Is Global Warming Extending Allergy Season?

Environmental factors may be producing more allergens and making them stronger

The source of your seasonal allergies, called allergic rhinitis or hay fever, depends on the month and which plants are flowering at the time. People who are allergic to trees suffer throughout spring, while summer is sneezy for those allergic to grass, and the fall is uncomfortable for people with ragweed allergies. It seems like there only a few months out of the year that allergy sufferers get a reprieve. The bad news is allergy season is starting earlier, ending later, and could get even worse. Mounting evidence suggests that global warming is playing a role, and could even lead to new allergies in people previously unaffected.

"Allergies in general are skyrocketing. I see people all the time who never had allergies before who are suffering,” said Katherine Shea, MD, MPH, an adjunct professor of maternal and child health at the Gillings School of Global Public Health at the University of North Carolina, Chapel Hill.

Compared to 40 years ago, allergy and asthma season has lengthened by a range of 12 to 20 days in the Northern Hemisphere, says Paul Epstein, MD, MPH, associate director of the Center for Health and the Global Environment at Harvard Medical School. At the same time, the carbon dioxide levels blamed for heating up the earth have gone up 22 percent over the last 50 years. That carbon dioxide acts as a kind of fertilizer, making allergenic plants more numerous and more noxious.

“Unlike several other health consequences we’re already measuring with regard to climate change in the developing world, such as malaria and malnutrition, the allergy problem is affecting the developed world,” says Shea, who co-authored a 2008 study on the health effects of global warming in the Journal of Allergy and Clinical Immunology. “It’s one of the few early problems we’re going to suffer from no matter how rich we are and how good our healthcare is.” To test theories about global warming’s effect on seasonal allergies, scientists from the U.S.

Department of Agriculture (USDA) grew ragweed in an environment that mimicked the levels of carbon dioxide projected for the middle of the 21st century. The plants produced pollen at quadruple the rate compared to ragweed grown at 19th century carbon dioxide levels, and it was significantly more allergenic.

Moving the experiment to the real world garnered similar—but not identical—results. The team planted ragweed in downtown Baltimore because its temperatures and carbon dioxide levels reflect the consequences of global warming. They also found increased pollen counts, though they could not determine if the pollen itself was more allergenic.

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