Covid-19 aerosol and the cigarette smoke analogy

by Professor Stephen Goldson and Dr Joel Rindelaub
Crowd of people walking street wearing masks during covid19 coronavirus pandemic

Professor Stephen Goldson, Koi Tū Associate member

Dr Joel Rindelaub, School of Chemical Sciences, University of Auckland

In the early days of the Covid-19 pandemic masks were not thought to be effective in preventing transmission. Initially, health officials were more worried about large droplets ‘splashing onto people’s faces’ due to close contact rather than referring to any smaller particles.  Indeed, this brief fixation on large droplets had an historical context, but it was not based on data from the latest aerosol measurement techniques. As it turns out, smaller aerosol particles are actually all-important. Reports have indicated that in an infected patient’s breath when talking, breathing or coughing, about 85% of the Covid-19 RNA detected is in minute particles of less than 200ths of a millimetre. Further, these have been found to contain more SARS-CoV-2 virus than their splashier counterparts. The creation of such infectious particles is thought to relate to air interacting with the virus-infected thin liquid coat in the respiratory tract and then exiting straight out on the breath, while in contrast, although the larger droplets do contain the virus, this is diluted by the saliva.

As well as the recognised aerosol component, it has unfortunately been found that the infectious Alpha variant of the Covid-19 virus causes patients to suffer from copious amounts of Covid-19 virus in their nose and throat. This results in 43 times more viral RNA appearing in the tiny aerosols than people infected with the older variants. There is little doubt that the Delta variant impacts are similar or worse, with early studies estimating Delta viral loads to be hundreds to a thousand times greater than the original variant.

This unfortunate trend has been highlighted in the recent New York Times column Is the coronavirus getting better at airborne transmission? It makes a point that the strains that produce higher volumes of infectious aerosols are simply outcompeting the older variants. This could well intensify should advantageous viral mutations continue to accumulate.

The downside of this is that the large droplets sink to the floor within metres of their source, whereas the smaller aerosols can float in the air for some time while dispersing widely the via slightest of air currents. An unfortunate and tawdry metaphor for this could be comparing the virus to how cigarette smoke behaves. Obviously, in an enclosed area, this completely defies any two-metre rule. For those who can recall, a room or a public bar used to have a blue haze of smoke that quite often remained for many hours. Conversely, there was no such trace outside in the beer garden, except maybe for a few metres downwind. In short, this analogy does help understand the aerosol behaviour of the Covid-19 cloud and assist in its avoidance. Finally, and somewhat reassuringly, even loose-fitting cloth masks have been found to reduce virus-laden fine aerosols by 48% or more if properly worn. Apparently, the virus strain does not make any difference. But tight-fitting masks designated as N95 may reduce exposure to Covid-19 virus by 96%, if everyone is wearing them properly as indicated in a recent Koi Tū report on improving mask use.

However, this good news may not persist in a room saturated with Covid-19 aerosol. Hence, the particular concerns about indoor spread. Ventilation is now an all-important consideration. To better protect the community, we need to rethink our airflow and filtration strategies to ensure that clean air is a constant in indoor environments. We need to ensure that exhaled breath – and the virus-laden particles within it – does not accumulate to unhealthy levels reminiscent of our smoky past.

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