Ventilation plays an important role in keeping our indoor environment safe and sound. In an enclosed space, people have the effect of reducing the air quality while ventilation using out-door air counteracts that effect. But can the ventilation system itself spread diseases?
The current consensus is that the Corona virus is predominantly spread by large droplet transfer. This means droplets that are caused by coughing and sneezing that are too large to remain airborne for long. Studies show that other people can be infected within a range of about 2 meters in this way. Surfaces are another mode of transmission whereby an infected person might cough or sneeze into their hand and then leave infective material on a door handle, for example, that someone else touches and transfers it to their own mouth or eye.
But can there be potential for airborne transmission? Perhaps if the virus that are included in the droplets that are produced are small enough, they could stay airborne for a long time. Studies shows that virus could remain active at common indoor conditions for up to 3 hours in indoor air and 2-3 days on indoor surfaces so it's not impossible that infectious particles in the air could stay airborne long enough to be transmitted in various ways.
But is transport of the virus through ventilation systems likely? What physical forces or mechanical obstructions are there to prevent it?
There is currently no scientific proof that the Corona virus can be transported through ventilation systems. On the contrary, most experts indicate that we should run ventilation even more during this period of disease. There are methods of securing good indoor quality and controlling airborne infection that have proof of efficiency.
Ventilation
First, bringing in more outdoor air in buildings with full fresh air ventilation systems helps dilute airborne contaminants, making infection less likely. A study published just last year found that ensuring even the minimum levels of outdoor air ventilation reduced influenza transmission as much as having 50 percent to 60 percent of the people in a building vaccinated.
In buildings with mechanical ventilation systems extended operation times are recommended. Change the clock times of system timers to start ventilation a couple of hours earlier and switch off later than usual. An even better solution is to keep the ventilation on 24/7, possibly with reduced ventilation rates when people are absent.
Ventilation systems can be broadly divided into constant volume (CAV) and variable air volume (VAV) systems. CAV systems should be kept running at the set airflow or increase the airflow a little if possible. For VAV systems that are demand controlled using some kind of method to sense the number of people it might be a good idea to raise the minimum airflow during these times to ensure that the building is purged during unoccupied periods.
Recirculation of air is not uncommon in buildings but it can increase the risk of spreading infection from room to room. By eliminating unintentional recirculation and restricting the use of mixing we can minimise the risks.
Viruses in return ducts could re-enter the building where centralized air handling units are equipped with recirculation sections. It is recommended to avoid the use of central recirculation during virus episodes wherever possible and appropriate.
It is also important to prevent the possible risk of “external recirculation”. That is the exhaust air from the building remixing with the intake air on the outside. This can be avoided by ensuring adequate distance between discharge and intake taking prevailing wind direction into account.
Humidity
Humidity has an effect on how viruses are spread in buildings. There are some different theories, but it is clear that humidity plays a role. The recommendation for a long time has been to try to keep minimum relative humidity between 40 and 60 percent, because viruses are least viable in that range and our body’s capability to resist viruses is much higher within that range of humidity.
Humidity control in buildings is not common but during winter the humidity can fall below 30% and in cold climate below 10%. Clearly, it would be a good idea to humidify the ventilation air to keep a more hygienic and comfortable quality of air.
Filters
Any filter that removes particles from the air has some potential to reduce exposure to virus. The question lies in what efficiency of filter is required to be effective and have a significant impact on how likely people are to get infected by the airborne virus.
Filters are designed trap particulates and contaminants from PM10, PM2.5 to PM1 (particles that have a diameter from 10 to 1 microns) The smaller the contaminant’s diameter, the denser the filter media needs to be.
In health care applications we need to be assured that virus and other pathogens will not pass through the ventilation systems and in such applications high-efficiency particulate arresting filters (HEPA) are used. As a further step, ultraviolet germicidal irradiation (UVGI) may be used. Strong ultraviolet lite will inactivate microorganisms and can help to keep surfaces safe and hygienic.
Summary
So, what can we do to secure good ventilation and prevent virus to be spread?
- Increase ventilate rate by time and/or volume
- Don’t recirculate air – minimise recirculation leakages
- If necessary and possible, add moisture to increase the humidity in the room
- Filters may catch some virus but only HEPA filters will be to a broader extent effective
So far no studies shows that ventilation systems are one of the main carriers of disease and virus transmission, so keep on using ventilation to preventing diseases by using air as purifier and having control at eventually potential risks.
Bibliography
Assessing the Dynamics and Control of Droplet- and Aerosol-Transmitted Influenza Using an Indoor Positioning System - Timo Smieszek, Gianrocco Lazzari & Marcel Salathé
Ventilation and airborne diseases – WHO
Coronavirus Prompts Response in HVAC Industry - Tagg Henderson
REHVA COVID-19 guidance document
How coronavirus spreads on a plane 2020 - National geographic
Microbiological cleanliness in the operating room - Preventing airborne contamination - Guidance and fundamental requirements – Swedish institute for standards
ASHRAE Position Document on Airborne Infectious Diseases
Combating the virus in the air - CAMFIL
