Museum storage buildings often have low air exchange rates. For such buildings, the indoor level of air pollution is primarily dependent on the air exchange rate, and the sorption capacity of indoor surfaces. This paper illustrates the effect of the air exchange and the sorption reactions, as well as mechanical ventilation with active filtration, by using data from pollution measurements in three Danish museum and archive buildings. Typically indoor ozone levels will increase when the air exchange rate increases, as ozone enters buildings from outside. Pollutants generated indoors, such as organic acids, will dilute in concentration as the air exchange rate increases. However, the source strength and the surface sorption capacity are dominating influences when the air exchange rate is below about 1 h-1. Active filtration is an efficient method for pollution removal, especially by internal re-circulation units.