The Coronavirus is an airborne virus.
In poorly ventilated indoor spaces we all share air, and this unfortunately is also how we share many common viruses and germs including the Coronavirus.
The virus is spread by hitching a lift on tiny particles called aerosols which are coughed, sneezed or simply breathed out by someone who is infected. These aerosols are so small that they can float in the air for hours, creating an infection risk for anyone who breathes them in.
So how can air purifiers help?
Outdoors, airborne transmission of viruses and germs is much less of a problem than indoors. This is because wind quickly disperses the aerosols they use to hitch a lift, while strong sunlight degrades the viruses and germs in them.
Indoors is a different story. In poorly ventilated indoor spaces, aerosols can accumulate and remain infectious for hours.
It is in these poorly ventilated indoor spaces that air purifiers can really help by sucking or blowing the contaminated air through a filter which traps almost all of the aerosols.
In this way it is possible to clean aerosols out of the air, reducing the risk that they spread infection.
Ultimately our aim is to make indoor air as covid-safe as outdoor air, using this and other technologies.
There are 2 things scientists consider when rating an air purifier.
The first is how efficiently its filter traps the aerosols, and the second is how often the air is passed through its filter.
Imagine a room with 1,000 aerosol particles floating in the air. Once all of the air in the room has passed through the filter once, then 95% of the aerosol particles have been removed. This leaves only 50 aerosol particles. The second time the air is passed through the filter, another 95% of particles are removed, leaving only 2 or 3 in the air. You can see from this that it is very important both to filter the air efficiently on each pass through the filter, and also to ensure that all the air in the room passes through the filter as many times as possible.
Together, the efficiency of the filter and the number of times air is passed through it in an hour make up the Clean Air Delivery Rate (or CADR) of the air purifier, usually expressed in m3/h.
Studies on ventilation have shown that completely replenishing indoor air 6 times per hour reduces Covid transmission by 82%.
As a guideline, if you are aiming to replenish the air in a room 6 times per hour and have no external ventilation, then calculate the volume of your room by multiplying its length by width by height (L x W x H) in metres. Then multiply this figure by 6. This is the minimal CADR in metres squared (m3) you would need to achieve 6 air changes per hour, or ACH. If you do have some ventilation and know how many air changes this gives you, then this can be added to your number of air changes. As a general rule it is better to have an air purifier that is too big than too small, and as many air changes per hour as you can cost effectively achieve!
Take a look at our calculator to help you decide (Link to Stefan’s calculator)
Evidence that air purification can work versus Covid is being gathered all the time, but the best evidence is simple physics. Air purifiers can remove almost all the aerosols from the air which the virus uses to spread. If it is filtered out of the air, then it cannot spread.
It is likely that the effect of air purification is greatest on far field transmission. This is transmission that happens over longer distances. We’ve all heard about cases of Coronavirus infecting people in offices, on cruise ships or at restaurants who had no close contact with each other. This is far field transmission.
See our Resources for studies showing how filtration can reduce the amount of Coronavirus (and other viruses and germs) in the air, and how ventilation, a close proxy for filtration, can reduce transmission:
Air purification is a risk reduction measure. Although we believe it can help a lot, it is one layer in the several layers of protection we should use. Layers of risk reduction are the best way to protect yourself, your family, your staff or your business.
As well as using air purifiers please also get vaccinated, ensure spaces you operate are as well ventilated as possible and adopt and encourage good mask wearing and hygiene practices.
Covid is not going to be easy to live with, but if we are sensible we can reduce its impact on our lives by reducing its transmission. Air purification can be a major tool in achieving this.