In the wake of the U.S. Centers for Disease Control and Prevention (CDC) proclaiming two masks better than one for halting the spread of the sometimes deadly SARS-CoV-2 virus, it only seemed inevitable that someone would up the ante.
And now there is a peer-reviewed study by the Indian Institute of Science, the University of California San Diego, and the University of Toronto declaring three-layer masks are best at stopping viral emissions. It was published Friday by the American Association for the Advancement of Science (AAAS).
“Three-layer masks can effectively block these (SARS-CoV-2 bearing) droplets and thus could be ubiquitously used as a key tool against COVID-19 or similar respiratory diseases,” they concluded.
They did not distinguish whether a specific, three-layer masked is needed or whether three, single-layer masks would work as well, though they did note that “many commonly available or makeshift masks contain single or double layers.”
After subjecting these masks to “carefully designed experiments involving high-speed imaging along with physics-based analysis, we show that high-momentum, large-sized (greater than 250 micrometer) surrogate cough droplets can penetrate single- or double-layer mask material to a significant extent,” they reported.
“The penetrated droplets can (then) atomize into numerous much smaller (less than 100 micrometer) droplets, which could remain airborne for a significant time.”
They went on to warn that pressure exerted by someone coughing or breathing could force these aerosolized particles “through the microscale pores in the fibrous network of the single/double-layer mask material” and into the surrounding air.
Aerosolized SARS-CoV-2 has been the subject of much debate with the CDC generally sticking to the belief that most of those who come down with COVID-19, the disease caused by a SARS-CoV-2 infection, have been exposed to virus-packing droplets.
Some independent scientists have, however, argued that there is no reason to believe aerosols are any worse or better at spreading the virus than larger droplets. And a peer-reviewed study published late last month in the Proceedings of the National Academy of Sciences of the United States (PNAS) concluded that a significant number of the infections aboard the cruise ship Diamond Princess were due to aerosols.
Passengers aboard the Diamond Princess were infected at the very beginning of the global spread of SARS-CoV-2 in early 2020. Harvard scientists who went back and reconstructed the movements of the passengers on the ship and the air circulation concluded that “short-range, long-range, and fomite transmission modes” of infection accounted for “35 percent, 35 percent and 30 percent, respectively.”
Fomites are objects or surfaces contaminated with the SARS-CoV-2 virus. A Chinese study early in the pandemic suggested the possibility elevator buttons could be a surface to which SARS-CoV-2 viruses stuck and then were spread.
Fomites are the reason for the much-repeated safety advice on handwashing. Short-range, non-fomite-linked infections have been largely attributed to large droplets which are blocked by masks.
Of the droplets large and small in the air aboard the cruise ship, the researchers concluded that 41 percent would likely be categorized as large and 59 percent as small.
“Our results demonstrate that aerosol inhalation was likely the dominant contributor to COVID-19 transmission among the passengers,” they reported. Their modeling of viral spread, they added, came to this conclusion even if they assumed “high ventilation rates and no air recirculation conditions for the cruise ship.”
A scientist from China and a colleague from Australia warned of the danger of air-recirculation systems spreading SARS-CoV-2 in June, but were largely ignored.
“Despite this, no countries or authorities consider airborne spread of COVID-19 in their regulations to prevent infections transmission indoors. It is therefore extremely important that the national authorities acknowledge the reality that the virus spreads through air, and recommend that adequate control measures be implemented to prevent further spread of the SARS-CoV-2 virus, in particular removal of the virus-laden droplets from indoor air by ventilation.”
Governments failed to respond, other than to order wide-spread masking. Residents of the Southern U.S. states began moving inside to the comfort of air-conditioned buildings as temperatures climbed. SARS-CoV-2 infection and death rates in the South skyrocketed.
Some scientists at that time warned air conditioning units could be driving the spread of the disease, but it wasn’t until fall that the CDC accepted that idea and then only reluctantly so.
Months later, it did recommend a “layered strategy to reduce exposures to SARS-CoV-2” that included recommendations to improve the ventilation in buildings, but even then it offered only recommendations and suggested to businesses that they would face additional costs for actions aimed at improving air quality.
The latest study indicates that three-layer masks are best at blocking droplets and reducing the aerosolized SARS-CoV-2 escaping through the masks, and “thus could be ubiquitously used as a key tool against COVID-19 or similar respiratory diseases,” the researchers said.
But it is unclear as to how long the masks will block the release of aerosols. Queried about the specifics of blocking the virus, professor Saptarshi Basu, the chair of Mechanical Engineering at the Indian Institute of Science and the lead author of the study, was unsure of what would happen in the world outside the laboratory.
He was asked these two questions via email:
1.) How many virions can the three-layer mask hold? ie. at what point does the mask reach its carrying capacity after which exhalations begin forcing surplus virions out of the mask?
2.) What happens to the pathogen carrying capacity of the mask when it wets out given that moisture should ease the flow of virions through the mask?
His answer was brief:
“Your questions are interesting, but I have no definitive answers to those. How moist masks behave is a subject of ongoing work.”
The moisture problem
Moisture accumulation in protective masks has long been known to be an issue. Humans exhale about half a liter of water per day. A 2015 study of medical masks in use in a Hanoi, Vietnam hospital warned against their use because “moisture retention, reuse of cloth masks and poor filtration may result in increased risk of infection.”
The peer-reviewed study was published in BMJ Open, a journal of the British Medical Association. Health Care Workers have long been advised to change their masks whenever they get wet.
A 2017, peer-reviewed study of the deadly Ebola virus published in Nature concluded the chances of that pathogen penetrating a wet mask – including high protection N-95s – was about seven times greater than for a dry mask.
Despite these known issues with masks, there are no national standards today governing their uses by workers who may be subject to requirements they wear masks for eight hours per day or longer in the workplace.
Neither is there much information on how masking, double-masking, triple-layer masks or hours spent in poorly ventilated spaces might affect worker health, although a May study of workers in a Massachusetts grocery store found a 20 percent of them had been infected despite requirements requiring everyone entering the store to wear a mask.
Meanwhile, while one group of scientists say three layers of mask are the way to go, another group says it supercomputer found negligible differences between one mask and the two masks suggested by the CDC.
Researchers from Kobe University and the Riken Center for Computational Sciences said they found a single surgical-type mask of non-woven material blocked 85 percent of large particles. Adding a polyurethane mask on top of that boosted the effectiveness, but only to 89 percent.
The examination contained the usual caveat that the results were dependent on the mask being “worn tightly around the nose and face.” The researchers did not report on what became of the large particles the other study described being “atomized” when they hit the mask.
Studies of what sorts of face coverings are most community protective are ongoing, but for personal protection, there appears a scientific consensus that clean, dry, N95 masks properly fitted are the gold standard although no face covering offers as much protection as avoidance of other people and closed spaces frequented by significant numbers of other people along with washing your hands after touching public surfaces.