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As food allergies become increasingly common, a new study offers the first proof that they may be linked to pesticides found in tap water.

Researchers at the American College of Allergy, Asthma and Immunology used existing government data to see whether people with more dichlorophenols in their urine were more likely to have food allergies. Dichlorophenols are a kind of chlorine in certain pesticides that are known to kill bacteria, and in theory, they could be killing the naturally occurring bacteria in humans' digestive systems, causing food allergies.

"We wanted to see if there was an association between certain pesticides and food allergies, and we were specifically interested in dichlorophenols because those were the ones that had this antibacterial effect," said lead researcher Dr. Elina Jerschow. "When researchers have compared bacteria from the bowel in healthy kids versus bacteria in the bowel for kids that have lot of allergies, they've noticed a big difference."

The number of children and teens with food or digestive allergies in the United States has increased 18 percent between 1997 and 2007, according to a 2008 study by the Centers for Disease Control and Prevention. That's about 3 million people under 18 years old.

Eggs, fish, milk, peanuts, shellfish, soy, tree nuts, and wheat make up 90 percent of food allergies, according to the CDC report. Symptoms can range from mouth tingling to anaphylaxis, which is the swelling of the throat and tongue and can lead to death.

Jerschow clarified that the researchers were only looking for a statistical association, meaning they were not able to examine patients to see how these chemicals physically caused their allergies. Because it's only an association, these findings could mean that the chemicals caused the food allergies, or it could mean the food allergies caused the chemicals in the urine. That part is not yet clear.

"While the study does not allow concluding that pesticides are responsible for the allergies, it certainly raises the possibility and justifies pursuing the kinds of studies that can help sort of if these pesticides are, indeed, the cause," said Dr. Kenneth Spaeth, who directs the Occupational and Environmental Medicine Center at North Shore University Hospital. He was not a researcher involved in the study.

Spaeth said the study findings fit in with a growing evidence that pesticide exposure can damage the immune system, which could increase allergies as well.

Researchers were surprised to find that dichlorophenol levels in urine didn't vary between urban and rural areas, Jerschow said. They concluded that even those who opted for bottled water instead of tap water could ingest the pesticide chemical from eating fruit, fruit juices and foods with cocoa powder, like chocolate.

As such, Jerschow said the research is still too preliminary to suggest that Americans should change their eating or drinking habits.