Covid-19: Do Air Purifiers Really work?

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John Wilton-Davies

[Aerosol: A suspension of fine solid particles or liquid droplets in air]

Six months into the UK’s experience of the pandemic, it’s finally being grasped that yes, you can catch covid from the air.

Original, and still, to a degree, ongoing advice from the World Health Organisation and most governments and national health services, was that you could mainly only catch covid from either direct transmission (largely avoided by social distancing) or indirect transfer from infected surfaces (addressed by enhanced cleaning). Transmission by aerosols (tiny droplets exhaled by an infected person so small they remain suspended in the air) was considered “not a major source of infection” and no advice was issued on how to reduce this risk.

Jumping ahead to a few weeks ago and the viewpoint is changing.  Aerosol and infectious disease specialists have long argued that this stance made no rational sense.  The official position implied that there was some discrete difference between ‘large’ infectious droplets, that might travel up to 6 feet before falling to the ground, and ‘small’ droplets, which either didn’t exist, or wouldn’t be infectious.  Yet, as the experts were pointing out, you don’t get two sizes of droplets in the real world, you get an infinite variety of sizes, and there was no reason to expect smaller particles would not be equally capable of carrying infection.

With lockdowns (hopefully) being unwound, official advice remains disturbingly slow to catch up with the evidence.  Aerosols remains the only recognised infection route that has had virtually no official guidance attached, so is perhaps the greatest danger to a successful return to business as normal.  Advice to employers re-opening businesses extends as far as telling them to ensure ‘plentiful’ ventilation.

But what exactly is ventilation.  And how much is plentiful?  The layman thinks of ventilation as opening windows, or turning on air conditioning.  And how much is the right amount?  To the occupant of a room it’s usually what makes them feel comfortable.  Which of course isn’t anything like the same as safe.

All employers, before bringing staff or customers back into their premises, are required to carry out appropriate risk assessments, and to take all reasonable steps to address any issues identified.  Now, most responsible employers are, of course, carrying out these risk assessments.  But are they even considering the risks of airborne infection?  No, partly because no one’s specifically told them to, and partly because they haven’t thought of it.

There’s a suspicion that official guidance on this area might be lacking on purpose.  It’s now in the government’s interests to get the economy back to work, and raising the suggestion that the workplace might not be as safe as you thought isn’t going to help restore confidence.  Yet virtually every outbreak of covid around the world begins indoors, and many of these are despite careful social distancing and cleaning.  So there is a real risk.  And whilst it can’t be entirely eliminated, it can be substantially reduced once you understand the factors behind the risk.

Perhaps surprisingly, the most important factor behind the risk of indoor infection is not the number of people in the room (think every other desk unoccupied) but what those people are doing.  The amount of virus emitted by an infected person depends considerably on what they are doing, and two things push the numbers up enormously - physical activity and the volume of speech or singing.  Someone running around shouting (think, perhaps, an exercise class) can emit hundreds of times as much virus as the same person sitting quietly at a desk.  Generally, most businesses can’t make significant changes to people’s activity levels - if you’re a gym, you’re a gym), but they often can make big reductions in people’s noise levels.  For example, turning down background music in a venue can mean people need to raise their voices less.

The next most important factor - definitely outside the control of individual businesses - is the background community infection rate.  The higher the proportion of the public that are currently infected, the greater the risks, obviously.  Everyone just has to live with this until the rate reaches the point the government imposes new restrictions.

Thirdly, and where we at Plain Air come in, is in the area of ventilation.  If there’s virus floating around, the more you change that air for cleaner air, the more you reduce the risk.  There’s two ways to do this - bring in air from outside, or clean the air already inside - and one way to definitely not do it - move air between different rooms in the building.  More on all of this below.

And the fourth main factor, which businesses and workplaces can definitely influence, but usually at the cost of efficiency, income and profit, is how much volume of room is occupied by each person, and how long it is occupied for.  The fewer people in a particular space, the lower the risk but, as any restaurant owner will tell you, that’s not very comforting from a business perspective.

There are many other factors influencing the risk of airborne infection, from humidity and temperature to immunity rates and the efficiency of any PPE used but, for the purposes of this article, we want to explore the practical steps any indoor business can take to reduce the risks without any negative impact on their profits or customer satisfaction.

So back to ventilation...To dilute any virus in the air you either need to change the air or remove the virus.

Many modern commercial premises have mechanical ventilation systems that push air around a building, normally mixing a little outside air on each circuit to remove carbon dioxide and excess humidity.  So most of the air blowing out of the vent into your office or cafe or studio or factory is likely to have passed through the room before, and the room before that.  And if anyone in one of those rooms has covid…

Contrary to many people’s instincts, it’s usually a very bad idea to use intra-building ventilation.  Of course if this same ventilation system includes the air-conditioning, then you may have an unpleasant choice to make - unless, of course, you’re the Health & Safety officer who actually understands the risks and then becomes even more unpopular.

Which leaves most businesses, and those without ventilation systems, to rely on opening the doors and windows.  An easy solution, surely?

Well yes, perhaps for the rest of the summer.  And if you don’t work near a road. And it’s not too wet or windy.  And if your windows even open, or open enough.

But even if everything fits, what’s going to happen in Autumn.  Are you really going to have all the windows open and the heating on full blast every day.  And no one will sit near a window - or more likely will close the windows once the Health & Safety manager goes to lunch.  In winter the heating won’t even be able to keep up.

The amount of clean air you need to reasonably reduce the risks is more than you’d expect, and more than many buildings can provide from window opening in any event.  Which is where we come to air purification.

A good air purifier (and no, the majority of purifiers you’ll find online are definitely not up to the job) will produce the same virus-reducing effect as open windows but without any of the downsides.  The clean air coming out has come from inside the room and so doesn’t need reheating.  They're much quieter than having the windows open and cost less to both buy and operate than even a few months of increased heating bills.  You’ll still want some windows open some of the time - to remove carbon dioxide and humidity - but the internal air will be much less polluted by virus and other irritants - such as road pollution, pollen etc.

So what would I need to look for in a purifier? Well it’s not just a question of plugging in a purifier and watching all your problems disappear.  Each room or building needs consideration as to the best balance between the factors listed above.  Plain Air can give you or your employer a bespoke assessment - often carried out remotely - and you can ask us more by contacting us HERE.

The scientists tell us that individual coronavirus particles are around 0.12 microns across (that’s 0.0000012 metres, or 0.0012 mm) so you need a filter on a purifier that will capture something that small.  In fact you want one that will catch pretty much everything that small.  Lucky for you, they do exist.  Purifiers described as having HEPA filters are defined by their ability to capture 99.97% of particles 0.3 microns across.  Now wait, you say, but 0.3 microns is bigger than a coronavirus.  Indeed, but that’s the minimum specification of these filters, and many of them will catch virtually everything down to 0.1 microns or less.

And of course not all purifiers are the same.  In a quiet office environment you don’t want the sound of a jet engine next to your ear, or to discover that the cost of replacement filters are so high you end up throwing the pesky thing into a cupboard.  But the right purifier can be quiet, cheap to operate, make staff and customers safer, look good and actually save you money.

But do your homework first.  Or, of course, talk to us.

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