In the present study, the critical re-breathed fraction in rooms with different occupancy levels and the associated critical indoor CO 2 concentrations above background (outdoor CO 2 concentration) were calculated from Equation (2), both as a function of exposure time ( Figure 5(a) ) and quantum generation rate ( Figure 5(b) ). Thus Figure 5 predicts the critical indoor CO 2 concentrations beyond which infectious disease will spread. The family of curves in Figure 5 (a) describes the trends of the critical indoor CO 2 concentrations above outdoor values (C-C 0 ) as a function of exposure times for risk of respiratory infections. The quantum generation rate used was 2/h. The critical CO 2 concentration above the background levels off if the common cold lasts more than 3 weeks (exposure time 8 hours/day, totally 168 hours) (Fig 5(a) ). This indicates that even for less infectious agents with quanta generation rate no more than 2/h, a full fresh outdoor air system without recirculation of indoor air needs to be used in environments where people spend extended time (for example bedrooms, dorms, schools, daycare centers) in order to prevent viral infections. In Figure 5 (b), the exposure time was assumed to be 56 hours (8 hours/day, i.e. 7 days). It shows that the current ASHRAE standard of 700 ppm above the background level [26] would not prevent the infection from being spread in a dorm room with 6 occupants unless the quantum generation rate of infectious agents is no more than 1 quantum/h (Fig. 5(b) ).