WATCH | Study reveals how fast conversation droplets spread in a building

  • Engineers calculated the velocity of aerosol droplets when talking and breathing
  • A 30-second conversation can propel droplets more than two metres
  • They hope this research will help the health sector curb the spread of SARS-CoV-2

Feeling safe to chat with your mask off in a restaurant? New research shows how animated conversation can propel droplets through a building – one of the main routes of the new coronavirus.

This is especially true in the case of pre- or asymptomatic spread through close-proximity talking and breathing – without the help of coughing or sneezing. This has already been well-documented, taking place at super spreader events like birthdays, family reunions and weddings.

READ | Covid-19: What is a super spreader event? 

Engineering experiment

A team of engineers from Princeton University devised an experiment where they recorded the fluid dynamics of speech, using a laser sheet to measure the velocity of droplets and publishing their findings in PNAS.

"Droplet emission occurs during speech, yet few studies document the flow to provide the transport mechanism. This lack of understanding prevents informed public health guidance for risk reduction and mitigation strategies," the engineers explained to justify their experiment.


While hard sounds like "p" cause droplets to travel up to one metre, they found that a train of sentences creates a cone-shaped, turbulent, jet-like flow that can propel droplets over two metres in 30 seconds.

These exhalations exit through the mouth and nose, where viral load tends to be highest.

READ MORE | Even more scientific evidence that masks help protect us against coronavirus-laden droplets 

They also analysed how far droplets can go in the case of slow breathing, as seen in the video below:

Source: PNAS

The droplet emission rate also increases the louder you talk. The longer the conversations in enclosed spaces, the further people need to sit apart from each other, starting at two metres. They also advise that one should stand to the side of the speaker to avoid the "speech vortex", especially if masks aren't used.

"With social situations in mind, in hindsight, it should perhaps not be surprising that droplet and aerosol generation and possible virus transmission are enhanced during rapid and excited speech at parties, singing events etc."

READ | Covid-19-infected droplets spread further in cold, humid climates

Static talkers

However, the experiment doesn't account for movement of the speaker's head or body and any background air motion like air conditioning, a draught, and other speakers.

This work will prove useful in understanding the important role of ventilation and mask-wearing in buildings as restrictions continue to ease around the world.

An earlier study found that those who exercise also gulp in six times more air, and combined with this study, shows how vital physical distancing in gyms is.

The engineers emphasise that, although they are not "trained in public health", they do believe their work is useful for this sector.

"Building on the understanding of the fluid dynamics of viral and pathogen transmission we believe it will be possible to design potential mitigation strategies, in addition to masks and vague physical distancing rules, and link to poorly understood issues of viral dose to better manage societal interactions prior to the introduction of a vaccine."

READ MORE | Worried about catching coronavirus from surfaces? The city you live in may matter 

Image credit: Pixabay

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