ISO 16814:2008 is intended to specify methods to express the quality of indoor air suitable for human occupancy, to allow several acceptable target levels of indoor air quality, depending on local requirements, constraints and expectations. ISO 16814:2008 applies to the design of new buildings and their systems and the retrofit of existing buildings and systems, indoor environments where the major concern is that of human occupants, buildings having any combination of mechanical and natural ventilation, commercial and institutional buildings. ISO 16814:2008 does not apply to residential buildings, industrial buildings and hospitals although those parts of such buildings that are similar to commercial buildings are covered. The requirements of ISO 16814:2008 might not achieve acceptable IAQ for all people in all buildings, due to one or more of the following sources of uncertainty. The outdoor air brought into the building can be unacceptable or might not be adequately cleaned. Indoor air has a wide diversity of sources and contaminants. There are many factors that affect occupant perception and acceptance of IAQ, such as air temperature, humidity, noise, odours, lighting and psychological stress. There is a range of susceptibility and preference in the population.