The report gives results of the development of particulate emission factors based on cutoff size for inhalable particles for external combustion sources. After a review of available information characterizing particulate emissions from external combustion sources, the data were summarized and rated in terms of reliability. Size specific emission factors were developed for these data for the major processes used in combustion. A detailed process description is presented with emphasis on factors affecting the generation of emissions. A replacement for Sections 1.1 (Bituminous and Subbituminous Coal Combustion), 1.2 (Anthracite Coal Combustion), 1.3 (Fuel Oil Combustion), 1.4 (Natural Gas Combustion), 1.6 (Wood Waste Combustion in Boilers), and 1.7 (Lignite Combustion) of AP-42 was prepared, containing the size specific emission factors developed under this program.