U.S. parents are reporting more skin and food allergies for their kids than ever, according to a survey by the Center for Disease Control and Prevention (CDC) - although the cause of the uptick remains uncertain.

"We don't really have the answer," said Dr. Lara Akinbami of the CDC, the senior author of the report released Thursday.

According to the reports from parents, one in 20 children have food allergies, a figure up 50 percent from the late 1990s. Parents report eczema and other skin allergies in one out of every eight children, which is up 69 percent from the late 1990s. Hay fever and respiratory allergies are reported at the same rate.

The survey, which compares parental responses from 1997-1999 with responses from 2009-2011, reports parents' responses to questions asked during in-person interviews. The figures do not take into account medical records or the opinions of doctors.

Experts believe that aside from allergy rate increases that can be attributed to reporting bias, there is also a real, if unexplained, increase in the number of cases of food and skin allergies.

One popular explanation for the rise of food and skin allergies among American children is the "hygiene hypothesis."

The idea is that exposure to germs and parasites early in childhood prevents the body from developing allergies later in life, but because American childhoods are characterized by disinfectants and antibiotics, kids born in the United States may be more prone to develop allergies. This hypothesis is supported by studies that have found lower rates of food allergies and eczema in foreign-born American children.

Another explanation is that big cities, which show higher allergy rates, have air pollutants bombarding kids' skin and acting as an unrecognized trigger, said Dr. Peter Lio, a pediatric dermatologist who specializes in eczema at Northwestern University.

Another hypothesis is that changes in the ways of food growth and production are contributing to increased rates of allergies, but tests have not supported this claim, said Lio.

It used to be that for families with histories of eczema and food allergies, parents were advised to wait for years before introducing certain foods to their children that might cause severe allergic reaction, like peanuts, milk, and eggs. However, a few years ago, professional associations began recommending the opposite approach after new research suggested that allergies were more likely to develop when parents delayed the exposure of certain potential allergens.

It turned out that the conventional wisdom "was exactly the wrong thing to do," said Dr. Morton Galina, a pediatric allergist at Atlanta's Emory School of Medicine.

The CDC report also found that food and respiratory problems are more common among high-income families, whereas eczema and skin allergies were reported as more common among the poor. Additionally, more black children were reporting as having skin problems (17 percent) compared to white children (12 percent) and Hispanic children (10 percent).