Indoor ultrafine particles of outdoor origin: Importance of window opening area and fan operation condition

Inhalation exposure to ambient ultrafine particles (UFP) has been shown to induce adverse health effects such as respiratory and cardiovascular mortality. Human exposure to particles of outdoor origin often occurs indoors due to entry of UFP into buildings. The objective of the present study is to investigate entry of UFP into a building considering building operational characteristics and their size-dependent effects on UFP concentrations. Indoor and outdoor UFP concentrations along with air change rates were continuously measured in a full-scale test building. Estimates of infiltration factor, penetration coefficient, and deposition rate have been made for a range of particle sizes from 4 to 100 nm. The results show that UFP infiltration factor varies with particle diameter, window position, air change rate, and central fan operation. When the central fan was on continuously, the average infiltration factor ranged from 0.26 (particles <10 nm) to 0.82 (particles >90 nm) for two large window openings, and from 0.07 to 0.60 for two small window openings. Under the central fan-off condition, the average infiltration factor ranged from 0.25 (particles <10 nm) to 0.72 (particles >90 nm) for two small window openings, while it ranged from 0.01 to 0.48 with all windows closed. Larger window openings led to higher infiltration factors due to the larger extent of particle penetration into the building. The fan operation mode (on vs off) also has a strong impact, as the infiltration factor was consistently lower (up to 40%) when the fan was on due to additional particle deposition loss to the furnace filter and duct surfaces.