For most people, the microscopic particles that seep into our bodies this time every year are harmless.
But for 35 million of us, the pollen that permeates the air turns into a natural poison in our nostrils and lungs, triggering the immune system in ways that send us to doctors and drug stores for relief.
"Pollen is a pollutant that gets into the air. We think of pollutants as man-made, but this is a naturally occurring pollutant," said Dr. Richard Lockey, an allergy specialist at the University of South Florida College of Medicine.
Although pollen can travel hundreds of miles to find its victims, experts say the allergy season that begins this month usually follows a pattern. Tree pollen season begins in mid-April, with oak pollen causing much of the misery. In May, grasses begin to pollinate and by the late summer, ragweed spreads its misery.
Allergists say May is a particularly noxious month because the seasons for tree and grass pollens overlap. But they emphasize that April - particularly if the weather stays warm and dry - is the month when they begin to see the runny noses, watery eyes and scratchy throats that herald this season of discontent.
Seasonal allergies affect younger people more than the elderly but can strike at any age. For Andrew Wirth, a 48-year-old Edgewood resident, allergy season means weekly trips to his doctor's office for injections of a serum designed to increase his resistance to the timothy and Bermuda grasses that make him sick.
Gradually, the shots build up his immune system so he can do his job - he's a sales representative for a lawn-care equipment supplier, which means frequent trips to golf courses. Without the shots, he says, his allergies would drive him crazy.
"It can make driving hard, or makes it hard just being outside. You're always sneezing and sniffling," Wirth said. "It just makes you feel like you have a cold."
For some, allergies are a year-round scourge.
"In the last year or two, it's gotten to a point where I feel like I had to do something," said Nina Gills, a day care program director in Howard County.
Gills, 29, had food allergies as a child and is now allergic to fresh fruit and freshly cut grass. She sleeps with a humidifier to minimize sinus problems, spends $300 a month on prescription and nonprescription medications and still has missed 25 days of work in the past six months. Currently, she's undergoing skin tests to determine the source of her allergies. "I have no way of knowing exactly what it is," she said.
Overreacting to allergens
Generically, an allergen is a substance that's harmless to most people but triggers an overreaction in the immune systems of others.
Pollen is an allergen because it contains microscopic cells that are rich in proteins. Once inhaled, the pollen grains rupture, allowing the proteins to reach moist nose and lung membranes.
In the allergic person, the body's immune system treats the proteins as foreign substances and produces antibodies, according to pollen experts. The antibodies trigger the release of histamines and other chemicals, leading to the cold-like symptoms familiar to allergy sufferers.
Pollen allergies are as widespread as they are because this time of year, pollen particles are so pervasive.
From an evolutionary standpoint, wind-borne pollination is one of nature's least efficient forms of reproduction. The trees and grass that use wind to reproduce have no way to direct their pollen, so they produce huge quantities of it.
To make sure that at least some of the pollen grains reach their destination, nature has given them aerodynamic designs - some as smooth as pingpong balls, others as spike-covered spheres - that make them capable of sailing long distances.
"It's kind of a shot in the dark to get such a tiny grain to a plant that may be a long distance away," said Peter Van de Water, a researcher with the U.S. Geological Survey.
In a study published last year, government scientists said that pollen from grass raised on test plots in Oregon traveled up to 13 miles. But in previous studies, researchers at the University of Tulsa, in Oklahoma, found that cedar pollen from central Texas makes people sick 200 miles north in Tulsa - and sometimes traveled hundreds of miles farther north to the Canadian province of Ontario.
A single oak tree can produce up to 10 billion pollen grains in a few days, according to Vaughn M. Bryant, a pollen researcher at Texas A&M University. During oak season, he said, the average person inhales 7,200 pollen grains every 24 hours.
"I don't think people realize the enormity of the amount of pollen that's out there," he said.
Researchers say that tree pollen's ability to travel makes it hard to pinpoint the source of an allergy. Those runny noses and itchy eyes don't necessarily come from plants in the back yard. And humans can be surprisingly sensitive - some people need only 15 to 20 grains of pollen per cubic meter of air to generate symptoms.