The article discusses the dependences of carbon monoxide (II) concentrations on the height of the building façade based on the results of in situ tests with the goal of preventing the inflow of pollutants into the ventilation system. Carbon oxide (II) may enter the premises when the inlet ventilation system is working through the inlet chamber from external sources of emission. i.e. vehicles and low and medium-power boilers in the urban environment. We have constructed nomograms to adjust the height of the receiving hole for air intake by the ventilation system depending on the quality of the outdoor air on the height of buildings from external sources of different heights in relation to the building. We have suggested guidelines allowing to design ventilation systems depending on the quality of outdoor air over the entire height of the building, which cannot be done at the design stage.