The Conversation, May 13, 2021
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What does the evidence say?
SARS-CoV-2 is a respiratory virus that multiplies in the respiratory tract. So it is spread by the respiratory route — via breathing, speaking, singing, coughing or sneezing.
Two other coronaviruses — the ones that cause MERS (Middle Eastern respiratory sydrome) and SARS (severe acute respiratory syndrome) — are also spread this way. Both are accepted as being airborne.
In fact, experimental studies show SARS-CoV-2 is as airborne as these other coronaviruses, if not more so, and can be found in the air 16 hours after being aerosolised.
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So why has airborne denialism persisted for so long?
The role of airborne transmission has been denied for so long partly because expert groups that advise government have not included engineers, aerosol scientists, occupational hygienists and multidisciplinary environmental health experts.
Partly it is because the role of airborne transmission for other respiratory viruses has been denied for decades, accompanied by a long history of denial of adequate respiratory protection for health workers. For example, during the SARS outbreak in Canada in 2003, denial of protection against airborne spread for health workers in Toronto resulted in a fatal outbreak.
What’s the difference between aerosols and droplets?
The distinction between aerosols and droplets is largely artificial and driven by infection control dogma, not science.
This dogma says large droplets (defined by WHO as larger than 5 micrometres across) settle to the ground and are emitted within 2 metres of an infected person. Meanwhile, fine particles under 5 micrometres across can become airborne and exist further away.
There is in fact no scientific basis for this belief. Most studies that looked at how far large droplets travelled found the horizontal distance is greater than 2 metres. And the size threshold that dictates whether droplets fall or float is actually 100 micrometres, not 5 micrometres. In other words, larger droplets travel further than what we’ve been led to believe.
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Leading aerosol scientists explain the historical basis of these false beliefs, which go back nearly a century.
And in further evidence the droplet theory is false, we showed that even for infections believed to be spread by droplets, a N95 respirator protects better than a surgical mask. In fact airborne precautions are needed for most respiratory infections.
Why does this difference matter?
Accepting how SARS-CoV-2 spreads means we can better prevent transmission and protect people, using the right types of masks and better ventilation.
Breathing and speaking generate aerosols. So an infected person in a closed indoor space without good ventilation will generate an accumulation of aerosols over time, just like cigarette smoke accumulates.
Masks work, both by preventing sick people from emitting infected aerosols, and by preventing well people from getting infected. A study in Hong Kong found most transmission occurred when masks weren’t worn inside, such as at home and in restaurants.
Coughing generates more aerosols
The old dogma of droplet infection includes a belief that only “aerosol generating procedures” — such as inserting a tube into someone’s throat and windpipe to help them breathe — pose a risk of airborne transmission. But research shows a coughing patient generates more aerosols than one of these procedures.
Yet we do not provide health workers treating coughing COVID-19 patients with N95 respirators under current guidelines.
At the Royal Melbourne Hospital, where many health worker infections occurred in 2020, understanding airflow in the COVID ward helped explain how health workers got infected.
Think about it. Airborne deniers tell us infection occurs after a ballistic strike by a single large droplet hitting the eye, nose or mouth. The statistical probability of this is much lower than simply breathing in accumulated, contaminated air.
The ballistic strike theory has driven an industry in plastic barriers and face shields, which offer no protection against airborne spread. In Switzerland, only hospitality workers using just a face shield got infected and those wearing masks were protected.
In hotel quarantine, denial of airborne transmission stops us from fixing repeated breaches, which are likely due to airborne transmission.
We need to select quarantine venues based on adequacy of ventilation, test ventilation and mitigate areas of poor ventilation. Opening a window, drawing in fresh air or using air purifiers dramatically reduce virus in the air.
We need to provide N95 respirators to health, aged-care and quarantine workers who are at risk of high-dose exposure, and not place them in poorly ventilated areas.
It’s time to accept the evidence and tighten protection accordingly, to keep Australia safe from SARS-CoV-2 and more dangerous variants of concern, some of which are vaccine resistant.