Many studies report that adverse health effects are most strongly correlated with ultrafine particulate matter (< 2.5 microns diameter), originating from ambient emissions mainly derived from fossil fuel combustion.  This research examines the infiltration of ultrafine particles into mechanically-ventilated indoor environments.  Controlled laboratory experiments will be conducted in the Larson Laboratory, Joint Center for Energy Management at CU.  With the use of an Ultra High Sensitivity Aerosol Spectrometer (UHSAS), data will be collected that can be used to compare and correlate indoor and outdoor ultrafine particulate matter concentrations.  First and foremost, this study provides important data needed to understand exposure and health risks associated with inhalation of fine particulate matter.  In addition, the information provided by this study can improve understanding of filtration requirements in buildings.  The correlation of particulate concentrations between indoors and outdoors can be used to evaluate the performance of heating, ventilation, and air conditioning (HVAC) systems in conditioning the outside air make up as well as re-circulated air.