ABSTRACT The report contains results from a study of gas spread from outdoor exposure. It is dealing with problems related to hospitals in war situations. There were two categories of modern hospital buildings, large and tall buildings and buildings of medium height. The results presented in the paper concern these two types of buildings. The report answers to questions like: - How does the gas concentration vary in a building when a toxic gas cloud drifts into a hospital area? - How does airtightness of outer walls, roofs and internal partitions affect the gas spread? - How does the height of the building affect the gas spread? To limit the costs we combined a minimum of experimental testing with calculating a number of cases with varying values of parameters. Calculation was possible thanks to a calculation model by N N (1985).