The Science Behind the Pollen Count

Pollen looks like powder because it of consists lots of tiny grains that are the male gametophytes of seed plants. These grains can be as small as 10 micrometers or as large as 100 micrometers, which still means they are all microscopic. Just a fingertip full collected from the hood of your car could contain thousands of pollen grains. These little grains, which produce sperm, are needed for fertilization.

"Without pollen, there would be no seeds and no fruit," Dr. Estelle Levetin, professor and chair of Biological Science at The University of Tulsa, said when we spoke to her in 2019. She's also a member of the aerobiology committee that oversees the National Allergy Bureau (NAB), part of the American Academy of Allergy, Asthma & Immunology (AAAAI).

Levetin explains that in flowering plants, pollen is transported from the male parts of the plant, the anthers where the pollen is produced, to the carpel, the female part of the plant that produces the ovules. Pollen gets transported by animals, like bees, or by the wind.

"Wind-pollinated plants are small and inconspicuous and produce large amounts of lightweight pollen that is easily carried by wind," Levetin says. "Insect-pollinated plants tend to be large and showy. They often have brightly colored petals, are fragrant and produce nectar." Generally, airborne pollen — the kind that attacks allergy sufferers — is from wind-pollinated plants. "Occasionally we will see a few pollen grains from insect-pollinated plants in our air samples."

If you tune in to a television weather forecast during pollen season, you'll likely hear them talk about the day's pollen count. You maybe even wondered whether the "count" was some sort of estimate of air quality. Literally counting microscopic grains of pollen each day might sound a bit farfetched, but that is precisely what is happening, plus a lot more.

"The pollen count is the number of pollen grains in a cubic meter of air over a 24-hour period," says Katie Walls, meteorologist at Spectrum News 1, a 24-hour cable news channel in Charlotte, North Carolina. (When we talked to her in 2019, she was a meteorologist at WSB-TV, the ABC affiliate in Atlanta.) That's when she explained that certified counters from Atlanta Allergy & Asthma get up early and physically count the number of pollen particles on a glass slide that has been outside for the prior 24 hours. "The number you see on WSB-TV is the number from that morning's count and represents the pollen present in [metro Atlanta]."

More specifically, pollen is captured by a volumetric air sampling instrument, which means, according to Levetin, a known volume of air that is sampled. There are two types of instruments: rotating arm impactors, like the Rotorod Sampler, and Hirst-type spore traps, like the Burkard Sampler. They are both volumetric, but they operate on different principles, and microscopic analysis is needed to analyze both types of samples.

The rotating arm version has a head that rotates at 2,400 revolutions per minute. While it spins, two small, greased rods drop down, capturing pollen and spores. These rods are placed into a special microscope adapter and examined. The Hirst-type instrument, on the other hand, has a suction trap that sucks in air and particles that adhere to a greased microscope slide inside. In this case, the slide moves toward the intake orifice at 2 millimeters (0.07 inches) per hour so it's possible to see what was swirling through the air hour by hour during examination.

Using one of the volumetric air sampling instruments, some pollen stations sample air and collect pollen 365 days a year. Other stations run samples on weekdays or only collect three days a week. But not every city or town has the ability to count pollen, and pollen stations are operated in a variety of ways.

Some are run by the city or county public health departments, others by allergists like Atlanta Allergy & Asthma. "A few stations are run by academics like me who study airborne pollen as a research topic," Levetin says. "It takes training to learn the morphology of pollen, and it takes time to analyze air samples," she explains.

Certification in pollen counting is available through the AAAAI and NAB. The process requires pollen counters to take an approved pollen and spore identification course, pass a written exam, and pass a pollen and fungal spore identification test. Counters have to learn the microscopic morphology of individual pollen grains.

The training in pollen identification is critical because just counting pollen isn't enough.

"People with allergies look at the types of pollen that are mentioned in that morning count to know how they might be impacted during the day," Walls says. "Technicians decipher the different types of pollen particles using a microscope. That's how they know which types of trees, weeds and grass are pollenating and causing people issues. Not everyone is allergic to or irritated by the same thing."

Pollen may be to blame for months of discomfort, but remember, it also has an important job. It gives us our daily bread by way of fruits, grains and seeds. And it has a lot of other neat applications, too. Levetin says that pollen is used in forensic science because it can help determine where an object originated. (Just ask Jack Hodgins from the television series "Bones.") Archeologists also examine fossil pollen to study which plants early human societies used, and geologists use it to determine the composition of ancient plant communities. Exploration geologists even use fossil pollen to help locate oil deposits.

Learn more about pollen in "Pollen and Spores: Applications with Special Emphasis on Aerobiology and Allergy" by S N Agashe and Eric Caulton. HowStuffWorks picks related titles based on books we think you'll like. Should you choose to buy one, we'll receive a portion of the sale.