Can air conditioners spread coronavirus?
We need to understand how SARS-CoV-2 is transmitted18th Sep 2020
While the country slowly starts to open up after an extensive lockdown period, the spread and effect of the COVID-19 pandemic are far from over. As scientists begin to understand the transmission of SARS-CoV-2, the virus that causes COVID-19, new methods of potential spread are emerging.
With news reports suggesting that the virus may even be airborne, we decided to ask the question as to whether or not the current air conditioning systems in public spaces can be a potential source of infection, and what can be done to limit potential spread.
SARS-CoV-2 survival times
The persistence of viral particles plays an important role in its infection cycle. Viruses that become inactive outside the host quickly tend to have lower infection rates than viruses with higher persistence.
Unfortunately, SARS-CoV-2 has shown persistence of up to several days on various surfaces. The virus can remain infectious for up to four days on plastic at room temperature. The virus has similar persistence rates on metal surfaces, though the presence of copper reduced the persistence rate dramatically.
Similarly, SARS-CoV-2 is just as resilient as an aerosol, which reinforces suspicions that the virus may be an airborne pathogen as well as being transmitted through contaminated surfaces. Airborne transmission has huge implications for how we deal with the virus, and explains why bodies like the WHO and CDC changed their recommendations on mask use.
SARS-CoV-2 surface vs. airborne transmission
During the early days of the pandemic, scientists had to rely on their knowledge of other coronaviruses to make assumptions about the transmission and spread of SARS-CoV-2. Most coronaviruses spread in large mucous droplets that are too heavy to remain in suspension and instead settle on surfaces and objects. Anyone who handles these objects and then touches their face is at risk of developing COVID-19.
However, new evidence shows that SARS-CoV-2 is remarkably stable in aerosolised form as well. Aerosols are significantly smaller droplets that can float around in the air for extended periods of time, promoting the risk of airborne infections.
Recent studies have shown that SARS-CoV-2 particles are released during talking and even exhalation, often settling 1.5 m above the floor where they remain viable for hours.
The role of air conditioning
Air conditioners and HVAC systems often contain a mix of indoor and outdoor air at any given point. During times of high load, such as during the hot summer months, AC systems tend to recirculate indoor air that may contain SARS-CoV-2 particles to save energy and to put less strain on the system.
The second way that AC systems may accelerate the spread of COVID-19 is that any movement of air can push aerosolised particles further out, where they can infect more people. A dramatic example of this happened at a restaurant in Guangzhou, China, where one infected person infected five other people sitting at tables two metres away. Video footage showed that there was no physical contact between the parties, leaving airborne transmission as the likely culprit.
Potential steps to take
If you’re concerned about virus build-up indoors, or that your AC system may increase the spread of the virus in your home or commercial building, there are several steps that you can take.
The simplest step is to let outdoor air in, either by opening up windows or by setting your AC system to take in more outdoor air than usual. Unfortunately, due to the costs of running an AC system, stopping air recirculation is too expensive to be economically feasible, so you may have to turn to other supplementary options as well.
UV germicidal irradiation is a sterilisation method that uses UV to kill off all pathogens in the air in an HVAC system. To date, this method is most commonly used in hospital isolation rooms, where the presence of any pathogen can have devastating results.
HEPA filters are also very effective at reducing the number of airborne virus particles. In one study, a portable HEPA filter unit was enough to reduce the number of aerosols in an insolation room by 98%.
However, it’s important to do your research when looking for air cleaning units. Many have limitations on how much air they can process and how much virus they can remove. Also, be careful about ionising cleaners, as their ability to inactivate SARS-CoV-2 particles has yet to be fully tested and confirmed.
The evidence for the airborne spread of SARS-CoV-2 is rising, which means that businesses and public spaces need to re-evaluate their protocols for keeping people safe. Air conditioning systems may potentially increase the spread of SARS-CoV-2, either by pushing airborne particles further, or by concentrating the number of particles due to air recirculation.
Luckily, many protocols, such as mask use, can help reduce the airborne transmission of the virus. There are also several other options, like UV irradiation and HEPA filter units, that can help reduce viral load in busy indoor spaces.