Pexels/IvanSamkov
Pexels/IvanSamkov

Two layers better but three layers best when it comes to home-made face masks

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Experimental study: At least one thing in the experiment was changed to see if it had an impact on the subjects (often people or animals) – eg: changing the amount of time mice spend on an exercise wheel to find out what impact it has on weight loss.

Case study: A study involving observations of a single patient or group of patients.

People: This is a study based on research using people.

Home-made cloth face masks likely need a minimum of two layers, and preferably three, to prevent the dispersal of viral droplets from the nose and mouth associated with the spread of COVID-19, Australian researchers say. Using a high-speed camera and a lighting system the researchers studied the spray of airborne droplets produced by a healthy person during speaking, coughing, and sneezing while wearing each type of mask. The study found a 3-ply mask was the most effective and while a 1-ply mask reduced the spread of droplets while speaking, a 2-ply mask was better than a single layer at reducing the spread from coughing and sneezing. 

Journal/conference: Thorax

Link to research (DOI): 10.1136/thoraxjnl-2020-215748

Organisation/s: The University of New South Wales

Funder: This research was supported by NHMRC Centre for Research Excellence (Grant Number 1107393), Integrated Systems for Epidemic Response. CRM is supported by a NHMRC Principal Research Fellowship, grant number 1 137 582

Media release

From: The BMJ

Home-made face masks likely need at least 2 layers to curb COVID-19 spread

Reduces dispersal of airborne viral droplets from nose and mouth, but 3 layers preferable

Home-made cloth face masks likely need a minimum of two layers, and preferably three, to prevent the dispersal of viral droplets from the nose and mouth that are associated with the spread of COVID-19, indicates a video case study published online in the journal Thorax.

Viral droplets are generated during coughing, sneezing, or speaking. And face masks are thought to protect healthy people from inhaling infectious droplets as well as reducing the spread from those who are already infected. 

But worldwide shortages of personal protective equipment during the pandemic have led some health agencies, such as the US Centers for Disease Control (CDC), to recommend the use of home-made cloth face coverings as an alternative to surgical face masks.

Several types of material have been suggested for these, but based on little or no evidence of how well they work. 

A team of Australian researchers therefore compared the effectiveness of single and double-layer cloth face coverings (175 g/m² cotton fabric, with a thread count of 170/ inch) with a 3-ply surgical face mask (Bao Thach) at reducing droplet spread.

The single layer covering was made from a folded piece of cotton T shirt and hair ties; the double layer covering was made using the sew method, as set out by CDC:
https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/how-to-make-cloth-face-covering.html

The researchers used a tailored LED lighting system and a high-speed camera to film the dispersal of airborne droplets produced by a healthy person with no respiratory infection, during speaking, coughing, and sneezing while wearing each type of mask.

The video recording showed that the 3-ply surgical face mask was the most effective at reducing airborne droplet dispersal, although even a single layer cloth face covering reduced the droplet spread from speaking.

But a double layer covering was better than a single layer in reducing the droplet spread from coughing and sneezing, the recording showed.

This is just one case, added to which several other factors contribute to the effectiveness of cloth face masks, note the researchers. These include the type of material used, design and fit, as well as the frequency of washing.

Nevertheless, based on their observations, a home made cloth mask with at least two layers is preferable to a single layer mask, they say, adding: “Guidelines on home-made cloth masks should stipulate multiple layers.” 

And they emphasise: “There is a need for more evidence to inform safer cloth mask design, and countries should ensure adequate manufacturing or procurement of surgical masks.”

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