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updated 5/27/2011 1:24:48 PM ET 2011-05-27T17:24:48

Allergies are increasing in America, and a large portion of the overall rise may be attributed to an uptick in ragweed and mold allergies, a new study suggests.

The findings confirm previous research that has shown allergies in the United States are on the rise and provide more evidence to suggest global climate change may partly explain the hike, the researchers say. Both ragweed and mold are environmental allergens that may be influenced by changing global temperatures.

"We believe this is the first large national study to show that the growing prevalence of allergies, suggested by other studies, is largely due to increases in environment-based allergens previously associated with climate change," said Dr. Stanley J. Naides, medical director of immunology at Quest Diagnostics, the diagnostic testing company that conducted the study. "Given concerns about a warming climate, additional research is needed to confirm these findings and assess the possible implications for public health."

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Other experts agree climate change may play a role in the allergy increase. But whether this particular study actually provides evidence for the theory is a different story.

The researchers are saying allergies and global temperatures have both increased together, that there is an association between them, said Dr. Jacqueline S. Eghrari-Sabet, an allergist at Family Asthma & Allergy Care in Gaithersburg, Md. But you would also find an association between the increase in allergies and the economic collapse, she said. "But is there a link [there]? No."

Global warming is just one of several possible explanations for the allergy rise, she and other experts say.

Allergies in America
An allergy is a reaction of your immune system to what are usually harmless, common substances, such as pollen, cat hair or dust. An antibody known as IgE binds to the offending substance, called an allergen. This binding triggers a chain reaction that ultimately results in allergy symptoms, including sneezing, wheezing and coughing.

The new study is based on 14 million allergy blood test results from 2 million patient visits (some patients may have been tested more than once.)

The blood tests looked to see whether individuals had IgE antibodies in their blood that would bind to a particular substance. The 11 most common allergens tested included: egg whites, milk, peanut, soybean, wheat, common ragweed, mold, two types of house dust mites, cat skin and dog dander.

Between 2005 and 2008, the number of people found to be allergic, or sensitive to, at least one of 11 substances increased nearly 6 percent. The number of people who were sensitive to ragweed increased 15 percent and the number who were sensitive to mold increased 12 percent, the researchers said. [See 9 Weirdest Allergies ]

Not all allergy symptoms increased. For instance, sensitivity to dust mites declined over the four-year study period.

Eghrari-Sabet says the findings are true only of the particular population in the study, that is, people who were referred by their doctors to get an allergy test. Also, it's not clear whether some people in the study were simply tested more often for certain allergens. If people were tested more often for ragweed allergy than a dust mite allergy, it might look like more people are allergic to ragweed, she said.

Finally, the presence in the bloodstream of IgE antibodies to, say, pollen, indicates that you have the potential to be allergic to pollen, but you may not show symptoms unless you are exposed to large amounts of it. Thus, it's not clear whether participants actually exhibited allergy symptoms to the substances they were sensitive to.

Other research has noted a rise in food allergies. From 1997 to 2007, the number of children with food allergies rose 18 percent, according to the Centers for Disease Control and Prevention.

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Global warming
Rises in carbon dioxide levels in the atmosphere along with the associated climb in global temperature create ideal growing conditions for plants. Studies have found the growing season for some plants, including ragweed, has increased in recent years. For instance, a study published in the March issue of the journal Proceedings of the National Academy of Sciences found the blooming season for ragweed was a month longer in 2009 than it was in 1995 in some parts of the country.

A prolonged blooming season may exacerbate allergies: Studies suggest increased exposure to ragweed may increase the risk of developing more severe allergies to the plant.

The prevalence of mold may also be affected by changes in rain patterns, another side effect of global warming. More mold may explain the rise in mold sensitivity seen in the new study.

Other theories for the increase include the "hygiene hypothesis," or the idea that extreme cleanliness in developed countries has made people's immune systems more sensitive to benign substances.

Also, it's possible the rise is due to more people being diagnosed with allergies. This happened with asthma when doctors started to recognize the disease as a distinct condition. "People were just as sick as they ever were," Eghrari-Sabet said. "They just had the right diagnosis."

But more diagnosis alone can't explain the whole allergy increase, particularly the rise in food allergies, Eghrari-Sabet explained in an interview last year.

These hypotheses are all hard to prove and so the real reason for the allergy increase remains a mystery, she said.

Pass it on: Ragweed and mold allergies may play a large role in America's increase in allergies. Global warming may contribute to the allergy rise, but it's unlikely to be the only explanation.

9 Weirdest Allergies
Americans Sneeze More As Allergies Mysteriously Increase
10 Surprising Results of Global Warming

Follow MyHealthNewsDaily staff writer Rachael Rettner on Twitter @RachaelRettner. 

Video: Alleviate allergies with simple fixes

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