The effect of external pollution levels on indoor air quality is a subject of growing interest, especially with the increasing application of natural ventilation in urban areas. The analysis described in this paper allows the effects of varying external pollution levels to be estimated for any configuration of ventilation system, natural or mechanical. The analysis takes into account filter efficiency and position, internal pollutant sources, degree of re-circulation and varying fresh air rates.

This Update is the first of two that discuss mechanical ventilation systems in houses. It explains why houses need to be mechanically ventilated, and examines the main characteristics of an ideal system from the standpoint of design and installation.

Current approach for design and installation of mechanical ventilation systems in houses is described in the framework of Canadian building codes and standards, which contain requirements about air change rates, air distribution, sound level, interference with other systems or building envelope.