This year's late spring is bringing a burst of warm days and beautiful flowers. Unfortunately, it's also made millions of allergy sufferers miserable. And, scientists say, the awful season could be a sign of worse suffering to come.

Unprecedented levels of pollen have been measured across the Eastern United States this April. On April 7, the Atlanta Allergy and Asthma Clinic in Georgia saw a near-record-breaking concentration of 5,733 particles per cubic foot. And in mid-April, Kansas City, Mo., recorded a pollen level of over 8,000 particles per cubic foot, the highest ever seen at that station.

To put that in perspective, 15 particles per cubic foot can cause sniffling and sneezing in those with bad allergies, said Jay Portnoy, the chief of allergy, asthma and immunology at Children's Mercy Hospitals and Clinics in Kansas City, Mo. At 100 particles per cubic foot, everyone with allergies gets sick. April's record levels went even further.

"The sheer number of particles in the air was enough to trigger symptoms even in patients who didn't have allergies, just because of the irritant effect," Portnoy said.

The culprit for this year's bumper crop of pollen is the weather, according to Portnoy. Temperatures stayed cool throughout February and March, preventing flowering trees from beginning their annual pollination ritual. Instead of a gradual, species-by-species release of pollen, the trees stored up until the weather got balmy. Then they all released at once.

For the 40 million Americans who have indoor/outdoor allergies, the pollen explosion translated into runny noses, scratchy throats and itchy eyes. Many of Portnoy's patients complained that their usual medications weren't working, but that wasn't quite true, Portnoy said.

"The exposure to the pollen was so great that it overwhelmed the medicine," he said.

A rising trend

The tree pollen burst has settled down somewhat, and allergists aren't yet sure how severe the grass pollen season, which starts in a few weeks, will be. Nonetheless, people with allergies might want to stock up on tissues.

Research suggests that the overall trend for pollen is up, and global warming could be to blame.

Both warmer temperatures and carbon dioxide trigger plants to grow faster and larger — and to produce more pollen. A 1995 study in the journal Grana found that birch pollen in Europe gradually increased over the previous 30 years. And a 2003 study published in the Journal of Allergy and Clinical Immunology found that ragweed growing near carbon-dioxide rich cities grew faster and denser than ragweed growing in the countryside. The urban ragweed also produced more pollen, said lead researcher Lewis Ziska, a plant physiologist at the U.S. Department of Agriculture.

Global warming lengthens the growing season, Ziska said, pointing to evidence that trees in carbon-dioxide rich cities flower earlier than those in the countryside. That could be good news for some farmers, but bad news for allergy sufferers.

"The combination of both increasing CO2 and, of course, warmer temperatures is likely to exacerbate both weed growth and pollen production from those weeds," Ziska said.

Halting hay fever

Allergies develop when pollen or another allergen triggers the immune system to produce an antibody called IgE. The tiny, y-shaped IgE antibodies then attach to large mast cells in the mucous membranes of the nose, throat, lungs and digestive system. When these primed mast cells encounter more pollen, they burst, spewing forth granules full of histamines and other chemicals. The result is the sneezing, sniffling mess of hay fever.

What scientists don't know is why something as ubiquitous as pollen makes so many of us sick. Allergies could be a byproduct of the way our immune system evolved: annoying, but not so detrimental that natural selection deletes the genes responsible. Another theory, dubbed the "hygiene hypothesis," notes that people in areas rampant with parasite infections have low allergy rates. IgE antibodies help defend the body against parasitic worms, the theory goes, so perhaps by curing ourselves of parasites, we've freed IgE to run amok, overreacting to every grain of pollen. 

The hygiene hypothesis is far from proven, but that hasn't stopped some online entrepreneurs from selling parasitic worms to allergy patients desperate for a cure. Fortunately for the squeamish, there are other, more reliable options.

Allergists have an arsenal of antihistamine sprays, pills and eye drops, and corticosteroids can soothe swollen airways. In some cases, allergen immunotherapy — better known as allergy shots — can help people control their allergies. In fact, scientists have a slew of tips for allergy sufferers.

The important thing, said Rebecca Piltch, M.D., an allergist in San Rafael, Calif., is that patients figure out which types of pollen set off their allergies. That way, they can prepare for the season by developing a treatment plan in advance.

"For most people with allergies, it is possible to achieve good control over symptoms, and it is possible to have a good quality of life, including outdoor activities," Piltch said. "So many people suffer for years or sometimes even decades, and that isn't necessary most of the time."