Removal of ultrafine particles by ventilation air filters

The fractional filtration efficiency for particles ranging from 5 nm to 100 nm was estimated based on previous publications. The downstream concentration and size distribution was calculated for each of four outdoor aerosols representing both urban and rural conditions. When interpreting data expressed as the total number concentration (dp>5 nm), the studied F7filter (corresponding roughly to MERV 13 according to the applicable US-standard) showed filtration efficiencies between 65% and 83%. The results also indicate that measurements with an instrument providing the number concentration of particles larger than 20 nm will underestimate the total UFP-filtration efficiency by about 10%-units, estimated as an average value for all of the studied filter classes.