Too often it falls upon ventilation to accomplish tasks for which it is not appropriate. The prime role of ventilation is to dilute and remove pollutants from unavoidable sources. In essence these are those generated by occupants themselves and by their essential activities. All other pollutants should be controlled by elimination or source containment. Some pollutants are chemically reactive, adsorbed on to surfaces, or have emission characteristics which are stimulated by the ventilation process itself. Such pollutants may not respond to the basic principles of ventilation, in which case ventilation may not be an entirely suitable control mechanism. Examples may include certain volatile organic compounds (VOC's), soil gases and moisture. Again source avoidance or containment are the best control strategies.

Ventilation cannot in itself deal with contaminants introduced into the supply air upstream of the point of delivery. Typical examples include outdoor contaminants, contamination of the ventilation system itself or contaminant sources located between the point of air supply and the ‘breathing’ zone. Filtration techniques combined with careful air intake placement may be necessary to cope with outdoor sources.