Droplets Loaded With Coronaviruses Last Far Longer Than Previously Thought

Experiments with dummy heads and masks. Credit: TU Wien

Tiny droplets full of viruses disappear more slowly after exhalation than previous models suggested. Experiments and simulations at the TU Wien (Vienna) can now explain this.

It is easier to get infected in the winter than in the summer – this is true for the Corona pandemic, for the flu and for other viral diseases. The relative humidity plays an important role in this. Outside, it is much higher in winter than in summer, as evidenced by the fact that our breath condenses into droplets in the cold air.

Previous models assumed that only large droplets pose a relevant infection risk because small droplets evaporate quickly. However, at the TU Wien (Vienna), in collaboration with the University of Padova, it has now been shown that this is not true: due to the high humidity of the air we breathe, even small droplets can remain in the air for much longer than previously believed. . The study is published in the scientific journal PNAS.

Simulations and plastic cups

Prof. dr. Alfredo Soldati and his team at the Institute of Fluid Mechanics and Heat Transfer TU Wien are researching currents composed of different components – the so-called ‘polyphase currents’. This includes the air an infected person exhales when sneezing: the infectious viruses are contained in liquid droplets of various sizes, with gas in between.

This mixture leads to a relatively complicated flow behaviour: both droplets and gas move, both components influence each other and the droplets can evaporate and become gas themselves. To understand these effects, computer simulations have been developed, in which the distribution of droplets and breathing air can be calculated at different environmental parameters, for example at different temperatures and humidity.

In addition, experiments have been carried out. A nozzle with an electromagnetically controlled valve is placed in a plastic head to spray a mixture of droplets and gas in a precisely defined way. The process was recorded with high-speed cameras to measure precisely which droplets remained in the air and for how long. Francesco Picano’s team at the University of Padua was also involved in the research project.

Moist breathing air makes droplets float longer

“We found that tiny droplets stay in the air an order of magnitude longer than previously thought,” says Alfredo Soldati. “There is a simple reason for this: the evaporation rate of droplets is not determined by the average relative humidity of the environment, but by the local humidity directly at the location of the drop.” The exhaled air is much more humid than the surrounding air and this exhaled moisture causes small droplets to evaporate more slowly. When the first droplets evaporate, locally this leads to a higher humidity, which further slows down the further evaporation process of other droplets.

“That does mean that small droplets are contagious for longer than assumed, but that should not be a reason for pessimism,” says Alfredo Soldati. “It just shows us that you have to study such phenomena in the right way to understand them. Only then can we make scientifically sound recommendations, for example about masks and safety distances.”

Reference: “Short-term exposure to airborne virus transmission and current guidelines” by Jietuo Wang, Mobin Alipour, Giovanni Soligo, Alessio Roccon, Marco De Paoli, Francesco Picano, and Alfredo Soldati, September 14, 2021, Proceedings of the National Academy of Sciences .
DOI: 10.1073/pnas.2105279118