Indoor air quality has an important role as health determinant: it relates to the health and comfort of building occupants. The present paper investigates the wind-forced natural ventilation rates of an office as observed with tracer decay, according to ISO 16000-8:2007 Standard, both with an experimental and numerical procedure: experimental data were used to validate the numerical method. Moreover the only infiltration rate case was analyzed. Natural ventilation was created by opening the window and the door of the office. A simulation model of the room was carried out by using the computational fluid dynamics (CFD) code Fluent. The experimental set up of the procedure was developed by a portable gas chromatograph. In both the numerical and experimental approach the mean age of air linking to ventilation efficiency was calculated. The defined experimental procedure can be applied to different situations, in order to evaluate the efficiency of both natural and forced ventilation existing systems. When the ventilation is clue to the opening of the window and the door, the simulation results are in a good agreement with the experimental ones, therefore the model could be applied to different situations, in order to reduce costs and time in the evaluation of indoor air quality. (C) 2011 Elsevier B.V. All rights reserved.