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

The decision to open or close a bathroom window depends on other aspects such as: 1) Is it the only provision to ventilate the bathroom?, 2) Is there also an extraction mechanically or naturally? 

Suppose the window is the only provision to ventilate. First, close the bathroom door. Assuming that the outside air coming in through the window is free of SARS-CoV-2, the opening the window to its’ maximum is recommended. Then, the dilution of any contaminant is at the highest level. The concentration will hence be at the lowest level. 

When the person in the bathroom is infected with the corona virus, there is a risk of spreading the virus to the rest of the dwelling  when there is an over pressure in the bathroom due to wind pressure. Nevertheless the concentration is as low as possible in when the window is wide open. 

Suppose the bathroom has a passive duct or mechanical extraction. First, close the bathroom door. The air extracted through the passive duct or the mechanical system determines the pressure between bathroom and the adjacent part of the dwelling. It is always better to strive for an under pressure to prevent the spreading of moisture. The same is valid in case of SARS-CoV-2. The dilution rate determines the concentration of the virus in the air and hence it is advised to open the window as wide as possible.


Willem de Gids, VentGuide