The basic principle in the ventilation of shelters is to ensure that the air of the shelters is suitable for the minimum living conditions of the people in it during the period they are in the protection environment. A positive pressure of 50 Pa relative to the external environment must be created indoors to prevent radiation, biological and chemical dust and particles from leaking into the shelter. The shelter air should not be heated, cooled or humidified. Shelters should include both normal ventilation and protective ventilation systems.