This paper introduces the effective depth of fresh air distribution in rooms with single-sided natural ventilation. A numerical method for the determination of the effective depth is described. The numerical method is based on the CFD technique involving the governing equations for air flow and the transport equation for the local mean age of air. The renormalisation group k - epsilon model of turbulence is used with equations for the conservation of mass, momentum and energy to predict turbulent buoyancy-induced room air movement. The air now through a large opening is derived from the Bernoulli theory. The predicted air flow pattern, air temperature and local mean age of air are used to determine the effective depth of fresh air distribution in a naturally ventilated room with a window opening for summer cooling. It is shown that the effective depth for thermal comfort may not coincide with that for air quality and in summer, the requirement for thermal comfort is the limiting factor to the effective room depth. The effects of window opening levels and room heat gains on the air flow rate and effective depth are investigated. (C) 2000 Elsevier Science S.A. All rights reserved.