Determines ventilation rates and intercell flow rates in naturally ventilated office building using multiple tracer gases. Subdivides the building into 3zones and seeds each zone individually with a different tracer gas. Monitors the time histories of the concentrations of all gases in each zone using non-dispersive infra red gas analysers. Calculates air flow rates from experimental data. Uses an in-house computer program which predicts the dispersion of a tracer gas in a multi-zoned environment, to compare the predicted time histories of concentrations with those obtained experimentally.