Spring showers bring … pollen.
That's the surprising discovery made by researchers when they measured tree pollen fragment concentrations during and after spring rains of varying intensity in Iowa City between 17 April and 31 May 2019.
Rain fell on 28 days of the study period, which is prime tree pollen season. There were light rains, thunderstorms, and a severe storm that spawned a tornado.
The researchers found that pollen fragments can hang in the air for as long as 11 hours after heavy rains, and can get deep into the lungs and worsen spring allergies.
Avoid going outdoors
"Our results show that while pollen grains decrease substantially during rain, peak concentrations of submicron pollen fragments occur during rain events and then persist for several hours," said study corresponding author Elizabeth Stone, an associate professor in the University of Iowa's department of chemistry.
"People who are sensitive to pollen in season should avoid going outdoors during rain events, especially thunderstorms, and for several hours afterwards," she added in a university news release.
The study was published online recently in the journal Environmental Science & Technology Letters.
Pollen grains are sturdy but can rupture when subject to high humidity, which can happen when a storm's updraft carries the grains to the cloud base, where humidity is quite high. The fragments are then carried down by falling rain and the storm's downdraft, the study authors explained.
The researchers said that pollen fragment levels remain elevated from 2.5 to 11 hours after a rain. The heavier the rain, the longer the levels remained high.
Intact pollen grains are larger and settle to the ground while pollen fragments are smaller and often remain in the air.
"Our study shows clearly that rain decreases intact pollen concentrations. But it can also increase pollen fragments," Stone said. "The interesting thing about the pollen fragments is the really high concentrations only last for a short period of time, primarily when it's raining and during the peak of the storm."
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