested are environmentally significant. Particulate sulfate emissions from coal- and wood-fired sources and SO3 emissions from two coal-fired sources are also significant. In addition, emissions of several trace elements are of concern: As, Be, Cl, Co, Cr, Fe, K, Li, Na, Ni, P, and Pb from coal-fired external combustion sources; Ni, Cr, P, and V from distillate oil sources; and Cl, Cr, Na, Ni, Si, and V from residual oil sources. Several potentially hazardous POM compounds were tentatively identified in the emissions from solid-fuel-fired sources; however, further study is needed to positively identify the compounds.