Diseases transmitted by droplets are more easily infected in winter than in summer. The reason for this is well illustrated by the exhaled vapor pulls that become visible in the cold.
For influenza and coronavirus, today’s health recommendations and restrictions are based on 1934 measurements made by William Firth Wells of the United States, who studied the role of droplets in the wake of the Spanish flu epidemic. The staff of the Vienna Technical University, in cooperation with the University of Padua, showed that we underestimated the mist clouds a lot.
Previously, coughing or sneezing was considered more dangerous because the tiny mist droplets dissipate very quickly. The new model showed that smaller droplets remain in the air much higher than previously thought at higher humidity, according to an article in the Journal of the American Academy of Sciences, PNAS.
In the cold, they go away a lot
Professor Alfredo Soldati and his team study multiphase flows made of different components. The air exhaled by an infected person also consists of such droplets of different sizes and exhaled air.
In addition to computer models, the researchers also built a plastic head model powered by an electromagnetic valve, whose operation was recorded with high-speed cameras, so they could measure exactly how long the droplets remain in the air.
The smaller droplets stayed in the air longer than we previously thought. The reason is simple: the evaporation of the droplets does not depend on the relative humidity but on the local humidity.
Professor Soldati explained.
This means that small droplets continue to infect more than we assumed, but this is not a reason for pessimism, it just shows that we need to properly study such phenomena to understand them. Only on this basis can we make scientifically sound suggestions, for example, regarding mask wearing or the optimum distance.
Exhaled air is more humid than the surrounding air, so the drops evaporate more slowly, and when this happens, the evaporation of each drop raises the local humidity, slowing the loss of the rest. The difference compared to Wells’ data was also marked at 20 degrees and 50 percent humidity, but at 5 degrees and 90 percent humidity it became dramatic. Smaller droplets may have survived up to two hundred times, and droplets older than one second can travel much longer distances.