Tree pollen, released between February and May for reproduction, is a primary cause of spring allergies, often triggering symptoms like itchy eyes, runny nose and sneezing.

Pine trees are often mistakenly blamed for spring allergies simply because they release pollen grains that are more visible, appearing as a layer of yellow dust on cars, patios and other outdoor surfaces — and sometimes even as drifting clouds in the air.

But the real culprit is hardwood tree pollen, according to Robert Bardon, a professor of forestry and environmental resources and the associate dean for Extension in the College of Natural Resources at North Carolina State University.

Unlike pine tree pollen, which is usually too large and smooth to penetrate deeply into the respiratory tract, hardwood tree pollen is smaller and has a rough, spiked surface.

“Hardwood pollen is two to three times smaller than pine pollen and remains invisible to the naked eye,” Bardon said, adding that it is about one-third to one-half the thickness of a human hair.

As a result, hardwood pollen is more likely to be inhaled deeply into the respiratory tract, where its rough, spiked surface can irritate mucous membranes and trigger the release of inflammatory chemicals that cause the familiar symptoms of spring allergies.

Hardwood trees such as oak, birch, ash, walnut and hickory are especially allergenic, according to Bardon. These species produce “massive amounts of lightweight, dry pollen that is easily dispersed by the wind,” he said.

Bardon added that its ability to remain airborne over long distances makes it particularly likely to be inhaled and cause allergic reactions. Its rough, spiked surface also allows it to cling more easily to clothing, hair and pets, spreading allergens indoors.

In contrast, species such as tulip-poplar, black cherry, black locust and catalpa produce heavy, sticky and clumped pollen grains that are dispersed by insects, making them far less likely to become airborne, spread and trigger allergic reactions.

Oak, birch, ash, walnut and hickory are common in rural and suburban forests, woodlands and agricultural edges, where their lightweight pollen disperses widely, often contributing to higher overall airborne pollen counts than urban areas.

Urban areas, however, may experience locally elevated pollen concentrations from ornamental trees such as maples, London plane and poplars, which are frequently planted along streets, in parks and in residential yards.

Trees release pollen throughout the day, with levels usually peaking in the mid-morning. Humidity and wind significantly affect pollen release: warm, dry and windy days spread the most pollen, while cold, rainy days temporarily clear pollen from the atmosphere.

According to Bardon, pollen exposure can be reduced by scheduling outdoor activities later in the day. It also helps to change clothes and shower upon returning home and to regularly bathe pets that spend time outdoors.