Abstract From a viewpoint of safety assessment of insecticides being released with an electric vaporizer into a room a model of fluid dynamics was used to simulate the behavior of the evaporated active ingredient (ai), air flow and the temperature distribution at a steady and unsteady state under various conditions. The model delivered the behavior of the evaporated ai due to the variations of the air exchange rate, locations of the air inlet and electric vaporizer and room temperature. A simulation experiment showed that the calculated aerial concentration at a steady and unsteady state agreed well with the measured one. Thus, the model will be very useful to describe the behavior of ai by utilizing known, calculated, or easily available data of the rooms, use patterns and physicochemical properties (ai) without realization of experiments.