Before revision:  We create such a model assuming that  the concentration of airborne infectious particles is governed by an advection-diffusion-reaction equation . As schools and universities are intensively discussing how to operate their indoor spaces, we solve the model for a classroom, assuming one infectious person at the centre of the room and quantify the effect of several ventilation settings when the person is breathing or talking, both with and without a face mask . We compare our model with more complex models and with experimental data and find good agreement. The framework can be easily applied to any other high-risk location. We also generate maps of the Time To Infection (TTI) by airborne transmission, for various activities and ventilation levels. Using the TTI maps we quantify the airborne transmission risk and make recommendations for the Safe Occupancy Time (SOT), for a chosen risk tolerance level. As expected, we find that the SOT decreases with ventilation quality.
After revision:  We create such a model assuming that the concentration of airborne infectious particles is governed by an advection-diffusion-reaction equation with the particles advected by airflow, diffused due to turbulence, emitted by infected people and removed due to the room ventilation, inactivation of the virus and gravitational settling. We consider one asymptomatic or presymptomatic infectious person who breathes or talks, with or without a mask and model a quasi-3D setup that incorporates a recirculating air-conditioning flow. A semi-analytic solution is available and this enables fast simulations. We quantify the effect of ventilation and particle emission rate on the particle concentration, infection risk and the SOT decreases with ventilation quality.