This report is part of a project to characterize cotton gin emissions from the standpoint of stack sampling. The impetus behind this project was the urgent need to collect additional cotton gin emissions data to address current regulatory issues. A key component of this study was focused on EPA emission factors for particulate matter with a particle diameter nominally less than or equal to 10 µm (PM10). The 1996 EPA AP-42 emission factors were assigned quality ratings, from A (Excellent) to E (Poor), to assess the quality of the data being referenced. Emission factor quality ratings for cotton gins were extremely low. Cotton gin data received these low ratings because they were collected almost exclusively from a single geographical region. The objective of this study was to collect additional PM10 emission factor data for overflow systems at cotton gins located in regions across the cotton belt based on EPA-approved stack sampling methodology, Method 201A. The project plan included sampling seven cotton gins across the cotton belt. Key factors for selecting specific cotton gins included: 1) facility location, 2) production capacity, 3) processing systems, and 4) abatement technologies. Three of the seven gins had overflow systems where the exhaust airstreams were not combined with other major systems. Another sampled gin had an overflow system where the exhaust was combined with a trash handling system prior to the cyclone. In terms of capacity, the three gins were typical of the industry, averaging 24.8 bales/h during testing. Some test runs, included in the analyses, had cotton lint fibers that collected in the ≤ 10 µm samples. This larger lint material can affect the reported emissions data, but EPA Method 201A does not suggest methods to account for these anomalies. The overflow system average emission factors for PM10 and total particulate were 0.013 kg/227-kg bale (0.029 lb/500-lb bale) and 0.033 kg/bale (0.072 lb/bale), respectively. The system average PM10 emission factor was higher than the emission factor currently published in EPA AP-42, whereas the system average total particulate emission factor was about equal to the AP-42 factor. Overflow system PM10 emission rate test averages ranged from 0.12 to 0.42 kg/h (0.26-0.93 lb/h). The ratio of overflow system PM10 to total particulate was 40.3%.