An infographic in the Chem vs COVID timeline series. On 6 July 2020, scientists called for greater recognition of the airborne transmission of SARS-CoV-2. People infected with COVID-19 expel droplets when they talk, cough, sneeze, or even breathe. Large droplets travel short distances and can contaminate surfaces, though research has suggested this is a less significant mode of transmission than initially thought. Very small droplets dry and form aerosols (particles suspended in air) which can travel greater distances and spread the virus through the air. Chemists have produced antiviral coatings, containing metals or polymers, to reduce surface transmission. Ventilation and air purification technologies such as air filters, UVC light and photocatalytic devices can destroy the virus in the air in buildings. Overall, understanding of transmission and preventative technologies have helped limit infections, make activities safer, and improved technology to combat viruses in the future.
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How does COVID spread? Of all the questions about the pandemic, this seems like it would be a fairly simple one to answer. But, even several months into the pandemic, the guidance from public health organisations did not recognise the potential for COVID to be transmitted through the air. The latest edition of the Chem vs. COVID series with the Royal Society of Chemistry highlights the point at which scientists called for this mode of transmission to be more widely recognised.

All the modes of transmission start with someone who’s infected with COVID-19. When an infected person coughs, sneezes, talks, or even breathes, they can propel droplets containing the virus into the air.

Some of the droplets the infected person ejects are quite large, approximately 100 micrometres in diameter. To give some perspective, this is a tiny bit bigger than the average diameter of a human hair, which is around 75 micrometres. Because these droplets are large, they fall to ground quite quickly, and can contaminate surfaces with the virus.

Since the beginning of the pandemic, there’s been a lot of emphasis on surface contamination as a means of transmission for the virus. Lots of the guidance issued by public health organisations has focused on the importance of hand-washing and regular cleaning of surfaces to prevent infections. The trouble is that, while our understanding of the importance of surface transmission has developed, the guidance has remained largely unchanged.

It’s certainly the case that the virus can contaminate surfaces, but studies after the onset of the pandemic increasingly showed that the risk of surface transmission was likely being exaggerated. Attempts to grow the virus from contaminated surfaces have been largely unsuccessful, suggesting that surfaces are unlikely to be a major route of transmission.

While the role of surfaces has been overestimated, other public health guidance around distancing and the wearing of masks address another mode of transmission: droplets. The droplets propelled into the air by an infected person can infect others if they breathe them in.

Most droplets between 60 to 100 micrometres in diameter travel no further than 2 metres from the infected person – which is why this is the figure used for social distancing measures. If you’re infected, wearing a mask can prevent you from spraying these droplets everywhere, reducing infection risk for others.

But it’s the smallest droplets expelled by those infected with COVID that had scientists concerned this time last year. We’re talking droplets around 5 micrometres in diameter here, smaller than the water droplets in fog or mist. These droplets can travel much greater distances – 6 metres or more – and often dry and form aerosols (solid particles suspended in air) before they hit the ground. These aerosols can carry the virus even greater distances, and it’s them we’re referring to when we talk about airborne transmission.

At the start of the pandemic, public health organisations were resistant to the idea that COVID could be airborne. Guidance had always been that particles bigger than 5 micrometres couldn’t lead to airborne transmission of infectious viruses or bacteria.

Meanwhile, evidence for airborne transmission of COVID continued to accumulate. Analysis of an outbreak in a restaurant in Guangzhou in China using a tracer gas as a model for exhaled aerosols showed the infection pattern was consistent with airborne transmission as opposed to the other modes. Another study of two hospital in Wuhan, China, found viral RNA in aerosols in parts of the buildings, though did not evaluate whether or not this was infectious.

On 6 July 2020, over 200 scientists signed an open letter to national and international health bodies, calling on them to recognise the potential for the airborne transmission of COVID-19 and to reevaluate public health advice accordingly. While conceding that the evidence for airborne pathways remains incomplete, they suggested that further emphasis on ventilation as a precautionary measure against the disease should be communicated.

It would be fair to say that the initial response to the letter was muted. The World Health Organisation did respond (although apparently the timing of their statement was coincidental) by stating that aerosol transmission couldn’t be ruled out in spaces that were crowded and poorly ventilated. It wasn’t until 30 April 2021 that the WHO updated its Q&A page to state “The virus can also spread in poorly ventilated and/or crowded indoor settings, where people tend to spend longer periods of time. This is because aerosols remain suspended in the air or travel farther than 1 metre (long-range).”

Even now, public health advice seems to largely neglect the importance of ventilation, with lots of emphasis still being placed on the prevention of surface transmission which we now know to be less significant. And ventilation isn’t the only way in which airborne transmission can be combatted; air purification technologies can also help, as I previously explored in this graphic for Chemical & Engineering News.

Chemists have been working to prevent surface transmission, too, using antiviral materials and coatings. Metals such as copper and silver can damage virus structures or genetic material, destroying the virus. A large variety of polymer coatings or composites can also be used. These work in a variety of ways, from charged groups which deactivate the virus, through to modifying surface properties like pH or how well the virus can stick to them.

UK universities have received government funding to continue to investigate and develop antiviral coatings for surfaces. While surface transmission may be less important than originally thought, such coatings could also be applied to PPE surfaces to help us save on waste from what are currently single-use items.

Most importantly, understanding of the transmission of COVID-19, and development of ways to combat it, will undoubtedly help us all be more prepared the next time a pandemic comes along.

This graphic was developed in partnership with the Royal Society of Chemistry. See the full #ChemVsCOVID series of graphics here.

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References/further reading

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