A method for determining indoor contaminant emission rates was described. The method was based on determining individual emission rates from various sources within the environment, known as source activity factors. Air concentrations of condensation nuclei (CN), respirable dust (RP), total dust, nitrogen-dioxide (10102440) (NO2), carbon-monoxide (630080) (CO), carbon-dioxide (124389) (CO2) inside and outside a welding shop, and CN, RP, and CO in ambient air of a lunch room were measured. Welding shop samples were collected during 50 4 hour sampling periods, and the lunch room samples during 16 4 hour periods. CN, RP, and NO2 were selected for emission rate analysis in the welding shop study, and CN and CO2 were measured in the lunchroom study. Observations of emission source activities, such as the number of welders and space heaters in the welding shop, were made for 10 minute intervals. Overall emission rates were calculated with a pollutant mass balance model. Multiple source emission rates were determined using linear regression analysis by comparing overall emission rates to the source activity factors. Experimental and literature values of emission source rates for the welding shop and lunch room were tabulated. Average source emission rates were, generally, consistent with literature values. Experimental and literature values for NO2 emission from heaters in the welding shop were 1.5 and 4.3 milligrams/minute, respectively. Experimental CN and RP emission rates from welding were larger than literature values because they included emissions from painted and galvanized metal bases. The authors conclude that the method can satisfactorily determine source emission rates in the presence of multiple sources and variable conditions for 4 hour sampling intervals.