Ultrafine Particles in the Indoor Environment: Field and Laboratory Measurements

Clean air is one of the most fundamental human needs since poor air quality affects the health and the environment. It has been shown that people spend on average over 85 % of their time at work, home, school etc. Therefore, human exposure to air pollution may occur mostly indoors. Despite this fact, outdoor air pollution is also important. Indoor exposure to airborne particles will not only depend on emissions from the various indoor sources, but also on the outdoor air which is linked to the indoor air through ventilation and infiltration. Assessing human exposure requires knowledge of the identity and the concentration of the pollutants. However, the available information is still limited, especially for particles smaller than 0.1 µm. The purpose of this study was to determine the indoor concentration of ultrafine particles (UFPs) in various non-industrial buildings, to identify the indoor sources and to clarify the contribution of outdoor UFPs to the indoor concentration. The measurements were conducted in a laboratory as well as in non-industrial buildings. The buildings concerned are located in Sweden and Denmark. The measurements were made continuously with a 1-minute sampling interval using two condensation particle counters. In the field studies, indoor and outdoor concentrations of UFPs were measured simultaneously. Indoor-outdoor 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 size distribution data for different particle fractions in the laboratory and outdoors, respectively. Size distribution data revealed that particles below 0.1 µm in diameter dominate the number concentration both indoors and outdoors. The concentration of UFPs may change rapidly, e.g outdoors by a factor of 2 within a few minutes. UFPs generated outdoors are supplied to buildings with the ventilation air and by infiltration. Often, such UFPs are the major contributors to the indoor particle levels unless a strong indoor source is present. Measurements in buildings without pronounced indoor UFP sources revealed rather strong indoor sink effects, leading to indoor-outdoor concentration ratios between about 0.5 and 0.75 (expressed as values averaged over the working hours). In buildings with indoor UFP sources the indoor-outdoor concentration ratio approached unity. Measurements conducted in a full-scale chamber indicated that burning candles and cigarettes, and frying are stronger UFP sources than the other sources examined. Cigarettes for example produced a concentration of about 160 000 particles cm-3. This value exceeds the highest outdoor concentration observed during the field measurements. The study clearly indicates that a substantial fraction of the exposure for UFPs can occur indoors, and that the exposure indoors is different from that outdoors, not only regarding concentration levels, but also with respect to the composition of the aerosol