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.