Being typical of the 70+ “vulnerable” generation, my main interest in the Covid-19 pandemic lies simply in how to avoid catching it. So, following the example of our political leaders, I looked into the “science” of the transmission of the virus, only to be underwhelmed and sadly disappointed with the information I have found so far. It all seemed to be based on the transmission of influenza and involves three main mechanisms:
a) Direct transmission: by person to person, intimate contact, kissing, hugging, just shaking hands or even bumping ‘well-coughed-on’ elbows.
b) Indirect transmission: picking up the virus on “fomites” from surfaces contaminated by droplets from the coughs and sneezes of those already infected.
c) Airborne transmission: by inhaling the droplet excretions from the coughs and sneezes, from those infected, or direct contact with eyes and teeth.

The advice on how to avoid the disease seems to be based on these mechanisms:
• Direct transmission: stay in your bubble and don’t touch anyone else.
• Indirect transmission: now wash your hands and disinfect everything afterwards.
• Airborne transmission: “social distancing”, according to the World Health Organisation (WHO) staying 6 feet apart.
In the eastern world, the wearing of face coverings, or masks, has been seen as essential from almost the start of the pandemic. The explanation being not to protect the wearer but to protect others, just in case you might be in the infectious pre-symptom or pre-diagnosis stage of the virus.

World Health Organization’s advice
It is not surprising that the paucity of this advice has provoked questions about the relative importance of the three different mechanisms in the various circumstances of every-day life. Many scientists and engineers have been concerned about the lack of information on airborne transmission. At the World Health Organization’s 7th July Media Briefing, a reporter asked about the New York Times report of a letter from 239 scientists, many of them engineers, who were concerned about airborne transmission of the virus. WHO admitted that this question was not new, having been raised on the 1st April. Not surprisingly, as Covid-19 is a respiratory pathogen, WHO acknowledged that there was emerging evidence that the modes of transmission could include airborne aerosols or droplets. They had been producing a scientific brief that they hoped would be published soon. On the 9th July WHO updated their 29th March ‘Newsroom report’; “Modes of Transmission”. This updated report didn’t help me at all:

“Respiratory infections can be transmitted through droplets of different sizes: when droplet particles are >5-10 microns in diameter, they are referred to as respiratory droplets, and when then(sic) are<5microns in diameter, they are referred to as droplet nuclei1. According to current evidence, COVID-19 virus is primarily transmitted between people through respiratory droplets and contact routes2-7. In an analysis of 75,465 COVID-19 cases in China, airborne transmission was not reported7.

It would appear that by introducing this definition WHO was opening the door to the possibility that long range airborne transmission could occur but was highly unlikely. They continued:

“Droplet Transmission occurs when a person is in in(sic) close contact (within 1m) with someone who has respiratory symptoms (e.g., coughing or sneezing) and is therefore at risk of having his/her mucosae (nose and mouth) or conjunctiva (eyes) exposed to potentially infective respiratory droplets. Transmission may also occur through fomites in the immediate environment around the infected person8.

Airborne Transmission
Any engineer or scientist involved with occupational hygiene or air pollution working with precautions against dangerous dusts and hazardous aerosols such as asbestos or pesticides might suspect that it would be ‘less than professional’ to live with sources of potentially infected droplets at separation distances of only two metres, without PPE. There appears to be a growing consensus of opinion that WHO has got this very wrong. Airborne transmission is perceived as being a reality and there is strong circumstantial evidence that it occurs, but there is a lack of test data to prove or disprove its relative importance.

Airborne droplets can vary in size from large drops that can travel only 1 or 2 metres from a source to very small ones that can travel hundreds of metres. I have been looking for information about the range and scale of the minimal viral load required to overcome an individual’s resistance to infection. This might also help with estimation of the size of the particles or droplets that cause transmission. There seem to be plenty of ideas about it but very little data to provide real guidance.

In particular I have been impressed with the argument that most of us “shielding” at home here in the suburban UK, seemed to have escaped infection for months now. This suggests that the risk of contracting the Covid-19 virus infection from droplets is significantly less than “hay fever” sufferers developing respiratory problems from 30 micron tree pollens at distances of hundreds of metres from the source. Could it be that only large droplets are involved in transmission of the Covid-19 virus? Is this the reason that, so far, we have avoided infection through airborne transmission? Is this why “social distancing” at only 2 metres appears to be so effective?

