What’s With All This Pollen?

Why some days are more allergenic than others.

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High rates of pollen production have made this an especially bad year for allergies. Sales of over-the-counter hay-fever remedies boomed in the United Kingdom, where the worst day of the allergy season should arrive next week. Why do pollen counts fluctuate?

The weather. Allergists measure pollen with something called a Burkard volumetric spore trap, which sucks air through a slit and picks up the floating grains. (The pollen count refers to the average number of grains per cubic meter of air over the course of a day.) Grains are more likely to float in dry conditions, and they’re more likely to fly around if there’s a breeze. In general, you’ll find higher pollen counts in your spore trap on windy days with low humidity.

Pollen counts measure only those windblown grains that reach the nose, where they can cause hay fever. These come from male trees, grasses, and weeds that disperse their pollen into the air to reproduce. Flowering plants that rely on insects for pollination produce heavier, stickier stuff, which rarely makes it into your nostrils.

Even a light rain can wash some of the airborne pollen out of the sky or keep it from being released. Moist air can dampen flowers and stop them from opening up at all. Sometimes a shower will actually produce a spike in the pollen count. The rush of cool air that precedes a summer thunderstorm might produce high winds close to the ground that can pick up and concentrate grass pollen as they go.

Most plants release pollen in the morning—after the morning dew has dried, and in time for the midday breeze. Some grasses pollinate in the late afternoon or early evening, and some trees do it throughout the day.

The allergy season typically begins in the early spring for trees, with grasses pollinating in the summer and ragweed in the fall. Yearly fluctuations depend on the recent weather conditions. Grasses and weeds respond to rainfall within the past few days; trees release earlier if the preceding winter was especially wet or mild. (Weather conditions from the previous summer and fall can also affect pollen counts in a given season.) Some kinds of trees have multiyear cycles of pollen production, so you may see more of one variety every fourth or fifth allergy season.

A few recent studies have suggested that global warming could contribute to rising pollen counts from year to year. A warmer winter will hasten the start of the allergy season and perhaps extend it over a longer period. High levels of atmospheric carbon dioxide can also lead to more pollen: A team of researchers at Harvard showed that ragweed produces 50 percent more of the allergenic grains when its exposure to CO2 is doubled.

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Explainer thanks Estelle Levetin of the University of Tulsa and Richard Weber of the National Jewish Medical and Research Center.