This report describes Risøs contribution to the TEARHS project. The aim of the TEAHRS project is to develop and assess methodologies to determine the acute toxicity of inhalation of fluctuating concentrations of hazardous substances as a contribution tothe improvement of quantitative risk assessment. A mathematical model is described that predicts the physical and chemical processes in the airways during short exposures to toxic substances. Based on this modelling, it has been assessed how fast and towhat level tissue in the airways is exposed to the substance. These time scales turn out to be small, in the order of a few seconds. Considering the aspiration pattern, it is concluded that relevant time scale for absorption in the airways is 5 to 10seconds. In real atmospheric, toxic gas clouds, fluctuations at this time scale are large. There is still a large gap between the empirical information on toxic effects and the relevant time scale for exposure. Concentration-time-fatality relations areobtained from experiments with rats down to 5 minutes of exposure. If the information from these relations is extrapolated down to 5 to 10 seconds using different assumptions, the predicted mortality is quite different. This study indicates thatatmospheric dispersion models for acutely toxic substances need to provide information about concentration fluctuations of time scales of 5 to 10 sec. Final conclusions can only be drawn when it becomes clear how the gap betweenconcentration-time-fatality relations and short duration fluctuations can be closed.