On the basis of known data a preliminary evaluation has been made of the limiting values of air temperature, relative humidity, oxygen and carbon dioxide content, which can be permitted to prevail in a standard shelter for civilians at full occupancy during a period of seven days and nights. With the assumption that the temperature tolerance limit may correspond to a wet bulb temperature of + 28° C and neglecting the heat transmission to the solid surroundings the ventilation necessary for cooling pro person and hour has been calculated for summer and winter conditions giving following results. Summer: 8,8 CFM/person (input: 10 g water/kg air) (output: 22 g water/kg air) Winter: 4,13 CFM/person (input: 1 g water/kg air) (output: 22 g water/kg air) The importance of maintaining internal vigorous air movement and having a supply of at least 200 g of water and one gram of sodium chloride pro person and hour is stressed. The magnitude of the air input mentioned will keep the oxygen content of the shelter air above 19 % and the carbon dioxide content well below 0,3 %.