&Bullet; physics 14, 78

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

Cough comparison. Simulations of a cough at two ambient temperatures:

10C.

(above) and

30C.

(below) – see videos below. A vapor-filled cloud of air that surrounds the droplets has higher humidity (white) at colder temperatures, which causes the droplets to grow. This initial growth allows droplets to survive longer in the air before evaporating compared to those in the air

30C.

Simulation.Cough comparison. Simulations of a cough at two ambient temperatures:

10C.

(above) and

30C.

(below) – see videos below. A vapor-filled cloud of air that surrounds the droplets has higher humidity (white) at colder temperatures, which means that the droplets … show more

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).

Simulation of a cough at a relative humidity of 90% and an ambient temperature of

10C.

. The local relative humidity is indicated with a color scale from dark red (90%) to white (110%), and the droplet diameters are indicated with colors from dark green (less than 10 micrometers) to dark red (100 micrometers). The video covers about 0.7 seconds in real time and the field of view is 1 m wide. The white and light red oversaturated area (humidity over 100%) is not available in

30C.

(see video below) and allows droplets to grow in size in their early moments (not apparent from these videos).Simulation of a cough at a relative humidity of 90% and an ambient temperature of

10C.

. The local relative humidity is indicated with a color scale from dark red (90%) to white (110%), and the droplet diameters are indicated with colors from dark … show more

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

10C.

((

50F.

) but continuously decreases when it is

30C.

. Growth in the

10C.

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

30C.

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.

As above, but the ambient temperature is

30C.

.

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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