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Allergy Risks by Geography continued...

In the middle of Mississippi, everything sits, Wolbert says, so pollen is likely to be worse.

Plants around river basins vary in different parts of the country. For instance, in the lower Mississippi, ragweed and chenopods thrive, Wolbert says.

In the Mountains

In the mountains, there are fewer plants, Wolbert says, perhaps explaining why some mountainous states are absent from the list. "The higher the mountains, the fewer the plants,'' Wolbert says, resulting in less pollen overall.

And pollen from evergreens is typically heavy pollen, so it falls to the ground relatively quickly, he says. It poses less of an allergy problem simply because it is airborne for a shorter time.

Near the Coast

While some seaside towns made the list, in general a sea breeze helps reduce allergens, Leftwich says. The closer to the sea the better. "If you can afford to live in that first quarter mile from the beach, it's great. Pollens are not so much a problem there."

In coastal areas that are densely populated, however, the pollution can make allergies worse despite the sea breeze, Wolbert says. One exception: In Miami, he says the sea breeze is strong enough to reduce pollen-triggered allergies, despite the population.

Allergy Risks by Region of the U.S.

It's difficult to pick out one region of the country as "better" or "worse" for allergies, according to the experts. Why? Even within a region the trees, grasses, and weeds that typically provoke allergies can differ.

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.

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