A similar, but less persuasive, argument can be made from our experience that the risk of infection in the out-door environment is much less than indoors, particularly where indoor ventilation is restricted, allowing internal contamination levels to increase. It is argued that outdoors the greater atmospheric dispersion potential is sufficient to decrease the risk of inhaling sufficient virus load to overcome initial individual resistance and complete transmission of the infection. Much more will be learned about this in next few months of the virology research programmes. For us, until the roll-out of the vaccine programmes, the best means of protection against airborne transmission are to maximise social distancing and avoiding poor ventilation conditions indoors.

Face Coverings
For me, the question about the size range of the droplets that create a risk of transmission is important because it changes the function of the ‘face mask’. We know that ‘face masks’ are not very effective at filtering out small particles. They are not very effective in affording protection against tree pollens causing allergies. However, from experience we know that they provide protection against larger dust particles, particularly the “nuisance dusts”. If the droplets involved in Covid-19 transmission are so large that 2 metre separation distances are effective their filtering capabilities would provide the user with protection against Covid 19 infection, and not just be useful in protecting others. Indeed, the efficiency of protection could be increased further by the use of more than one layer of material. Further improvements can be gained by using hydrophilic filter materials, standard clinical grade, and ensuring a good fit at the sides. If in doubt wear two masks, one on top of the other. So what are the disadvantages … ?

How to “hide”
We “Oldies” do not like to be reminded of the Covid fatality statistics and this tends to colour our individual risk assessments. The “vulnerable” cannot be blamed for being sceptical about the precautionary measures designed for society as a whole or for being sceptical about the assurances from WHO. We all know that social separation distances of 2 metres are safer than I metre and that 3 metres would be safer still. The safest way of avoiding infection by direct transmission is to stay inside your own bubble and not to mix with people enjoying other bubbles, especially those at the pub or school. However, we have to accept that even with social distancing there are occasions when there is a significant residual risk of infection through airborne droplet transmission.

It seems to me that the only practicable measure available to reduce this risk is to be critical about the conditions and to recognise when and where to seek the protection afforded by wearing an effective face covering. Even if it only provides protection against the larger droplets, it should be remembered that it is these larger droplets that carry the bigger viral load and therefore could be the most hazardous.

David Shillito CEng FIChemE FEI FRMetS

WHO References:

1 World Health Organization. Infection prevention and control of epidemic- and pandemic-prone acute respiratory infections in health care. Geneva: World Health Organization; 2014 Available from:

2 Liu J, Liao X, Qian S et al. Community transmission of severe acute respiratory syndrome coronavirus 2, Shenzhen, China, 2020. Emerg Infect Dis 2020

3 Chan J, Yuan S, Kok K et al. A familial cluster of pneumonia associated with the 2019 novel coronavirus indicating person-to-person transmission: a study of a family cluster. Lancet 2020 doi: 10.1016/S0140-6736(20)30154-9

4 Li Q, Guan X, Wu P, et al. Early transmission dynamics in Wuhan, China, of novel coronavirus-infected pneumonia. N Engl J Med 2020; doi:10.1056/NEJMoa2001316.

5 Huang C, Wang Y, Li X, et al. Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China. Lancet 2020; 395: 497–506.

6 Burke RM, Midgley CM, Dratch A, Fenstersheib M, Haupt T, Holshue M,et al. Active monitoring of persons exposed to patients with confirmed COVID-19 — United States, January–February 2020. MMWR Morb Mortal Wkly Rep. 2020 doi : 10.15585/mmwr.mm6909e1external icon

7 World Health Organization. Report of the WHO-China Joint Mission on Coronavirus Disease 2019 (COVID-19) 16-24 February 2020 [Internet]. Geneva: World Health Organization; 2020 Available from: source/coronaviruse/who-china-joint-mission-on-covid-19-final-report.pdf

8 Ong SW, Tan YK, Chia PY, Lee TH, Ng OT, Wong MS, et al. Air, surface environmental, and personal protective equipment contamination by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from a symptomatic patient. JAMA. 2020 Mar 4 [Epub ahead of print]