Defines ventilation effectiveness as a measure of the performance of the ventilation system in terms of maximising the use of a given fresh air input in reducing indoor generated pollutant concentrations. Describes a laboratory-based examination of the ventilation effectiveness of three different types of mechanical ventilation arrangements. Measures the difference between extract only, wall-mounted supply grilles and floor mounted supply twist outlets. Also investigates different rates of fresh air supply, a with a simulated occupant producing the contaminant. Concludes that the overall fresh air ventilation rate is the key parameter, with ventilation effectiveness having a secondary influence on the acceptability of the indoor air quality Notes the differences in the ventilation effectiveness of the three systems, with the largest difference of 28% between extract only and floor-mounted twist outlets, the latter providing the more favourable result.