Indoor–Outdoor Relationships of Particle Number and Mass in European Cities

Human exposure to air pollutants is often characterized by measured or modeled outdoor concentrations. In Western societies, subjects spend about 90% of their time indoors, of which a large fraction in their own home. Hence indoor air quality is an important determinant of the true personal exposure for many components. Indoor air quality is affected both by infiltration of outdoor air in buildings and indoor sources such as smoking, gas cooking, and use of consumer products. In this chapter we separately describe the impact of indoor sources and outdoor air on indoor pollution. We first illustrate differences in outdoor and personal exposure using data on real-time particle number concentrations from a recent study in Augsburg, Germany. We then present a model of indoor PM concentrations, illustrating the factors that affect indoor air quality. We summarize empirical studies that have assessed indoor–outdoor relationships for particle mass, particle number, and specific components of particulate matter. Outdoor air pollution significantly infiltrates in buildings. Combined with the large fraction of time that people typically spend indoors, a major fraction of human exposure to outdoor pollutants occurs indoors. Understanding the factors affecting infiltration is therefore important. Infiltration factors have been shown to vary substantially across seasons, individual homes and particle size and components. Important factors contributing to these variations include air exchange rate, characteristics of the building envelop (e.g., geometry of cracks), type of ventilation, and use of filtration. Penetration and decay losses are particle size dependent with the lowest losses for submicrometer particles and higher losses for ultrafine and especially coarse particles. The largest infiltration factors are consistently found for sulfate and black carbon. Volatilization and chemical decay may also result in losses of specific components, including nitrates and organic components. The large variability of PM2.5 infiltration factors reported may further be due to different composition of PM across locations. In locations with relatively high sulfate and EC contributions, higher infiltration factors can be anticipated than in locations with high nitrate and OC concentrations.