Wearing two masks or a close-fitting one offers significantly better protection against catching and spreading Covid-19, according to a new US study released on Wednesday.
The impact of mask-wearing has been a hotly-debated topic worldwide during the pandemic, with the United States -- where 468,000 people have died -- now urging everyone over the age of two to wear masks in public.
The Centers for Disease Control and Prevention (CDC) study examined fitting a cloth mask over a medical procedure mask, and also knotting the ear loops of a medical mask to improve fit.
"Each modification substantially improved source control and reduced wearer exposure," the study concluded.
"These experiments highlight the importance of good fit to maximize mask performance."
In one test, an unknotted medical mask alone blocked 42 percent of particles from a simulated cough, and the cloth mask alone blocked 44 percent.
But the combination of the cloth mask covering the medical mask blocked 92 percent of cough particles.
Adding a cloth mask over a medical mask, or knotting and tucking the medical mask to fit better, reduced the cumulative exposure of the unmasked receiver by 82 percent and 63 percent.
Scientists agree the main way the virus is spread is through the air, rather than surfaces, and there's growing evidence that small droplets from breathing and speech that can travel many meters (yards) are a common mode of transmission.
Emerging variants also require a smaller viral load to spread Covid-19 compared to the more common strain.