While we might anxiously await springtime rainstorms to wash pollen off our cars and out of the air. Recent research shows that these rainstorms might actually make pollen allergies worse. April showers can bring pollen allergies, but those allergies are usually not from May flowers. Most airborne pollen, which is often what we breathe in, comes from trees and grasses.
What is pollen?
Apart from serving as a sneeze-inducer, pollen is a small grain which many plants use to reproduce. Even though most pollen looks like a blanket of yellow powder to us, this blanket is actually a mixture of many different microscopic grains. Each plant type has a unique pollen grain, and they can look quite different on a microscopic level (Figure 1). Though their shapes and sizes may differ, pollen grains generally consist of an outer protective wall and an inner core containing the genetic material needed for plant reproduction.
Many types of pollen exist, but they can largely be grouped into two types: (1) pollen distributed by wind and (2) pollen distributed by pollinators (insects, birds, etc). Pollen distributed by the wind is small and lightweight, ideal for being carried in the air. This pollen deposits on your car, your driveway, benches, and pretty much everything else outside. Because it’s air-borne, it is easily inhaled and often leads to pollen allergies.
By contrast, pollen carried by pollinators is larger, heavier, and often slightly sticky. These characteristics allow it to adhere to pollinators that will carry it to a new plant. Flowers have evolved to reflect the likely mode of pollen transport. Pollinator-dependent plants have much showier blooms to attract pollinators, while airborne-pollen-dependent plants have more subdued, less evident flowers. A lot of trees and grasses fall into the latter category.
Why does pollen make us sneeze?
A person experiences pollen allergies when their body’s natural defense mechanisms (antibodies) interact with specific pollen allergens, causing an allergic reaction. These interactions usually occur as a result of someone inhaling pollen, which means that allergies will increase when more pollen is in the air. However, each plant’s unique pollen contains different allergens, so allergies can also differ depending on which plant the pollen belongs to. Different plants produce pollen at different times of year, so people can experience allergies at different times.
We can get an idea of how bad pollen allergies might be on a given day from the local news. This website compiles local pollen forecasts in the US. Along with the weather, broadcasts often report a daily pollen count. This number is as simple as it sounds: a count of individual airborne pollen grains, usually expressed per cubic meter of air. They’ll often also report the main types of pollen contributing to this count.
When pollen grains are exposed to excess moisture, they can essentially explode and break their protective outer wall and inner genetic material into many smaller particles (see Figure 4). A single pollen grain (about 30 um in diameter, about the width of a human hair) can rupture into up to 700 smaller pieces, ranging in size between 0.6 – 2.5 um (smaller than a human red blood cell). These smaller fragments, no longer protected, are useless to plants, and they don’t necessarily show up in a pollen count. However, they still contain proteins and other pollen signatures that can cause allergic reactions in humans. Their smaller size enables the fragments to penetrate deeper in the respiratory tract, causing more severe reactions. Researchers hypothesized that high rupture rates and concentrations of fragments can cause thunderstorm asthma, or increased allergic reactions during thunderstorms.
Researchers at the University of Iowa found evidence that airborne pollen grains rupture more readily during rainstorms. The research team monitored the size distribution of pollen during thunderstorms, severe weather events, and light rainfall during the springtime in Iowa. They separated particles by size and chemical signatures to distinguish, count, and measure pollen grains and pollen fragments. How did pollen grain and fragment numbers change during storms? While the count of pollen granules decreased during rain events, pollen fragments increased significantly. Higher total rainfall and rainfall rates increased the concentration of fragments. Although lightning was not required for pollen to rupture, concentrations of pollen fragments were highest in thunderstorms. This result is consistent with the idea of thunderstorm asthma. They also found that the smaller pollen fragments can linger in the air over 2.5h after a storm ends. Although increased humidity has been shown to increase pollen rupture, and increased asthma attacks have been correlated with thunderstorms, this is the first time a team has successfully measured pollen fragments during a thunderstorm.
So, while rain might appear to “wash away the pollen”, it’s only removing larger pollen grains from the air. There are still elevated levels of pollen fragments suspended in the air during and several hours after a rainstorm that can cause allergic reactions. If you’re particularly sensitive to pollen, you might want to stay inside when it rains on high pollen days, and keep your home’s air filter clean by changing it regularly.
Is pollen rupture out of our control?
In many ways, yes; we can’t stop the rain. However, strong rain events are projected to increase with increased global warming. And the speed at which that occurs is in our hands. Additionally, remember that the vast majority of pollen comes from plants that don’t rely on pollinators. Supporting pollinator health, through limiting the use of pesticides and growing pollinator gardens, will keep plants with “sticky pollen” healthy and also keep air-borne pollen producers in check.
D. Hughes, C. Rampage, L. Jones, Z. Liu, and E. Stone. Environ. Sci. Technol. Lett. 2020, 7, 6, 409–414. https://pubs.acs.org/doi/10.1021/acs.estlett.0c00213
Cover image source: https://commons.wikimedia.org/wiki/File:Lightning_Pritzerbe_01_(MK).jpg
Reviewed by: Christina Marvin