Climate change extending ragweed season, hay fever misery, study finds

You’re not imagining that sneezy nose and those itchy eyes: Allergies have become increasingly prevalent in the last three decades, costing Americans about $21 billion every year. Researchers point to one possible factor (when it comes to hay fever, at least): climate change.

Recent increases in the length of the ragweed pollen season are associated with warming, they wrote in a study released Monday by the Proceedings of the National Academy of Sciences.

The team looked at pollen counts and weather data from 10 locations spanning more than 1,300 miles of the central part of North America -- from Georgetown, Texas (30.63 degrees north latitude), to Saskatoon, Canada (52.07 degrees north latitude).

Poring over the data, they determined that from 1995 to 2009, as latitude increased, so did the frost-free period -- and the number of days of the pollen season.

Five sites, extending north from LaCrosse, Wis. (43.80 degrees north), to Saskatoon had significant increases in the length of the ragweed pollen season of at least 13 days. Saskatoon’s increase was 27 days -- a full month’s worth of facial tissues and antihistamines.

The trend tracks Intergovernmental Panel on Climate Change projections, as well as shifts in plant hardiness zones in the upper Midwestern U.S., the authors reported.

Weed season for most of the U.S. is in the summer and fall. At least 10% of the U.S. population is ragweed sensitive. The paper cited one report that ragweed may cause more seasonal allergic rhinitis -- hay fever -- than all other plants combined.

A longer pollen season and the higher levels of pollen it brings could increase hay fever by increasing the number of people who become sensitized to pollen or by increasing the duration and severity of symptoms in people who already have the allergy, the authors wrote.

RELATED: More on pollen from the Los Angeles Times.