Ventilation terms in the context of COVID19

Ventilation is different from circulation and internal leakage is different from recirculation. Ventilating by opening the windows is an emergency measure and, overall, there is a need to clarify the different terms to avoid being used interchangeably.

Ventilation is the intentional replacement of used indoor air with fresh, outdoor air. The air is used by the people inhabiting the space, who inhale oxygen (O2) and breathe out carbon dioxide (CO2). The concentration of CO2 serves as a measure of the quality and freshness of indoor air. However, the concentration of virus particles in the air also depends on how many sick people are inside.

A ventilation system facilitates the introduction of fresh outside air in an intentional and controlled manner.

Fresh outside air can typically enter the building through infiltration, which is considered unconscious or non-intentional ventilation. A way to temporarily increase ventilation through infiltration is to manually open a window or door. By consciously making larger openings in the façade, more fresh outside air will flow in, allowing for better ventilation. More fresh air indoor leads to lower CO2- concentration and reduces the possible concentration of harmful virus particles.

However, there are major drawbacks to opening the windows, include reduced energy efficiency because of increased heat requirement and pollution in terms of outside particles, insects and even noise. It also leads to drier air especially during the winter, which makes inhabitants more susceptible to respiratory infections including COVID19. We recommend maintaining indoor relative humidity between 40% and 60%.

In the reporting surrounding COVID19 and ventilation, internal leakage in a central ventilation unit is often confused with recirculation. Following greater awareness on the Sick Building Syndrome (SBS) of the 1990s, we no longer use recirculation systems. SBS was caused by often too little ventilation and too much recirculation to save energy. Together with inadequate maintenance of the systems, this resulted in buildings where inhabitants became ill.

Recirculation is largely 50%-90% reduction of "used" air. Essentially, the air that is extracted from a room is returned to the same or other rooms. This is done consciously in new systems for clean rooms, surgery rooms and operating theatres, where the recirculated air passes through a very good filter, such as HEPA filters, so that the supply air to the room will be rinsed clean.

If there is a concern that virus particles could be spread again through the exhaust air from a certain room throughout the building, it is, in principle, possible to upgrade recirculation systems with virus-killing UV lamps or photocatalysis filters.

An important condition for the application of UV and photocatalysis is to make sure that there is no ozone (O3), which is carcinogenic, or other residues dangerous for humans are formed in the supply air. That could make the remedy worse than the disease. By installing a balanced air handling “fresh air ventilation” unit a good and healthy indoor climate can be guaranteed.