Pollen, the notorious spoiler of summer evenings, has the potential to act like an airborne raft that helps viruses like SARS-CoV-2 spread between people, according to a new study.
As reported in the journal Physics of Fluids, a duo of fluid dynamics experts from the University of Nicosia in Cyprus have used computational modeling to understand how tree pollen might influence the spread of an RNA virus, such as SARS-CoV-2, the virus that causes COVID-19.
"To our knowledge, this is the first time we show through modeling and simulation how airborne pollen micrograins are transported in a light breeze, contributing to airborne virus transmission in crowds outdoors," Professor Dimitris Drikakis, study author from the University of Nicosia, said in a statement.
Airborne pollen could help facilitate the spread of viral infections by acting as a floating raft that holds hundreds of virus particles at a time. Since the pollen is specially adapted to drift distances with the wind, it’s possible that the virus particles hitching a ride can make relatively long journeys and make their way into the respiratory systems of humans.
To dig into this idea, the scientists simulated a number of outdoor gatherings of roughly 10 or 100 people, a small number of whom were infected and shedding SARS-CoV-2 particles, about 20 meters (65 feet) from a mature willow tree. On a heavy day, such trees can pump out some 1,500 grains of pollen per cubic meter into the surrounding air, so their models also saw the introduction of some 10,000 pollen grains.
They modeled conditions similar to those seen on an average spring day in the US and found the pollen had drifted through the crowd in less than one minute, picking up virus particles as it floats by. While it’s not possible to precisely gauge how high the risk is (this depends on an array of factors, such as the individual’s immune system) the researchers claim this could significantly affect the virus load carried along by the breeze and likely increase the risk of infection. So much so, a day of heavy pollen might be enough to undermine the “2 meter (~6 foot) rule” used for social distancing in public, they argue.
This research is not the first to raise this concern. In March 2021, scientists at the Technical University of Munich in Germany found that increased SARS-CoV-2 infection rates occur at times when airborne pollen levels are higher. In this piece of research, the study authors hypothesized this link could be explained by high pollen counts resulting in a weaker immune response in the airways.
While a dampened immune response is still likely to play a role in this link, the new study highlights another possible way in which pollen counts might impact viral infection rates.
"When studying the spread of SARS-CoV-2, environmental factors such as pollen must be taken into account. Increased awareness of these effects are an important step in preventing and mitigating the impact of Covid-19," Athanasios Damialis, lead author of this second study and Chair of Environmental Medicine, Technical University of Munich, said in March 2021.