For cost reasons, passenger cars sold in moderate climatic zones frequently come without factory-installed air conditioning. Therefore, the temperatures in the vehicle occupant compartment may rise above the thermal comfort range during hot summer months. In order to provide for a maximum of thermal comfort under those circumstances, occupant compartment ventilation should satisfy certain clearly defined requirements. The paper submits the physiological rationale and a concrete description of these requirements.A special method, the so-called isotach method, was developed in order to permit a quantitative determination of the vehicle occupant cell air flow and to verify compliance with the air flow requirements. The isotach method is used to measure the air flow distribution in a lateral cross section of the vehicle occupant compartment front section. The method is described and explained.While the application of the isotach method to given vehicles does not pose any problems, there are frequent requests for air-flow data projections during the first stages of new vehicle development. The paper presents a computing process that permits prognoses of the air-flow velocities downstream from the fresh air outlets.