In a recent study led by experts from the Technical University of Vienna (TU Wien), it was found that virus-laden droplets disappear more slowly when exhaled, which refutes the patterns suggested above.

According to experiments and simulations conducted at TU Wien, it is easier to become infected in winter than in summer by the covid pandemic, flu, or other viral diseases. Relative humidity plays an important role in this.

Outdoors, humidity is much higher in winter than in summer, as evidenced by the fact that our breath condenses into droplets in cold air.

Previous models assumed that only large droplets had a significant risk of infection, as small droplets evaporated. In TU Wien, however, this turned out to be incorrect: due to the high humidity in the air we breathe, even small droplets can stay in the air much longer than you think.

 

The experiment: Valves and simulations

Professor Alfredo Soldati and his team from the TU Wien Institute for Fluid Mechanics and Heat Transfer have been studying the flows formed by different components, called "multi-stage flows". This includes the air that an infected person breathes out when sneezing; in this case, infectious viruses are found in liquid droplets of different sizes, with gas in the middle.

This mixture leads to a relatively complex flow behavior; the droplets and gases move, affect each other and the droplets can evaporate and become gas. To analyze these effects, simulations have been developed, in which the movement of breathing air droplets can be calculated with different parameters, for example at different temperatures and humidity.

In addition to this, experiments were carried out at TU Wien, involving the installation of an electromagnetically controlled valve nozzle on a plastic head, to spray a mixture of droplets and gases in a precise and defined manner. This process was recorded with high-speed cameras, so it was possible to know exactly which droplets were kept in the air and for how long they did so.

 

Believe it or not, the moisture you breathe makes droplets fly longer

According to Professor Soldati, "there is a simple reason why the evaporation rate of the drops is not determined by the relative environmental average, but by the local humidity at the location of the drop. The exhaled air is much more than the ambient air, and this exhaled moisture causes the small drops to evaporate more slowly. As the first drops evaporate, this leads locally to an increase in humidity, which further slows the process of evaporation of the drops".

Despite the observed findings, Professor Soldati insists on not being pessimistic about it: "We simply have to study these phenomena in the right way to understand them. Only then can we make scientifically sound recommendations, for example about masks and safety distances".

To access the full study, go to: https://www.pnas.org/content/118/37/e2105279118