The indoor air quality is a very important issue and it generally depends on the airtightness of theshell, the ventilation rates, the deposition and the resuspension rates of the particles and the internalsources as well.The whole experimental procedure took place in a PASSYS test cell, which is located at the University Campus of Athens. At the internal door of the test cell, a blower door system was mounted, in order to control the pressure difference between the internal and external environment as well as the equivalent airflow through the internal of the test cell.The aim of this study is the investigation of the balance of the particles concentration in two cases;firstly, by keeping the pressure difference between the external and the internal environment near tozero and then by changing the pressure difference as follows: 0.1-4 Pa, 4.1-8Pa, 8.1-12Pa. Thecalculations of the deposition velocities have been performed for the following ranges of aerodynamic diameter: values less than 1?m, 1-2.5 ?m, 2.5-5 ?m, 5-10 ?m.In order to measure the infiltration rates the tracer gas decay method has been used with INNOVA1312 tool. The particles concentration has been measured through the HANDHELD 3016 IAQ as well.The deposition velocities range from 0.027m/h for values of particles less than 1?m to 0.670m/h forparticles of 5-10 ?m, in case of zero pressure difference. The deposition velocities have also beencalculated for the cases that the pressure difference varies in a specific range.