A new study has found that disinfecting homemade masks by heating them may not significantly reduce their effectiveness — and that some medical masks can undergo this process and still be effective.
The research, published in the Journal of the International Society for Respiratory Protection, may be valuable for reducing the pressure on stocks of medical masks during the coronavirus disease 19 (COVID-19) pandemic.
According to the World Health Organization (WHO), the virus that causes COVID-19, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is primarily transmitted by coughing and sneezing.
Droplets of saliva and discharge from the nose can carry the virus. It can be directly passed on if, for example, someone with the infection coughs or sneezes, and another person in close proximity breathes in the resulting droplets.
The virus can also survive on surfaces that the droplets land on. Transmission can occur if a person comes into contact with the droplets on a surface and the virus gains access to the person’s body — if, for example, the person touches their face.
There is also evidence that some people with the infection experience few, if any, symptoms and may still be able to pass on the virus.
As a consequence, the Centers for Disease Control and Prevention (CDC) have recommended wearing cloth face coverings in situations when it is difficult or impossible to maintain a distance of 6 feet between people.
They urge people to make their own face masks and reserve surgical masks and N-95 respirators for healthcare workers. The CDC have produced a guide on how to make a face mask at home.
In light of this advice, the researchers behind the present study set out to investigate ways of disinfecting masks safely — killing any SARS-CoV-2 without reducing the effectiveness of the masks over time.
They studied means of disinfecting homemade masks and those for healthcare workers, as reusing masks may help ease pressure on dwindling stocks.
The study authors cite previous research demonstrating that heating masks to 70oC (158oF) is effective at killing the SARS-CoV-2 virus.
While other disinfection techniques have also proven effective, heating can be done by anyone with access to a stove.
To test how effective masks are after disinfection by heat exposure, the authors applied different masks to the heads of mannequins designed to simulate the drawing of breath.
Because it had already been established that heating to 158oF could kill SARS-CoV-2, the researchers did not need to work with the virus. Instead, they used soot from a kerosene lamp, the particles of which are roughly the same size as SARS-CoV-2 particles.
The authors then tested two disposable N-95 respirator masks, one disposable surgical mask, and three homemade masks, from a cotton dress, a cotton sweater, and polyester cloth.
They measured the levels of soot outside the mannequin and in the air that had passed through the masks.
They repeated the tests, changing the tightness of the masks and using mannequins of different materials, plastic or silicone, to simulate the softness of the human face.
The researchers concluded that — if fitted tightly on a plastic mannequin — one N-95 respirator and the surgical mask could be disinfected by heat exposure ten times without losing their effectiveness.
The N-95 mask achieved a filtration efficiency of 95%, while the surgical mask achieved 70%.
The other N-95 respirator failed after being repeatedly applied to the mannequin.
However, when the authors used the masks on the softer, silicone, mannequin with a fit that a person would normally use, they found that filtration levels were significantly lower — at around 40% for both the surgical mask and the N-95 respirator.
When the team used homemade, cotton masks on the softer mannequin, they discovered that the filtration efficiency was 55% — better than either the surgical mask or the N-95 respirator when used with a regular fit.
Heating the homemade masks also had no effect on their filtration capacity.
Finally, the authors explored the effectiveness of homemade nose clips applied to the mask. They found that, when applied to disposable masks with a regular fit, the nose clips significantly increased filtration efficiency: to 98% for the N-95 mask and up to 88% for the surgical masks.
The researchers acknowledge that their study was relatively small and did not account for the ways in which a person’s breathing and movement change the fit of a mask.
Nonetheless, the authors — who hail from Columbia University’s Lamont-Doherty Earth Observatory, in Palisades, NY — note, “Everything we know so far suggests that wearing almost any kind of mask in public is better than nothing, that a tight fit is best, and that, with certain limits, many types of masks can be reused outside of medical settings.”
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