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How are pollen counts calculated?

It starts in March, and it seems to last forever: allergy season. For about four to five months, more than 60 million Americans have to choose between suffering from the symptoms of hay fever (technical name: allergy rhinitis) such as sneezing, runny nose, and an itchy nose, or take over-the-counter medications that make them very, very sleepy. Whatever the decision, those millions check on the daily pollen counts to figure out just how much they’ll be suffering that day. The higher the number, on a scale of 0 to 12, the more pollen there is in the air.

There’s no government agency that tracks the pollen count, and it’s not the job of the National Weather Service either. Private organizations, including IMS Health Incorporated and the American College of Allergy, Asthma and Immunology collect pollen data and distribute it as a public service (often underwritten by the makers of allergy medications).

There are many allergens in the air. That yellow haze that covers the world in early spring is primarily pine pollen, but it’s not what most allergy sufferers are most allergic to. Instead, it’s the microscopic particles in the air given off by walnut, oak, and ash trees, grasses, and mold spores. (Rule of thumb: The drier the area, the more pollen floating around. The wetter the place, the less pollen gets through.)

Here’s how IMS and the ACAAI figure out pollen counts. A researcher uses a transparent plastic rod coated in silicone grease and rotates it in the air for one minute. (Collection sights are generally at least one story above ground and away from pollen-heavy vegetation that could skew the results.) Pollen sticks to the rod. Many more samples are taken over the course of 24 hours and analyzed under microscopes. There, individual pollen grains are counted, and converted into a concentration of units of grains per every cubic meter of air.

Those numbers are then disseminated as a pollen count level: low, medium, high-medium, and high. “Low” is a count of 0 to 2.4; medium 2.5 to 4.8; high-medium 7.3 to 9.6; and high, 9.7 to 12.