Ultrafine Particles in Indoor Air. Measurements and modelling
Clean air is one of the most fundamental human needs. Since people spen on average over 85% of their time indoors at work, home, school etc., human exposure to air pollution may mostly occur indoors. However, indoor exposure to airborne pollutants will not only depend on emissions from varoius indoor sources, but also on outdoor pollutants supplied through ventilation and infiltration. Assessing human exposure requires knowledge about the identity and the concentration of the pollutants. However, most available information is insufficient, especially concerning ultrafine particles (particles below 0,1µm in diameter). The purpose of this study is to determine the indoor number concentration of ultrafine particles (UFPs) in various non-industrial buildings, to clarify the contribution of outdoor UFPs to the indoor concentration, to identify important indoor sources, and to predict particle number concentrations indoors and the strength of sources and sinks by modelling. The sampling of UFPs has been performed in a laboratory and in various non-industrial buildings. The buildings concerned are located in Sweden and in Denmark. The measurements were made continuosly over hours with a 1-minute sampling interval using condensation particle counters. In the field studies, indoor and outdoor concentrations of UFPs were measured simultaneously. Indoor-outdoor (IO) concentration ratios were calculated for each building studied. In the laboratory different sources of UFPs were examined. An optical particle counter and an electrical low-pressure impactor were used to collect particle size distribution data for different particle fractions in the laboratory and outdoors, respectively. The studies revealed that outdoor UFPs are the major contributors to the indoor particle number concentrations, unless a strong indoor source was present, and that the concentration of UFPs may change rapidly. in office buildings the UFP concentrations were typically lower than outdoors leading to IO concentration ratios between about 0.5 and 0.8 (values averaged over working hours), which suggested rather strong indoor sink effects. Filtration of the supply air seemed to influence the indoor particle concentrations as the lowest indoor-outdoor ratios were observed in the building equipped with the highest class of supply air filter. In residential buildings, the indoor concentration was strongly influenced by several indoor human activities, e.g. cooking, candle-burning. The IO concentration ratios was from about 0.7 up to 2.5 in the presence of significant indoor sources. The laboratory sampling showed that frying, burning candles and cigarettes were stronger UFP sources than the other sources examined. Cigarettes, for example, produced a concentration of about 160 000 particles per cm3. The modelling approach successfully demonstrated its applicability for predicting the number concentration of UFPs indoors and for determining the strength of sources and sinks. Size distribution data revealed that particles below 0,1µm in diameter dominated the number concentration both inddors and outdoors. The study clearly indicates that a substantial fraction of the exposure to UFPs occurred indoors, and that the exposure indoors was different from that outdoors, not only regarding concentration levels, but also with respect to the compsition of the aerosol.