It’s true – a recent study suggests that summer pollen could carry the COVID-19 virus. But are the scientists missing something right under their noses…?
It was suggested earlier this year that pollen may carry coronavirus particles. Scientists trying to make sense of recurring waves of coronavirus around the world made a link between pollen levels in 31 countries and the number of coronavirus cases. But they had no hard evidence that the two were connected, and other studies suggested the opposite, that peaks in the pollen season coincide with a decrease in respiratory diseases like coronavirus and influenza.
There are many theories as to the drivers of waves of coronavirus infection, including temperature and humidity. There’s also a theory that ultraviolet wavelengths in sunlight may act as a natural disinfectant, so coronavirus spreads more easily in the winter when there’s less sun. Sunlight, of course, also stimulates the production of Vitamin D in the body, strengthening the immune system to resist coronavirus.
Changes in the weather also affect human behaviour – when it’s particularly cold or hot we tend to stay indoors, making it more likely that infections will be passed around.
A study published in the Proceedings of the National Academy of Sciences, taking data from 248 sites in 31 countries where airborne pollen is counted, was adjusted for other factors such as population density, temperature, humidity, and lockdown orders. Yet it still found that when pollen count spiked, coronavirus infections increased about four days later. The study suggested that pollen seemed to account for around 44 percent of the infection rate difference between countries.
The mechanism causing it wasn’t so clear. It may not be as simple as viruses hitching a ride on grains of pollen and getting into our respiratory systems; it may be more that the pollen disturbs our immune systems, whether or not we suffer from the allergies commonly referred to as ‘hay fever’.
Study author Stefanie Gilles, PhD, chair of environmental medicine at the Technical University of Munich in Germany, said: “When we inhale pollen, they end up on our nasal mucosa and here, they diminish the expression of genes that are important for the defence against airborne viruses”. Her studies showed that mice exposed to pollen made less interferon and other protective chemical signals to the immune system. The mice infected with a respiratory virus also seemed to have more virus in their bodies compared with those not exposed to pollen, and the same effect was noted in human volunteers. The conclusion was that the effect of pollen was to reduce the body’s immune system response to the coronavirus.
“If you’re in a crowded room and other people are there that are asymptomatic, and you’ve just been breathing in pollen all day long, chances are that you’re going to be more susceptible to the virus,” said study author Lewis Ziska, PhD, a plant physiologist studying pollen, climate change, and health at Columbia University’s Mailman School of Public Health in New York City. “Having a mask is obviously really critical in that regard.”
Masks, of course, are as good at blocking pollen as they are coronavirus, so many people with pollen allergies wear them in times of high pollen counts anyway.
On the other hand, other studies suggest there’s no causal link between pollen levels and coronavirus infections. Martijn Hoogeveen, PhD, a professor of technical sciences and environment at The Open University in the Netherlands, says: “Just because two things happen at the same time doesn’t mean that one causes the other.” His studies in the Netherlands suggest that says the start of pollen season coincides with the end of flu season, and that COVID-19 infection peaks tend to follow a similar pattern – the opposite of the conclusion of the study in the Proceedings of the National Academy of Sciences.
Another study in the Chicago area suggests that as pollen climbs, flu cases drop, leading the researchers to think that pollen may actually compete with viruses in our airways, helping to block them from infecting our cells.
None of these studies may be wide or long enough to come to any meaningful conclusions, but it will probably be possible to draw better conclusions over the coming years. Part of the problem is that there are too many variables in play, such as whether the cities in the study are in lockdown or not.
In the most recent study, scientists from the American Institute of Physics (AIP) in Maryland noticed a correlation between coronavirus infection rates and high pollen concentrations. They used computer modelling to mimic how up to 1,500 grains of pollen per cubic metre from a willow tree can move through the air, picking up coronavirus particles and spreading with a light breeze.
“To our knowledge, this is the first time we show through modelling and simulation how airborne pollen micrograins are transported in a light breeze, contributing to airborne virus transmission in crowds outdoors,” said study author Professor Dimitris Drikakis. According to AIP’s scientists, pollen is known to capture particles of RNA of the sort found in coronavirus.
The study went as far as to create a computer model of a willow tree which “included thousands of tree leaves and pollen grain particles, hundreds of stems and a realistic gathering of a crowd of about 100 individuals at about 20m (65ft) from the tree.”
The results, published in the journal Physics of Fluids, suggested that pollen could pass through a crowd in under a minute, picking up coronavirus particles from people who are shedding the infection. The conclusion was that in pollen season, the social distancing measures don’t apply, and that “In the case of high pollen grains concentrations in the air or during pollination in the spring, the social distance of 2m (6.5ft) does not hold as a health safety measure for a crowd outdoors.”
“Thus, the public authorities should revise the social distancing guidelines.”
So there you have it – in pollen season, it might pay to keep your coronavirus mask on outdoors too.