Outdoor Air as a Source of Indoor Pollution
Modern populations spend the majority of their time indoors. For the most active working age population, the fraction of time spent indoors is around 85%, whereas for small children and the sedentary elderly the figure approaches 100%. Indoor air is, of course, particularly affected by indoor sources. Nevertheless, outdoor air is a significant contributor to indoor levels of the main classical air pollutants such as ultrafine and fine particles and nitrogen oxides, and often the only source of others such as ozone and sulfur dioxide. Indoor air quality is created by complex processes that involve outdoor air, ventilation, building tightness, filtration, mixtures of pollution, occupant behaviour and indoor emission sources ranging through building materials, soil, occupants, and customer products and appliances used in the building. The literature shows convincingly that the role played by outdoor air is probably the most significant single factor at the population level. In developed countries, the building stock is relatively well insulated, affecting the air exchange rates and infiltration processes. This chapter presents an overview of recent evidence on infiltration rates of outdoor air pollution and processes that affect them.