Indoor air quality is a major factor as nowadays people spend most of their time in buildings being exposed to the indoor level of pollution. The rate of pollutant entry through a leakage can be expressed by means of a penetration factor. Being a function of species, leakage path properties or pressure drop over the crack, the penetration factor states the level of outdoor particles found indoors. Present paper aims to present a numerical model proposed by the authors able to determine the particles’ mass transfer from outside environment to the inside one as a result of a pressure difference between the two zones. The studied crack has a simple rectangular geometry. The chosen crack’ height was 1mm with a length of 50mm. According to the reference data, three levels of pressure drops were studied: 3, 6 and 10Pa. The most dangerous particle diameters’ span for the human health varies from 0.35 to 2.5?m, and so we have chosen for our case this particles’ dimensions domain. The injected particles were made of a solid with the density of 1050 kg/m3 and had spherical shape. The penetration factor showed to be in the same range of values for a pressure drop of 6 and 10Pa. 75% of the outside particles will penetrate the crack for these pressure difference levels. If the pressure difference decreases to 3Pa, then the penetration factor reaches the value of 60%.