Introduction

Studies have shown that the general population typically spends 90% or more of their time indoors and that the most susceptible individuals, such as the elderly and those with pre-existing medical conditions, spend almost all of their time indoors (Hancock, 2002; Jenkins et al, 1992). Therefore, the quality of the indoor air is of great importance to their health, comfort and well-being.

Effective ventilation of buildings to provide optimum indoor air quality relies on good outdoor air quality in the locality of the building. However, in practice, outdoor air is often contaminated by pollutants from external sources. For example, in the UK, there are many areas, in particular in major cities such as London, where UK air quality standards for nitrogen dioxide and fine particles (PM10) are regularly breached (GLA, 2010). As a result, indoor air is likely to experience higher concentrations of common air pollutants from outdoor sources, especially if buildings have not been designed effectively to reduce their ingress (Kukadia and Palmer, 1998).

Furthermore, until recently, there has been little understanding of the impact of outdoor air pollution on indoor environments, and the complex interactions that occur between outdoor air pollution, ventilation and indoor air quality. As a result, there has been no methodology until now that has allowed the potential for ingress of external pollutants into buildings to be determined. Therefore, it has often been difficult to design buildings in such a way as to reduce the ingress of any outdoor air pollutants, and at the same time to provide high quality ventilation for occupants' respiration.