A person with coronavirus could infect others around them simply by saying one or two words in a calm and normal voice, a new study suggests.
When a Covid-19 carrier speaks, they emit tiny liquid droplets that contain infectious virus particles and could linger in the air - and be inhaled by others - for up to three hours, research has found.
The droplets behave like an aerosol - a suspension of particles or droplets in air or another gas - and can penetrate deep into the lungs when they are breathed in, scientists found.
A team from the US National Institutes of Health used a laser light-scattering experiment to demonstrate how droplets fill the air when a person speaks, and how masks can block them and prevent the spread of Covid-19.
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The study was published in the New England Journal of Medicine.
Molecular biologist Dr Matthew Meselson, of Harvard University, wrote: "Certain persons called 'super spreaders' produce many more aerosol particles than other persons.
"These particles are too small to settle because of gravity, but they are carried by air currents and dispersed by diffusion and air turbulence."
Inhaled aerosolised particles can penetrate deep into the lungs and infect cells in the alveoli, air sacs in the lungs which exchange oxygen and carbon dioxide molecules to and from the bloodstream, the study found.
The particles can be carried by drafts.
Dr Meselson added: "Aerosols from infected persons may therefore pose an inhalation threat even at considerable distances and in enclosed spaces, particularly if there is poor ventilation.
"The possible contribution of infective aerosols to the current pandemic suggests the advisability of wearing a suitable mask whenever it is thought that infected persons may be nearby and of providing adequate ventilation of enclosed spaces where such persons are known to be or may recently have been."
The researchers said some studies have shown that "the number of droplets produced by speaking is similar to the number produced by coughing".
Masks can prevent the droplets from filling the air, the scientists said.
A separate study in the New England Journal of Medicine in March found that aerosolised coronavirus could remain in the air for up to three hours.
Many countries are now urging people to wear masks when they go out in public to prevent the spread of the virus.
Harvard Medical School said coronavirus is primarily spread when a person breathes in droplets from an infected person's cough or sneeze.
People can also catch it when they touch a contaminated surface and then touch their eyes, nose or mouth.
People who do not have symptoms or have not yet developed symptoms can infect others.
"That's where masks come in," said Harvard Medical School.
