In this work the evaluation of indoor air quality in a classroom equipped with cross-flow ventilation is presented. A numerical methodology, based on comparison with experimental data, used in the evaluation of the air exchange rate, airflow rate and the age of the air, was applied in the first phase of this work. The evolution of carbon dioxide inside spaces, with different airflow typologies, was then predicted in the second part. The study was based on a school located in the South of Portugal. In the experimental methodology the tracer gas decay method was applied. In order to reduce the experimental time, the first minutes of the test were measured, while the remaining decay was obtained using a numerical exponential regression. Natural and forced cross-flow ventilation topologies were analyzed. In the case of forced ventilation, fresh air from the external environment was driven into the classroom through an air inlet using a supply fan. An extract strategy was also used in which stale air was mechanically extracted from the classroom. Natural ventilation consisted of opening perimeter and above-door windows. The forecast of carbon dioxide evolution was made using software that evaluates the thermal response of and air quality in a building with complex topology. The numerical model used to evaluate internal air quality was based on energy and mass conservation integral equations. These were solved using the Runge-Kutta-Fehlberg method with error control.