Covid is airborne — but preventing airborne spread is perfectly doable. Here’s how to do it

As Covid case numbers continue to reach record highs, Dr Adam Squires and Prof Christina Pagel detail what needs to happen next

Friday 14 January 2022 14:40 GMT
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Another year, another variant. This time, while vaccines mean far fewer deaths, the sheer rate of spread is causing overwhelmed and understaffed hospitals and is disrupting businesses, travel and schools. There is no longer any scientific doubt about how Covid spreads through the air, and we know what we can do in the short-term, medium term and long-term to stop it.

Covid spreads like any other airborne respiratory disease. The virus is carried in tiny particles called aerosols, which we breathe out constantly — especially when speaking loudly or singing. The particles stay in the room air like smoke, and if someone has Covid, their exhaled aerosols contain the virus and can infect someone who breathes them in. We stop the disease spreading by stopping people inhaling infected aerosol.

What can we do to protect ourselves from the airborne spread of Covid?

The first thing we can do is to wear masks. However, there are masks and there are masks. And, with Omicron, it is time for an upgrade. Disposable blue surgical masks are better than cloth masks, which are better than nothing. But both have gaps at the sides and mainly catch aerosols in the jet of breath on the way out. So, while they reduce spread to others if the wearer is infected, they don’t offer much protection to the wearer themselves.

Other European countries have set FFP2 as the minimum standard. We should do the same

However, “filtering facepiece respirator” masks are far better at catching particles and stopping transmission, both to and from the wearer. These folding, slightly thicker masks are called FFP2, KN95 or N95 depending on regulatory standards. They are made of a superior filter material and have a closer gap-free fit. They can be reused for weeks and are comfortable to wear and breathe through. Other European countries have set FFP2 as the minimum standard for shops and public transport, and provide them for free to maximise uptake and suppress Covid spread. We should do the same.

Distancing also helps. We used to think that the virus was contained only in drops of spittle that fly through the air and drop to the floor, and so keeping two metres away meant Covid couldn’t get you. The much smaller aerosol particles that float in the air mean that, unfortunately, this is not the case.

But keeping your distance is still helpful. This is because it gets you beyond the reach of the respiratory jets coming out of someone’s unmasked mouth (the visible puffs in cold weather). Within this “near zone”, fresh air helps less, just as open windows don’t help much if a nearby smoker breathes smoke into your face. Whereas, further away, fresh air helps more.

Distancing rules also limit occupancy, which reduces both the chance there’s someone with Covid in the room and the number of people they can infect. So, distancing helps, but (like so many things) is “not a magic bullet”.

What needs to happen next?

We need more. Distancing is not always practical. Masks cannot be worn when eating and drinking, and are not used in UK primary schools. Many people don’t wear masks in the workplace or while entertaining at home, and most of us would be happier if we didn’t have to. Masks require people to change their behaviour; safer environments do not. Much can be done by cleaning the air in the room, removing infectious aerosol before it can be inhaled.  We can do this with ventilation (replacing stale air with fresh air), filtration (removing the aerosols directly) or sterilisation (killing the virus in the aerosols).

Many spaces already have good ventilation, but some don’t, and the problem arises when we can’t tell which. Many modern buildings have “mechanical” ventilation which delivers a reliable amount of fresh air, but it is difficult for customers or employees to know if good mechanical ventilation is in place — is the box on the wall really blowing fresh air in, or just blowing the same stale air around, allowing infectious aerosol to build up and spread?

Meanwhile “natural” ventilation through open windows and doors is the main source of ventilation for older buildings, including many schools. Open windows are visible, but the amount of ventilation they deliver varies with the weather. It’s likely to have more ventilation if it’s windy or colder outside, and a quick blast of fresh air for a few minutes every half hour helps keep infectious aerosol from building up. But it’s often hard to judge whether the ventilation is enough, and how much to open windows while getting the right balance for comfort. We need a direct way of showing how much breath has built up in the room.

For this, we use carbon dioxide (CO2). As we breathe it out, CO2 levels build up in the room and fresh air can dilute it down closer to the same level you would find outdoors. With the right sensor, this is an easy thing to measure. We need more of theses sensors, ideally one in every naturally ventilated classroom. This would help the teacher to manage ventilation and temperature by opening and closing windows. We need more sensors offered to hospitality venues, so they can publicise the level of ventilation to help customers make informed choice — which in turn would provide an incentive to improve. And we should aspire to more ambitious targets than our current limits.

School guidelines currently set a limit of 1500 parts per million (ppm) CO2, which represents nearly twice as much exhaled air as the 1000 ppm limit in other countries. In comparison, outdoor values sit around 400 ppm CO2. Over a third of our classrooms would currently fail this higher standard, from the sensors already distributed. And the equivalent data for hospitality venues is unknown.

In the 21st century world of new pandemics, it’s time we tackled indoor air

In the meantime, where the ventilation is inadequate, we can supplement the clean air by filtering out the respiratory aerosol particles using small portable HEPA (“High Efficiency Particle Air”) filter units that just plug into the mains. HEPA filtration is long established and highly effective at filtering out aerosols, and was shown to remove nearly all infectious aerosol particles from the air in a hospital Covid ward.

Some HEPA units have been made available for schools, but the government-approved models are expensive. However, there are many available commercial home “air purifier” units that can provide filtered air equally well for a fraction of the price. Schools and businesses just need the guidance to help choose.

What are some long-term solutions?

If masks, distancing and windows are immediate options for safer air, and CO2 displays and HEPA units medium term, what of the long-term? New developments in ultraviolet (UV) technology can safely and efficiently kill airborne pathogens in large spaces such as canteens, gyms or theatres.

Infrastructure upgrades and new builds, necessary for zero carbon targets, can combine more energy-efficient ventilation with filtration to lower pollution. In the classroom and the workplace, clean fresh air has wider benefits on health and wellbeing far beyond our current airborne pandemic.

Clean air comes at a financial cost. But this should be weighed against the benefits of reducing ill health in the population and disruption from epidemic surges. In the 19th century, this reasoning gave us clean water free of water-borne diseases. Then we cleaned outdoor air in cities, tackling smog, industrial pollution, leaded petrol and now diesel. In the 21st century world of new pandemics, it’s time we tackled indoor air.

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