COVID-19: Can airborne pollen help spread the virus?

  • There appears to be a correlation between high concentrations of airborne pollen and high rates of infection with SARS-CoV-2, which is the virus that causes COVID-19.
  • A new computer model suggests that tree pollen could facilitate the transmission of the virus in a crowd of people gathered outdoors.
  • The model assumes that the virus can hitch a ride on pollen grains, though there is currently no direct evidence that this can happen.
  • An alternative explanation for the correlation between pollen levels and COVID-19 is that immune reactions to pollen can make individuals more susceptible to infection.

Two physicists have claimed that physical distancing rules designed to curb the spread of COVID-19 may be inadequate for people standing outdoors near major pollen sources such as trees and grass.

The researchers at the University of Nicosia in Cyprus modeled the spread of pollen from a willow tree through a nearby crowd of people, each standing at least 2 meters apart, at a wind speed of 4 kilometers per hour.

The model simulated the movement of 10,000 individual grains of pollen through groups of either 11 or 97 people around 20 meters from the tree.

The scientists found that the larger crowd of people temporarily trapped the airflow and the pollen it carried. This would have the effect of increasing potential contacts between pollen grains and the tiny droplets of saliva generated when people talk, cough, and so on.

Assuming that the virus can hitch a ride on pollen grains, the results suggest that the pollen could carry the virus from one person to others in the crowd.

Talib Dbouk, Ph.D., and Dimitris Drikakis, Ph.D., from the university’s Defence and Security Research Institute conducted the study, which has been published in the journal Physics of Fluids.

Lighter, drier particles

The researchers point out that droplets of saliva are relatively heavy and evaporate rapidly, whereas pollen grains contain little water, are lighter, and can travel tens of kilometers.

“People should avoid crowd gatherings close to some types of plants or trees known to be very active in pollen grains release during a pollination season,” Prof. Drikakis told Medical News Today.

“The public should be aware of the airborne transmission risks from both indoor and outdoor saliva droplets and the hazards arising from airborne pollen grains outdoors,” he added.

He said the safe physical distance to minimize transmission by pollen would vary according to environmental conditions.

The scientists were inspired to conduct their study after noticing correlations between high airborne concentrations of willow oak and white willow pollen in the United States in March–May 2020 and SARS-CoV-2 infection rates.

Other research has linked pollen levels to the transmission of SARS-CoV-2. A study published in March 2021 found that airborne pollen concentrations in 31 countries around the world accounted for 44% of the variability in SARS-CoV-2 infection rates.

However, when scientists in Leipzig, Germany, analyzed pollen from local air samples and conducted lab tests on purified pollen, they found no evidence that the pollen could carry or transmit SARS-CoV-2.

Susanne Dunker, Ph.D., of the Helmholtz Centre for Environmental Research in Leipzig, who led the research, said they were unable to completely rule out the possibility that pollen can transmit SARS-CoV-2.

She told MNT that for practical and safety reasons, they incubated pollen and the virus in a fluid rather than air.

“In reality, a binding would happen in the air and probably not in a fluidic system. We could only show that both do not form a “stable connection” when co-incubated in a suspension,” she said.

Their paper notes that electrostatic effects in the air could affect binding between the virus and pollen grains.

“Such effects might be of interest in future studies,” they write.

Effects of pollen on immunity

In their article, Dr. Dbouk and Prof. Drikakis cite a study that found pollen can carry RNA viruses. SARS-CoV-2 is an RNA virus.

However, this study analyzed the pollen collected from flowers by bees and stored in their hives, rather than wind-dispersed tree or grass pollen.

There may be an alternative explanation for the observed correlation between airborne pollen concentrations and SARS-CoV-2 infection rates.

Research suggests that pollen can reduce the production of antiviral molecules in a person’s respiratory tract, which in turn increases their susceptibility to viral infections.

More research is necessary to disentangle the relationship between pollen and SARS-CoV-2.

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