Tiny droplets of saliva that are sprayed into the air when people speak may be sufficient to spread coronavirus, according to US government scientists who say the finding could help control the outbreak.

Researchers at the US National Institutes of Health (NIH) in Maryland found that talking released thousands of fine droplets into the air that could pose a risk to others if the speaker were infected with the virus.

The scientists used laser imaging and high-speed videography to show how thousands of droplets that are too small to see with the naked eye are emitted in normal speech, even in short phrases such as “stay healthy”.

The work is preliminary and has not been peer-reviewed or published, but in a report the scientists claim the findings may have “vital implications” for containing the pandemic.

“If speaking and oral fluid viral load proves to be a major mechanism of Sars-CoV-2 [the official name of the virus] transmission, wearing any kind of cloth mouth cover in public by every person, as well as strict adherence to social distancing and handwashing, could significantly decrease the transmission rate and thereby contain the pandemic until a vaccine becomes available,” the researchers write.

The results will fuel the ongoing debate over whether or not healthy people should wear face masks in public. Recent advice from the US Centers for Disease Control and Prevention called for members of the public to wear cloth face covers when they visit places where it is hard to maintain physical distancing, such as pharmacies and grocery stores.

But the US advice contrasts with that from the World Health Organization, which reviewed its stance on face masks last week. In updated guidance published on Monday it restated that there was no evidence wearing a mask in public prevented people from picking up respiratory infections such as Covid-19.

The positions are not as conflicting as they seem, though. The WHO concedes that people who are infected with the virus but do not show symptoms can still spread the virus, and that a mask may reduce the number of droplets they spray into the air. Their primary argument is that wearing a mask in public is unlikely to protect the wearer from viruses, which can penetrate cloth masks, and the virus can still invade the body through the eyes.

No need for healthy to wear face masks, says WHO after review Read more

The US team, led by Adriaan Bax at the NIH’s Laboratory of Chemical Physics, repurposed laser instrumentation to detect otherwise invisible mists of droplets that are released when people speak. In a single 17-millisecond frame they counted as many as 360 saliva droplets when the phrase “stay healthy” was spoken.

The scientists have not analysed the droplets for their ability to carry coronavirus particles but said they were sufficiently large to carry a variety of respiratory pathogens including the measles and influenza viruses.

“A damp homemade cloth face mask dramatically reduced droplet excretion, with none of the spoken words causing a droplet rise above the background,” the scientists said.