A new study, released as a preprint on the medRxiv* server, shows that inexpensive portable air cleaners may boost the removal of aerosols containing the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from indoor spaces by as much as three times, compared to conventional Heating, Ventilation, and Air Conditioning (HVAC) systems.
HVAC and room air cleaning
It is noteworthy that HVAC has played little, if any, role in controlling indoor aerosol transmission. Many guidelines recommend using portable high-efficiency particulate absorbing (HEPA) filters to improve this function, but with little success.
An HVAC system brings fresh air into a room, after filtration, circulates it around the room, and then pushes it out via a vent, along with the contained gaseous aerosols and pollutants.
Conversely, an air cleaner filters the air from the room and sends it back cleaned, with air circulation being a byproduct of this process.
The measure of the air change brought about by these methods is the Air Changes per Hour (ACH), which is the ratio of the volume flow rate of the system to the volume of the room.
The current study used portable air cleaners fitted with HEPA filters to remove aerosol particles inside a room near the infected person. The name for this process is source control. The result is a dramatically improved aerosol clearance rate.
A portable air cleaner is meant to filter out dust and other polluting aerosols from room air, rather than remove it from the room altogether. Thus, a combination of both of these would lead to superior infection control.
The clearance time required to remove aerosols at 90% or above is an important parameter in the present situation. The researchers compared several commonly available portable air cleaners in combination with HVAC systems to explore their effectiveness in reducing aerosol particle counts.
With a small control experimental room, and high flow rates, the aerosol clearance was accomplished 4-5 times as fast with the combination of devices than with the HVAC alone. There was complete clearance by 12 minutes.
Even with low flow rates, the portable air cleaner-HVAC combination still surpassed the performance of the HVAC alone.
With the hospital room experimental setup, the HVAC system already produced a high flow rate. With the addition of two air cleaners, the clearance time dipped below 10 minutes, three times faster than with the HVAC alone.
At this setting, the hospital room ACH was 39, but composed of two quite different methods of clearance – HVAC-induced air ventilation and air cleaner-mediated air filtration.
The result is that both operate to clean the room air by different types of air changeovers.
If the ACH is high enough, at about 25, aerosol filtration occurs very efficiently. Fortunately, this is not hard to do with portable air cleaners.
Efficiency peaks at high ACH
The difference between the predicted and the experimental results were significant at low ACH values below 15, but comparable at higher ACH values.
At low ACH, the differences may be due to the presence of dead zones, areas in the room where the air recirculates and traps the aerosols, thus slowing down the movement of the particles during their clearance. Other airflow aberrations may also be present, and need to be identified.
At very low ACH, such dead zones are unlikely, since laminar airflow is predominant. Moreover, slower processes become more important at such low clearance rates, including condensation, air leakage through tiny gaps, and small differences in the outside pressure.
At high ACH, the flow rate keeps the air mixing homogeneously, without dead zones. The use of HVAC in this system may cause the formation of strong local flow fields that prevent the aerosols from leaving the room before being removed by the air cleaners.
Sometimes, the cleaners may filter the aerosol out of the room air even faster than the HVAC removes the air from the room, if their ACH is high enough compared to that of the HVAC. This has been suggested by earlier researchers and could indicate that in rooms with positive air pressure, such a combination would prevent the infectious aerosols from leaving the room and reaching nearby nursing stations or hallways, for instance.
Further studies should use other room designs with the same type of HVAC-air cleaner combination, as well as other smaller but more numerous air cleaners in a similar combination. This would help understand whether more and smaller devices do a better job than fewer high-flow air cleaners.
The optimal positioning of these devices should also be examined.
What are the implications?
The findings show that with increasing ACH, the aerosol clearance time is reduced. This indicates that the result of using the portable devices is to boost the clearance of the aerosols at a given ACH. When used at high flow rates, the clearance time went down markedly.
The study also demonstrates that at standard HVAC operation, aerosol clearance is probably inadequate to control the spread of infectious aerosols in such rooms. The addition of a portable air cleaner would make a significant difference in this situation, especially since it does not push out the aerosol but filters it out of the room air before it leaves the room.
The use of such a combination makes it possible to achieve about 25 ACH, which in turn allows the room air to be cleared of aerosolized particles in less than 10 minutes. This could transform the handling of virus transmission risks in small confined spaces, including hospital wards, classrooms and offices.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.