Not only do most people in the most polluted regions of the world live in urban areas, but they also spend nearly 90% of their time inside buildings, a further few percent in vehicles and only around 6% outdoors. Therefore, in this chapter we shall look briefly at special features of indoor air quality. Despite its importance and complexity, the indoor environment has been much less thoroughly studied than has outdoors. The general perception is that one is safe from air pollutants indoors, and many who are today aware of high NO2 concentrations near roads do not worry about cooking with a gas hob that potentially gives them an even higher NO2 exposure. The sources of air pollution that have been discussed in this book thus far have been characterised by their release into the atmosphere at large. In general, the concentrations of these pollutants inside buildings are lower than the concentration outside, because the pollutants are deposited to internal surfaces during the residence time of air in the building. However, there are many additional sources inside buildings, and the same reduced air turnover that protects from external pollutants may cause the concentrations from internal sources to rise to unacceptable levels. Furthermore, the species involved include not only the same gases and particles that penetrate in from the outside, but a whole range of new pollutants that only increase to significant concentrations in the confines of the indoor environment. Table 7.1 provides a list of major health-damaging pollutants generated from indoor sources. Additional sources include people and domestic animals, micro-organisms, moulds and fungi. The concentration of any particular pollutant at any time is the result of the external and internal source strengths, deposition velocities to different surfaces, air turbulence and building ventilation rate.