REMARK: This Q&A was part of the AIVC special COVID-19 newsletter published in February 2021. To subscribe to the newsletter please click here.

Assuming an infected person has occupied the room, the room should be ventilated until the concentration of infectious aerosols is decreased to an acceptable level before it is used again by other persons. At the time of writing there is however insufficient scientific knowledge to define what is an acceptable level.   

Still, it is possible to estimate how long a room should be ventilated to reduce the aerosol concentration down to a level relative to the initial concentration. Ventilation in most cases is mixing the air and diluting the contaminant. As a result the concentration decreases exponentially when no further contaminant production is present and ventilation occurs with uncontaminated air. Assuming complete mixing of air, and negligible influence of settling and inactivation of infectious aerosols, the decay from the initial concentration depends on the product of the air change rate of a room and the ventilation time. The air change rate expresses the rate at which a room volume is replaced by uncontaminated, typically outside air. Replacing the room volume with the same volume of outside air in a given time does not mean the room is completely clean after that time. Table 1 shows the ratio between the concentration at a given moment and the initial concentration as a function of time and air change rate.

Table 1: Relative concentration as a function of ventilation time and air change rate

Air change rate (per hour)
After 15 minutes88%78%61%37%
After 30 minutes78%61%37%14%
After 60 minutes61%37%14%2%
After 120 minutes37%14%2%0%


A faster reduction of contaminant concentrations requires increasing the air change rate, as is expressed in the following Equation:

t= (1/n)ln(C(t)/C0)

where t is the ventilation time (h), n is the air change rate (h-1), C(t) is the concentration at time t, and C0 is the initial concentration.

So, in order to reduce the contaminant concentration to 1% of its initial concentration a room should be ventilated for more than 9 hours at an air change rate of 0.5 air changes per hour, or for about 1 hour and 10 minutes at an air change rate of 4 air changes per hour.


Willem de Gids, VentGuide & Arnold Janssens, Ghent University


  1. AIVC. AIVC Newsletter Special Issue on COVID-19. February 2021.