&Bullet; physics 14, 78

In cool, moist air, the droplets released by coughing grow first according to simulations and then shrink according to simulations.

The COVID-19 pandemic has drawn attention to the droplets of breath produced by breathing, coughing, and singing. There are now numerical simulations of a cough [1] In cooler, more humid air, these droplets grow first before they evaporate and shrink – they don’t evaporate continuously, as previous research has shown. Growth occurs when warm, moist breath interacts with colder air and creates a cloud of water vapor-saturated air – an effect that leads to the familiar “frosty” breath on cold days. The enlargement of the cough drops in this cloud causes them to survive longer than they would at higher temperatures, potentially allowing an infected person to spread an airborne disease to people farther away.

In more recent simulations [2] Detlef Lohse from the University of Twente in the Netherlands and his colleagues showed that the moist, turbulent jet of breath when coughing enables the smallest droplets (around 10 micrometers in diameter) to survive up to 150 times longer than in isolation. These results were in line with previous evidence that the ubiquitous “6-foot rule” indoors is insufficient to avoid contact with the smallest droplets expelled by an unmasked person (see How Talk Spreads Viruses).

In their new simulations, the team varied both the ambient temperature and the ambient humidity. They found that at 90% relative humidity, the average droplet size increases for about 0.3 seconds at ambient temperature

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((

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) but continuously decreases when it is

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. Growth in the

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Simulations are caused by condensation in a cloud of air with a local humidity of over 100%. at the

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the air around the droplets is less humid. The team also developed a mathematical model that accurately predicts local changes in humidity and that can be used to predict the behavior of droplets.

David Ehrenstein

David Ehrenstein is Senior Editor for physics.

## References

1. CS Ng et al., “Growth of breath droplets in cold and humid air” Phys. Rev. Fluids6th054303 (2021).
2. KL Chong et al., “Extended Life of Breathing Droplets in a Turbulent Blast of Steam and Its Effects on Airborne Disease Transmission” Phys. Rev. Lett.126034502 (2021).

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