The ozone emission rate and the increase in indoor ozone concentrations from the use of electrically connected in-duct air cleaners were studied in this research. Electrically connected in-duct air cleaners (eight models) were tested on a laboratory test apparatus using a Standard Test Method that was developed for this project. Emission rates ranged from less than the method quantification limit of 2.3 mg h-1 to greater than 350 mg h-1. With some exceptions, emission rates were generally not sensitive to flow rate or temperature. Field tests of electrically connected devices were completed in 7 residential buildings (1 in Tulsa, OK, 6 in the Davis/Sacramento area of California) and one California classroom. The incremental increase in the room ozone concentration due to the operation of these devices ranged from undetectable to 194 ppb with devices operating normally, which is above the current California limit of 50 ppb set for portable air cleaning devices. The operation of one unit in gshockh mode elevated the maximum and steady-state concentration at a supply vent to 508 ppb. Estimated emission rates in field buildings ranged from undetectable to 414 mg h-1. For a Standard California house, model analysis predicts that an emission rate of approximately 150 mg h-1 would raise the indoor concentration by about 50 ppb. In an 'At-Risk-House' model analysis, an emission rate of 27 to 55 mg h-1 can raise the indoor ozone concentration by 50 ppb. Both Standard and At-Risk home simulations assume that the outdoor ozone concentration is zero. Therefore, some in-duct air cleaners generate ozone at rates that can increase indoor concentrations above accepted maximum levels.