A simple model has been developed to predict the indoor air concentration, indoor surface accumulation and the connection between the outdoor and indoor air concentrations. The model is valid for systems where the chemical reactions are slow or the aerosol size distribution is pretty constant, for example for the number concentration of radioactive aerosols (particularly the total activity) or fungal spores. In the model, several factors such as filtration, ventilation, deposition, re-emission, outdoor concentration and indoor sources are included. When simulating radioactive clouds the model results show that there is always a time period after which the radioactivity indoors is higher than outdoors. If the outdoor aerosol particles have a periodic behaviour, also the indoor aerosols behave periodically but smoother and with some delay. Indoor sources are shown to be able to increase indoor concentrations continuously.

• fungi: aerobiology, dispersion of spores,