This chapter shows special features of indoor air quality. Despite its importance and complexity, the indoor environment has been much less thoroughly studied than has outdoors. Odour is one of the most exacting indoor air quality problems to deal with. Some chemicals such as ammonia and mercaptans have extremely low odour thresholds; a ventilation rate that is entirely adequate for health and thermal comfort may still leave a perceptible smell in the air. US federal standards are based on particle counts per volume of circulated air, with the performance graded into classes. Ventilation of buildings controls the rate of exchange of inside air with outside air and hence not only the ingress of outside pollutants into the building but the release of indoor pollutants from the building. In developed countries, domestic solid-fuel combustion usually takes place in a grate with a chimney and, provided that ventilation is adequate, the increase of indoor pollutant concentrations should be small.