Steady state pollutant concentration: The pollutant concentration in a space depends on the rate of pollutant emission and the rate at which the space is ventilated. Provided the emission rate remains constant, then a steady state concentration is eventually reached which is independent of the enclosure volume. Under conditions of uniform mixing, the concentration throughout the space will be uniform, whereas if mixing is non uniform (e.g. displacement ventilation), the pollutant concentration will vary throughout the space.

Transient pollutant concentration - the building as a ‘fresh air’ reservoir: The time it takes for the steady state concentration to be reached depends on the rate of ventilation and the volume of enclosed space. Thus it may sometimes be possible to avoid immediate air quality problems by taking advantage of the fresh air already stored in a room. The capacity of a building to act as a reservoir is useful for absorbing the impact of transient pollution emissions and variations in ventilation rate.
It may also be used to advantage if the outdoor air becomes polluted for a short period (e.g. rush hour traffic) by temporarily restricting the rate of ventilation. Older naturally ventilated buildings are typically constructed with high ceiling heights to provide an air quality reservoir.