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Allergy Risks by Region of the U.S. continued...

People's sensitivities are very different, too. For example, one person may be allergic to tree pollen. Another person may be allergic to grass pollen. Your allergies react to the plants that surround you, no matter the region of the country.  Nonetheless, here are allergy triggers to look for, region by region.

The Midwest

The Midwest, known for its ragweed pollen, has several cities on the list including Dayton, Ohio, and St. Louis. Some experts believe global warming is making the ragweed season longer, Wolbert says, so pollen may just get worse.

''Ragweed thrives with higher carbon dioxide,'' he says. So the more air pollution, the hardier ragweed becomes. "I think ragweed will continue to worsen every year," he says.

Global climate change also appears to increase ragweed – and allergic disease – according to recent studies in medical journals, including the Journal of Allergy and Clinical Immunology.

In the Midwest -- Illinois, Indiana, Ohio, Minnesota, Wisconsin, Michigan, Iowa, Missouri -- tree pollen season is roughly March to June. Trees that typically trigger allergies include elder, alder, birch, oak, elm, and hickory, Wolbert says.

Overlapping the tree pollen season, grasses start to pollinate in the summer, Wolbert says. Grasses that can provoke allergy symptoms include Bermuda, Timothy, fescue, rye, and orchard grass.

Weeds pollinate in the fall, says Wolbert. "Weed season is pretty uniform," he says. The Midwestern states are known for lamb's quarter weed, pigweed, Russian thistle, and others.

The West

In Washington, Oregon, and California, tree pollination is usually in full swing from February to June.  Trees that are native cause the most allergy problems, says Richard W. Honsinger, MD, an allergist in Los Alamos, N.M., and a clinical professor of medicine at the University of New Mexico School of Medicine.

For instance, in the Pacific Northwest, pollen from the native alder and birch trees can make people with allergies miserable, he says.

In California, oak and walnut trees can be problematic for those with allergies, he says. Pine trees don't deserve their bad reputation, he says. "People always think pine pollen causes problems because they see it." When they park their car under pines in the mountain, he says, the car can be covered with the pollen.

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