The article describes the level of reliability of the methodologies for identifying the level of air pollution in case of accidental and operational emissions, which are the basis of the number of norms and standards. Methodological basis for regulatory determination of the level of air pollution in the rooms is the use of the mass balance equation. In this equation, as the input data the expenses averaged over space and time are introduced, which determine the inflow of harmful substances and their removal by forced or natural ventilation. This calculation model, which assumes that the process is quasi-static, is usually called a single-zone model. In some cases, faced by the designers, the principle of quasi-static inflow and removal of harmful impurities is violated, and strict design norms are based on average annual or average daily cost parameters, which are evenly distributed throughout the space of the room. In this regard, there is a need for a more detailed study of the issue of air pollution in the room under consideration. This issue often comes up when considering real emergency gas leaks in the rooms.  These leaks create explosive mixtures that cause domestic explosions. In the article, on the example of the car parks, the issue of their gas pollution by harmful emissions created by the parking cars is considered. Numerical calculation scheme is given, which allows determining the instantaneous gas contamination of the room under arbitrary initial and boundary conditions. Taking as a source of emission a random in time and space distribution of arriving and departing cars, as well as the actual location and capacity of the ventilation devices based on the numerical calculations it is possible to reliably determine the excess of maximum permissible concentrations of harmful emissions in the rooms, in particular at the car parks. In addition, the proposed methodology allows at the design stage to justify the number, location and capacity of the ventilation devices ensuring an allowable level of air pollution.