The concentration of gas species inside a fire exposed enclosure e g a shelter is calculated as a function of leakage, gas concentration and pressure differences between the enclosure and its surroundings. The gas concentration inside the enclosure is assumed evenly distributed. Thus the gas concentrations as a function of time may in many cases be given in a closed form analytical form. In some cases numerical solutions are required, Several examples are given and the obtained results are compared with hazardous concentration levels for carbonmonoxide.