Cloth Masks Reduce Omicron Risk by 50%

by James A. Bacon

More evidence is coming in that the Omicron variant of COVID-19 has very different properties than Delta and other variants, and that polices and practices deemed appropriate for earlier versions might not be so for Omicron. The latest revelation comes from Dr. Linsey Marr, a Virginia Tech engineering professor, who works in the once-obscure academic specialty of bioaerosols. She became famous during the COVID-19 epidemic after playing a role in determining that the virus did not spread by fomites (particles left on surfaces) but through aerosols in the air.

WTVR in Roanoke sums up her latest conclusions about the efficacy of wearing masks to prevent the spread of Omicron: “She said cloth masks are only about 50% effective in protecting against infected particles — good enough for earlier forms of COVID-19, but not against the more transmissible Omicron variant.”

Only 50%? Cloth masks don’t offer a 100% guarantee of protecting us from the virus? That’s the glass-half-empty version of the data. The glass-half-full version is this: hey, cloth masks are 50% effective in protecting against infected particles!! Which is a lot better than zero.

This information resonates with me because, although I have dutifully worn masks in public spaces, I was unsure about their efficacy and feared that I was engaging in COVID theater.

I’ve never seen the benefits of mask wearing expressed the way Marr does, and I think the data should be disseminated widely — along with any caveats she and her collaborators might have. This data needs to be incorporated into the thinking of businesses, local governments, schools, universities and most of all of individual Virginians.

The WTVR article is unfortunately sparse on details, and I cannot find a Virginia Tech press release summarizing her research. But you can consult Marr’s academic profile page, which lists selected publications, here,  as well as the “Applied Interdisciplinary Research in Air” page here.

Marr’s web page links to an article in The Journal of Hospital Infection, dated April 2021, that addresses six myths about the transmission of COVID-19 and masks. (I have appended a lengthy extract at the end of this post.)

A key takeaway for me is this: while it is true that individual viruses can easily pass through the fibers in a cloth mask, viruses come in clumps, or “particles,” that contain other materials found in saliva and mucus. Collectively, these clumps are more likely to be blocked. Furthermore, there is a physics phenomenon known as Brownian Motion (random motion of particles suspended in a gas) that increases the odds of bouncing into a cloth fiber.

Bacon’s bottom line: I’ve justified to myself the wearing of flimsy cloth masks in public spaces on the grounds that the masks will limit the amount of virus I cough, hack, wheeze, sneeze or otherwise expel into the air around me, thus limiting the exposure of others. I never expected any personal protection. If I can reduce the odds of exposure by 50%, I’m all on board!

Extract from “Dismantling myths on the airborne transmission of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2)”

Myth 5a. ‘If it is airborne, surgical masks (or cloth face coverings) will not work’

This statement is false because it is essentially presented as an oversimplified binary scenario [i.e. masks work (completely) or do not work (at all) against viruses in respiratory particles].

Several laboratory studies have already shown that surgical and home-made masks are somewhat (but incompletely) effective in limiting exhaled particles and in protecting wearers from inhaling particles from others. Surgical masks can contain, and therefore reduce, the dissemination of viruses shed by an infected wearer by up to 3–4-fold (i.e. approximately 67–75%), and even 100% in the case of seasonal coronaviruses.

When an infectious person wears a mask or face covering, the size of the exhaled plume is also reduced, and this also helps to reduce the risk of exposure to those nearby.

Surgical masks also protect the wearer by reducing the exposure to incoming droplets and aerosols from infected individuals by an average of 6-fold (range 1.1- to 55-fold). The filtration capacity of surgical masks in the micron size range is often considerable, although it varies between brands.

It is known that the filtration capacity of N95/FFP2 respirators is better if they have been appropriately fit-tested to avoid leakage of aerosols around the side of the respirator into the breathing zone.

Even home-made cloth masks (made from tea towels or cotton t-shirts) can reduce the exposure from incoming particles by up to 2-4-fold (i.e. approximately 50–75%)

This mainly depends on how the mask is made, what materials it is made from, the number of layers, and the characteristics of respiratory secretions to which it is exposed. Based on the evidence supporting a role for airborne transmission of COVID-19, the use of N95/FFP2/FFP3 respirators by front-line healthcare workers should be recommended. For those that cannot tolerate wearing these masks for long periods, the less restrictive surgical masks still offer some protection, but it needs to be acknowledged that these will not be quite as effective.

“Myth 5b: ‘the virus is only 100 nm (0.1 μm) in size so filters and masks will not work’”> Myth 5b: ‘the virus is only 100 nm (0.1 μm) in size so filters and masks will not work’

This myth is related to Myth 5a. There are two levels of misunderstanding to be considered for this myth. Firstly, there is a lack of understanding of how high-efficiency particle air (HEPA) and other filters actually work. They do not act as simple ‘sieves’, but physically remove particles from the air stream using a combination of impaction and interception (where faster moving particles hit and stick to mask fibres via a direct collision or a glancing blow), diffusion (where slower moving particles touch and stick to mask fibres), and electrostatic forces (where oppositely charged particles and mask fibres adhere to each other). Together, these create a ‘dynamic collision trap’ as particles pass through the network of air channels between fibres at various speeds.

The minimum filtration efficiency typically occurs for particles of approximately 0.3 μm in diameter. Particles smaller than this ‘most penetrating particle size’ are captured with greater efficiency because their Brownian motion (allowing diffusion at an atomic level) causes them to collide with fibres in the filter at a high rate. Particles larger than this limiting diameter are removed efficiently through impaction and interception.

Secondly, viruses that are involved in transmission of infection are not generally ‘naked’. They are expelled from the human body in droplets containing water, salt, protein and other components of respiratory secretions. Salivary and mucous droplets are much larger than the virus, and it is the overall size that determines how the droplets and aerosols move and are captured by mask and filter fibres.

HEPA (or ‘arrestance’) filters can trap 99.97% or more of particles that are 0.3 μm (300 nm) in diameter. Exhaled salivary/mucous droplets start from approximately 0.5 μm in size and are removed entirely by HEPA filters. Indeed, HEPA filtration is not strictly needed in the ventilation systems of most commercial buildings other than health care, where specialist areas such as operating theatres, clean rooms, laboratories and isolation rooms benefit from single-pass capture of particles. Stand-alone ‘portable’ air cleaners that filter room air through built-in HEPA filters are an option for non-specialist areas such as offices and classrooms, although their performance may be limited by imperfect mixing, noise and draught effects.