Education

University of Iowa scientists find bad weather worsens pollen counts

Allergies worse after a thunderstorm? Could be a link

University of Iowa associate chemistry Professor Betsy Stone
University of Iowa associate chemistry Professor Betsy Stone

If you’ve ever felt like your allergies get worse after a big thunderstorm, you may be onto something.

University of Iowa researchers, succeeding for the first time at finding and measuring fragmented pollen in the atmosphere, have discovered those splintered grains can linger in the air during and after spring rains for up to 11 hours.

Not only that – being smaller than intact pollen grains – fragmented pollen can sneak deeper into the lungs, potentially exacerbating allergies.

UI associate chemistry Professor Betsy Stone was corresponding author on a recent paper summarizing the research — published online in the journal Environmental Science & Technology Letters – and talked with The Gazette about its implications and next steps.

Q: When did you conduct the research – and what did you find?

A: The study, conducted in Iowa City and looking at tree pollen, went from April 17 through May 31, 2019.

Researchers long have known weather can influence pollen in the atmosphere – with hot and dry days boasting higher concentrations “because those are favorable for release and dispersal of pollen.” When it rains, on the other hand, those fully-intact pollen grains get washed out of the air, causing the pollen concentration to drop.

“But what we have learned is that when it rains, you can actually have pollen fragment into hundreds of tiny little pieces,” Stone said. “Laboratory studies have shown that pollen grain can rupture into 700 or more tiny fragments. And our research was able to show, for the first time, that this process actually happens in the atmosphere, and tiny pollen segments are generated during thunderstorms.”

Q: What causes a pollen grain to fragment?

ARTICLE CONTINUES BELOW ADVERTISEMENT

A: “The way that happens is the pollen grains are pulled up in the atmosphere through an updraft that takes warm air from the surface and it rises due to convection,” she said. “When the pollen grains have been exposed to very high humidity, they can rupture, releasing hundreds of tiny little fragments per pollen grain.

“And then as it begins to rain, the rain will actually cause a downdraft, which is a transfer of the air in the atmosphere that now contains these pollen fragments down to the Earth’s surface,” she said. “So our research characterized — what are the concentrations of these pollen fragments during rain events? And what are the conditions that surround the highest concentration?”

Q: And what are those conditions that create the highest concentrations of pollen fragments?

A: “The stronger the thunderstorm, the higher the concentration,” she said.

The UI research window encountered two severe weather events in 2019 – one on May 16, prompting a severe thunderstorm warning, and another on May 24, when a tornado developed nearby.

“So both of those very strong convective thunderstorms had the two highest concentrations of pollen fragments that we saw,” she said. “And we were able to show that the highest concentrations occurred at the peak of the downdraft, when you have a large change in temperature due to the cool air above coming down to the surface.

The May 16, 2019, storm produced a peak concentration of 1.3 million pollen fragments per cubic meter of air. The May 24, 2019, storm produced 960,000 pollen fragments per cubic meter of air.

“The transfer of air brought the pollen fragments down to the surface, which is the air that we breathe.”

Q: What is the difference between intact pollen and fragmented pollen?

A: “An intact pollen grain, those tend to be about 20 to 100 micrometers in size,” she said. “And that can rupture into hundreds of tiny pollen fragments that generally are less than two and a half microns in size.”

Q: So for someone with allergies, do fragments carry the same potential for irritation as intact pollen?

ARTICLE CONTINUES BELOW ADVERTISEMENT

A: “We can’t say anything specifically about allergies because we didn’t measure allergens yet for these samples, but that’s something we hope to be able to shed some light on in the future,” she said, adding, “There is certainly potential for these pollen fragments to carry allergens, especially those that are located in the cytoplasm – or on the inside of the pollen – that could be released when the pollen ruptures.”

Q: If you find pollen fragments can carry allergens, what could make that problematic for allergy sufferers?

A: “The important thing about pollen fragments that I want to convey is that pollen fragments have a couple of properties that make them very different from intact pollen,” she said.

An intact pollen grain will settle via gravity on the Earth’s surface, whereas pollen fragments are much smaller and thus don’t settle as quickly.

“They actually stay in the atmosphere much, much longer,” she said. “Our data shows that after it rains, pollen fragments can stick around for two and a half to 11 hours.”

And, she added, “The other important thing about pollen fragments is they can actually – because of their particle size – make it much deeper into the human respiratory tract than an intact pollen.”

Q: How does that happen?

A: “An intact pollen, because of its large size, will get impacted in the upper airway,” she said. “Whereas these small pollen fragments can make it much deeper into the lungs, where they may deposit and kind of trigger an allergic response in a different part of the lungs.”

Q: And this is brand-new research and discovery out of the University of Iowa?

A: “So people have observed pollen fragments in the laboratory before,” she said. “But we actually observed them in the environment for the first time. We made the first real-time measurements of pollen fragments in the atmosphere.”

Q: So what’s next?

ARTICLE CONTINUES BELOW ADVERTISEMENT

A: Because Iowa has a pronounced ragweed season in late summer, UI researchers collected measurements and samples last summer that they’re analyzing now.

“We’re repeating the same study to examine the occurrence of pollen fragments during ragweed season – and trying to understand what are their concentrations, their properties, and what are the conditions that occur in late summer that contribute to high concentrations of ragweed pollen fragments.

“And in that experiment, we are incorporating measurements of allergens. And so we hope to be able to make more direct conclusions about the potential health impacts of those particles.”

Give us feedback

We value your trust and work hard to provide fair, accurate coverage. If you have found an error or omission in our reporting, tell us here.

Or if you have a story idea we should look into? Tell us here.

Give us feedback

We value your trust and work hard to provide fair, accurate coverage. If you have found an error or omission in our reporting, tell us here.

Or if you have a story idea we should look into? Tell us here.