Can medical face masks be safely re-used?
With governments around the world advising citizens to cover their faces while in public places to help protect themselves and others from COVID-19 infection, the rise in demand for face masks has led to global shortages and makeshift solutions. But are these solutions effective?
Back in May, Associate Professor Richard Peltier from the University of Massachusetts Amherst received a grant to study the impact of various sterilisation techniques authorised for emergency use by the US Food and Drug Administration (FDA) for N95 respirators — surgical-grade masks that are designed to filter 95% of airborne particles. Normally intended to be single-use only, world health bodies and governments have recommended their use only by healthcare professionals as the masks are now in short supply.
Assoc Prof Peltier used state-of-the-art pollution detection instruments and a mannequin head in his lab to measure whether microscopic particles can pass through the masks after they are sterilised. He noted, “Respirators must be effective across a range of potential conditions to provide protection, since droplets that contain virus particles immediately start to evaporate and shrink.”
While the testing was limited by the availability of processed masks provided by hospitals in Massachusetts, Assoc Prof Peltier said he was able to draw several conclusions, as published in the journal Infection Control & Hospital Epidemiology. “Some treatments for decontamination had no impact on respirator performance, while other treatments resulted in substantial damage to masks,” he said.
Respirators that were treated between one and 10 times with specific vaporised hydrogen peroxide (vHP) sterilisers or up to five times with shorter decontamination cycles of gas plasma hydrogen peroxide (gpHP) retained their original filtration capabilities; treatments with high concentrations of gpHP or longer processing times degraded filtration performance below the requirement for N95 masks. A decontamination process using ultraviolent germicidal irradiance (UVGI) meanwhile slowly diminished filtration efficiency, reaching a level that warranted caution after nine repeated treatments.
For comparison, Assoc Prof Peltier also tested a KN95 mask, some brands of which have been removed from the FDA’s emergency-use list due to poor performance, and a four-ply polyester bandana. Neither had been treated with any decontamination technique and both performed below N95 standards. He also found that immersing an N95 mask in a 10% bleach solution degraded its performance.
Assoc Prof Peltier noted that his study did not address the masks’ fit or general integrity, including elastic function, corrosion on staples or compression of the respirator, all of which are important for proper functioning. “There are [also] still a number of steriliser systems that are being used on these masks which we don’t have information about and therefore can’t determine if they keep workers safe,” he added.
“We hope this work supports good decision-making that protects those who are on the front lines of this pandemic keeping us all safe,” Assoc Prof Peltier concluded. “Without them, none of us are safe.”
Other researchers are meanwhile developing new methods to safely re-use face masks, with a team at the King Abdullah University of Science and Technology (KAUST) designing an ultrathin, lightweight, porous polymeric membrane that can turn the N95 respirator into a re-usable mask while potentially improving its ability to keep out SARS-CoV-2.
KAUST electrical engineer Muhammad Mustafa Hussain and his team repurposed the N95 respirator by fabricating an attachable membrane that can be replaced after a single use. This is designed to facilitate re-use of the N95 mask, saving costs and resources while broadening its availability.
Importantly, it could also improve the mask’s filtration efficiency for SARS-CoV-2. Pores in currently available N95 masks are around 300 nm in size, while the SARS-CoV-2 virus is significantly smaller at 65 to 125 nm. The KAUST team’s approach facilitates the design of ultrathin polymeric membranes with pore sizes as small as 5 nm.
As described in the journal ACS Nano, the method first involves etching funnel-shaped pores into a silicon-based template, producing an array of 90 x 90 nm squares on one side and 5 to 55 nm-sized pores on the other. Hussain explained, “The etching method controls the distances between the pores and overcomes the problem of randomly spaced and oriented pores found in polymeric, nanoporous membranes developed by other researchers until now.”
The template pattern is then etched on to a 10 µm-thick polyimide film that is removed from the template and can be attached to an N95 respirator. The polyimide membrane is intrinsically hydrophobic, meaning that water droplets fall off it instead of being absorbed into it — and that particles don’t accumulate or collect on the mask’s surface.
The team’s theoretical calculations show that their repurposed N95 mask conforms to the breathability standards set out by the US National Institute for Occupational Safety and Health. They are now working with commercial partners to further optimise the mask’s breathability and filtration efficacy.
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