We've been asked by several CHOICE members whether an air purifier can help keep their home clear of germs, including the COVID-19 coronavirus.

The short answer is yes. Viruses like COVID-19 are usually transmitted by small aerosol particles when an infected person breathes out, coughs or sneezes, and these tiny droplets can stay in the air for up to an hour or more. In the right circumstances, a good air purifier can remove most of these particles from a room's air.

Nevertheless, the air purifier can only filter the air in the room that it's located in, so air in other parts of the house can remain untreated. And any droplets that settle onto surfaces won't end up going through the air purifier.

So it's still very important that you keep up the usual hygiene practices: washing your hands, cleaning hard surfaces, and of course, trying to avoid bringing germs into your home in the first place.

Let's look at how effective an air purifier filter can be when it comes to trapping a virus.

HEPA filters and COVID-19

HEPA (high efficiency particulate air) filters claim to filter particles down to 0.3 microns (0.3 micrometres, or 300 nanometres). 

Most viruses vary in diameter from a mere 20 nanometres (that's 0.02 microns, or 0.00002mm) up to 400nm (0.4 microns) – some are bigger still. A typical coronavirus is about 100 nanometres, or 0.1 microns, in diameter. That puts the coronavirus, and most other viruses, well below the particle size that a HEPA filter can claim to trap.

However, studies show that such tiny particles can still be trapped by a HEPA filter, and in fact the filter has a good chance of trapping most such minuscule particles that it encounters. Because particles of this size tend to move in random directions (rather than in a straight line), there's a good chance they'll still make contact with the filter fibres as they pass through it, and become attached to the filter surface just like a larger particle such as dust.

Small aerosol particles from a cough or sneeze can remain airborne for hours. An air purifier with a HEPA filter can help remove these

So it's likely that an air purifier with a HEPA filter will trap a lot of airborne viruses, including the COVID-19 coronavirus, that happen to pass through it. But it won't necessarily catch them all, or kill them. Even if trapped, the virus may stay alive on the filter surface for several hours or even days. However, the virus will probably eventually die there, unless you happen to remove the filter in the meantime, which might release the virus back into the air or onto your skin.

There is evidence that the small aerosol particles from an outward breath, cough or sneeze can remain airborne for hours, and these particles can carry viruses. While an air purifier with a HEPA filter can help remove these small particles from the air, viruses are also likely to be found on a person's skin, or on a hard surface that an infected person has touched. 

Larger droplets from a cough or sneeze are likely to settle to the floor or other surfaces rather than remain airborne for a long time. There's nothing an air purifier can do about particles that have settled out of the air.

So there are several reasons why an air purifier is not a perfect solution to keeping a home virus-free, but can certainly be a big help towards that goal.

Other filters: photocatalytic, carbon and more

• Photocatalytic air purifiers can also trap and kill bacteria and viruses, as they're caught and potentially broken down by the electrostatic and oxidisation effects of the ions generated by this type of filter.
• Ultraviolet (UV) light is also a good steriliser, and some air purifiers have a UV sterilising feature. But usually a virus or bacteria has to be exposed to UV light for several minutes to be destroyed, and it's not clear that's the case inside many air purifiers. If the UV is used to sterilise a filter that has trapped the germs, that could be very effective, but if the air is simply passed through a UV light it may not be exposed for long enough to be of much use.
• Catechin-based filters may also have antiviral properties, but we haven't seen many models with this type of filter. Catechin is a plant extract with antifungal and antibacterial properties.
• Carbon filters and other types of filters can be good for trapping odours, volatile organic compounds and other pollutants, but are unlikely to trap germs.

Again, though, any of these filters or sterilisers can only trap a virus that has actually been airborne and passed through the air purifier – it can't do anything about viruses on a person or on a hard surface.

Don't forget the basics: fresh air and good hygiene

In summary, a good air purifier can trap and even kill viruses in the air, and can be a useful supplement for helping keep your home's air virus-free and clearing it of dust, smoke and other pollutants.

An alternative to an air purifier is to simply keep the room naturally ventilated, with open windows and doors. Keeping the air circulating and regularly refreshed is the aim here. In circumstances where you can't do that, such as when the room doesn't have good natural ventilation, or the air outside is unsuitable (too hot or cold, too smoky or too polluted), then closing the house up and turning on the air purifier is a good option.

But viruses are also found on skin, in bodily fluids or on hard surfaces after human contact. So you still can't neglect the usual good hygiene practices for avoiding viruses and bacteria: avoid crowded spaces, minimise unnecessary contact, clean any hard surfaces that people often touch (door handles, light switches, taps, toilets and so on), and regularly wash or sanitise your hands.

Stock images: Getty, unless otherwise stated.

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