Double the layers
Homemade face masks need at least two layers to effectively stop viral droplets.
HOMEMADE 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.
This was indicated in 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 (see Mandatory mask on p5).
But worldwide shortages of personal protective equipment during the pandemic have led some health agencies, such as the US Centers for Disease Control and Prevention (CDC), to recommend the use of homemade cloth face coverings as an alternative to surgical face masks.
Several types of material have been suggested for these, but there is little or no evidence of how well these materials 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 three-ply surgical face mask (made by a Vietnamese company) at reducing droplet spread.
The single layer covering was made from a folded piece of cotton T-shirt and hair ties, while the double layer covering was made using the sew method as set out by US CDC.
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 three-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 factor among several others that contribute to the effectiveness of cloth face masks, the researchers note.
Other factors include the type of material used, design and fit, as well as the frequency of washing.
Nevertheless, based on their observations, a homemade cloth mask with at least two layers is preferable to a single layer mask, they say, adding: “Guidelines on homemade 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.”