People spend most of their time indoors being exposed to the pollution level determined by the inside sources, as well as by the change between the inside and outside environment through the building envelope. Undesired air flow can enter through the cracks of the building envelope. The leaks are present in the building envelope, even if they are undesired, around the windows, doors and at the connections between the walls and ceiling. Common cracks are characterized by changing properties in time and space being so very difficult to establish a relation between them and the ones that can be reproduced in laboratory tests or in numerical simulations. An overall characteristic number for the process of infiltration is called the "penetration factor" and states the degree of outside pollution level which is encountered in the indoor climate. 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 cracks had a simple rectangular geometry. The chosen cracks' heights was 120mm with a length of 50 mm. According to the reference data, three levels of pressure differences were studied: 3 Pa, 6 Pa and 10 Pa. The most dangerous particle diameters span for the human health varies from 0.35 to 2.5 μm, and so we have chosen for our case this particles' dimensions domain. The injected particles were made of a solid with the density of 1000 kg/m~3 and had spherical shape.
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