Aerosols, masks, infectiousness: Coronavirus myths debunked
Confusion about the virus that causes Covid-19 has caused problems in grasping the science
Politicians facing the spread of coronavirus, which causes Covid-19, lay claim to “following the science”, conferring a Rosetta Stone-like quality on the discipline: a monolithic key to indisputable facts.
But as the philosopher Mary Midgley observed, the value of science lies not in how we store facts but how we interpret them. And interpretations are “always shaped by background world-pictures”.
“Following the science”, therefore, entails navigating a labyrinth of multiple “world-pictures” freighted with tensions between language and meaning. As the biologist Dr Justin Pruneski notes: “Overcoming the complex and technical language used in science is a major barrier to scientists being able to communicate their work with the general public.”
A review in the Journal of Hospital Infection addresses this challenge in relation to the transmission of coronavirus – properly, severe acute respiratory syndrome coronavirus 2, or Sars-CoV-2. It cites confusion among healthcare professionals and the public over “the definition and application of relevant terms, such as droplets, droplet nuclei, aerosols and particles” that create “problems in understanding the science”.
Corresponding author Prof Stephanie Dancer is a consultant microbiologist working in NHS Scotland, and professor of microbiology at Edinburgh Napier University. She acknowledges the difficulty of communicating with people who lack specialist expertise. “It’s a gift, isn’t it? I usually tell my students to write their grant applications or theses as if they were explaining to a friend in the pub.”
And lead author Dr Julian W Tang – honorary associate professor/clinical virologist, respiratory sciences, University of Leicester – suggests that “it is the style that you present in that matters most to some people, especially those with preconceived ideas”.
Seventeen review co-authors outline what they consider as myths about Sars-CoV-2 transmission; explain why they are outdated; and highlight current evidence illuminating other directions. Droplets are defined as particles that fall to ground under gravity, the momentum of an infected person’s breath, or both; aerosols are particles in suspension due to size, environmental conditions, or both; and “particles” are droplets/aerosols in general.
Myths debunked include:
Aerosols are droplets with a diameter of five microns or less. (A micron is a millionth of a metre.)
This definition, favoured by the World Health Organisation, or WHO, is wrong. Rather, “Respiratory droplets over a wide range of diameters can remain suspended in the air and be considered airborne.”
Dr Tang explains that since, in bovine terms, a herd of cows whirling around in a tornado could be considered an aerosol, from a virological perspective 100 microns is “a more rational size threshold to distinguish droplets from aerosols, in terms of their physical behaviour and route of exposure”.
If the virus is airborne then surgical masks (or cloth face coverings) won’t work.
This wrongly implies that masks are either completely effective against viruses in respiratory particles, or completely ineffective. Cited studies show that surgical masks can contain, thereby reducing, the dissemination of viruses shed by an infected wearer by 67-75 per cent.
The virus measures 0.1 micron, so filters and masks won’t work.
Two levels of misunderstanding arise. First, high-efficiency filters are not simple “sieves”, but they physically remove particles from the airstream by enabling slower-moving particles to touch and stick to mask fibres; and electrostatic forces allow oppositely-charged particles and mask fibres to stick to each other. This combination creates a “dynamic collision trap” as particles pass through the network of air channels between fibres at different speeds. Second, viruses that transmit infection are not generally “naked” but held in respiratory secretions bigger than the virus, which increases the efficiency of capture by mask and filter fibres.
Unless it grows in tissue culture, it’s not infectious.
The decades-old diagnostic method of tissue culture – whereby certain viruses grew in glass tubes seeded with susceptible cells – has been superseded by more sensitive molecular techniques. However, this sensitivity difference is compounded by currently available air-sampling techniques, which generate high shear forces and “may damage viral surface proteins and stop them growing in culture. In contrast, natural human exhalation and inhalation flow velocities are much slower, which make them much less likely to cause shear stress damage to viruses.”
Failing to detect viable viruses in air samples doesn’t necessarily prove the absence of live virus in samples where viral nucleic acid was detected by molecular methods. Dr Tang explains: “We have the WHO saying that the virus is not airborne because you cannot culture from the air. But when evidence shows that we can culture from the air, WHO says that you cannot prove that there is enough viable virus in air samples to cause infection.”
More broadly, the widespread adoption of molecular techniques such as the polymerase chain reaction has seen the near-universal abandonment of traditional tissue culture methods in routine diagnostic virology labs. . . and a loss of expertise. What if a new virus with pandemic potential were discovered; one that grew well in traditional tissue culture? Dr Tang acknowledges that there has been a loss of cell culture skills, and Prof Dancer told The Irish Times that she “would agree that viral culture expertise has been downgraded over the past decade. However, it’s not just viral culture that has been lost, but the labs in which it was routinely performed.
“Indeed, the dismantling of the public-health network across England and Wales has been bemoaned by a group of retired microbiologists as a factor in the unco-ordinated and fragmented response to the pandemic on several fronts.”
Interestingly the review states: “Opening windows, subject to thermal comfort and security, provides more than a gesture towards reducing the risk of infection from lingering viral particles.”
Might Covid-19 force a rethink in future hospital/healthcare architecture that accentuates the benefits of fresh air and sunlight? Prof Dancer is clear: “I hope so! It’s already being discussed in my health board. A century ago, hospitals were built with attention to prevailing wind, airflow across wards from strategically positioned windows, and sun rooms or sunlit balconies for patients.”
Dr Tang agrees, adding: “It’s even in the United States’ CDC guidance – ‘increase circulation of outdoor air as much as possible by opening windows and doors if possible, and using fans’.”
This review also represents a breath of fresh air by highlighting the fact that “following the science” should not assume a straight trajectory.
As the economist John Maynard Keynes observed: “When my information changes, I change my mind. What do you do, sir?”