A study conducted by a team of Australian researchers from the University of NSW found homemade face masks likely need at least two layers to curb the spread of Covid-19.
The researchers compared the effectiveness of single and double-layer cloth face coverings with a 3-ply surgical face mask 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 the Centres for Disease Control and Prevention in the US.
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 while speaking, coughing and sneezing wearing each type of mask.
To induce a sneeze, the subject used tissue paper to simulate the mucus membrane of the nasal cavity.
The footage they captured showed that the three-ply surgical face mask was the most effective at reducing airborne droplet spread.
But even a single layer cloth face covering reduced the droplet spread from speaking.
The researchers found a double layer covering was better than a single layer in reducing the droplet spread from coughing and sneezing.
They noted several other factors contributed to the effectiveness of cloth face masks, including the type of material used, the design, fit and how often it was washed.
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"Guidelines on homemade cloth masks should stipulate multiple layers," the researchers said in the study published online in the journal Thorax.
"These visualisations show the value of using face masks and the difference between types of masks.
"There is a need for more evidence to inform safer cloth mask design, and countries should ensure adequate manufacturing or procurement of surgical masks."
Worldwide shortages of personal protective equipment during the pandemic have led some health agencies, such as the 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 based on little or no evidence of how well they work.
