Face masks reduce the risk of spreading large Covid-linked droplets when speaking or coughing by up to 99.9%, according to a lab experiment with mechanical mannequins and human subjects, researchers said Wednesday, December 23, AFP reports.

A woman standing two metres from a coughing man without a mask will be exposed to 10,000 times more such droplets than if he were wearing one, even if he is only 50 centimetres away, they reported in the journal Royal Society Open Science.

Large respiratory droplets -- which act like projectiles before being pulled toward the ground by gravity -- are thought to be the main driver of SARS-CoV-2 transmission, senior author Ignazio Maria Viola, an expert in applied fluid dynamics at the University of Edinburgh's School of Engineering, said.

Smaller ones, sometimes called aerosol droplets, can remain suspended in the air for longer periods.

"We continuously exhale a whole range of droplets, from micro-scale to millimetre-scale," Maria Viola said by phone.

"Some of the droplets will drop faster than others" depending on temperature, humidity and especially air speed, he said.

The study focused on particles larger than 170 microns in diameter -- roughly two to four times the width of a human hair.

Aerosol particles, which tend to follow currents in the air, are generally described as smaller than 20 or 30 microns.

Intermediate size droplets can behave either way, the study found.