Researchers have found that ordinary conversation creates a conical 'jet-like' airflow that quickly carries a spray of tiny droplets from a speaker's mouth across metres of interior space.

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"People should recognise that they have an effect around them. It's not just around your head, it is at the scale of metres," said study author Howard Stone from the Princeton University in the US. Although scientists have not yet fully identified the transmission mechanisms of COVID-19, current research indicates that people without symptoms could infect others through tiny droplets created when they speak, sing or laugh.

In the study, published in the journal 'Proceedings of the National Academy of Sciences', the research team wanted to learn how widely and quickly exhaled material from an average speaker could spread in an interior space. They concluded that for interior activities, normal conversations can spread exhaled material at least as far as, if not beyond, social distancing guidelines recommended by the World Health Authority (1 metre) and US officials (2 metres.) Their work examined particle flow in an interior space without good ventilation. The researchers also said that while masks do not completely block the flow of aerosols, they play a critical role in the disruption of the 'jet-like' airflow from a speaker's mouth, preventing the quick transport of droplets on large length scales bigger than 30 cm.

Using a high-speed camera, they filmed the movement of a mist of tiny droplets illuminated by a laser sheet in front of a person speaking several different phrases adjacent to the sheet. The phrases ranged from short statements like "we will beat the coronavirus" to nursery rhymes including "Peter Piper picked a peck" and "Sing a song of six pence." The researchers selected the phrases to include different sounds that affect turbulent flows in a speaker's exhalation. The researchers found that this airflow could easily and very quickly carry tiny particles away from the speaker.

They said that even a short phrase can move the particles past the 1-metre distancing recommended by the World Health Organisation in a matter of seconds. The researchers said the distance depends in part on the duration of the conversation. Someone speaking for more time will send particles farther. They said that the 6-foot distancing rule may not be a sufficient barrier in an interior space without good ventilation. The researchers said the experiment showed that a social distance of 6 feet (2 metres) did not work like a wall to protect people.

Over time, conversations can cause the material to move past the distance, particularly inside buildings. "If you speak for 30 seconds in a loud voice, you are going to project aerosol more than six feet in the direction of your interlocutor," Stone said.