NEW YORK - Parents are reporting more skin and food allergies in their children, a government survey found.

Experts are not sure what is behind the increase. Could it be that children are growing up in households so clean that it leaves them more sensitive to things that can trigger allergies? Or are mom and dad paying closer attention?

"We don't really have the answer," said Lara Akinbami of the Centers for Disease Control and Prevention, the senior author of the report released Thursday.

The CDC survey suggests that about 1 in 20 U.S. children have food allergies. That's a 50 percent increase from the late 1990s. For eczema and other skin allergies, it's 1 in 8 children, an increase of 69 percent. It found no rise in hay fever or other respiratory allergies.

Already familiar with the trend in food allergies are school nurses, who have grown busier with allergy-related duties, such as banishing peanuts at school parties or stocking emergency allergy medicine.

Sally Schoessler started as a school nurse in 1992 in New York state and did not encounter a child with a food allergy for a few years. By the time she left school nursing in 2005, "there were children in the majority of classrooms" with the disorder, said Schoessler, who now works at the National Association of School Nurses in Silver Spring, Md.

It's been difficult getting exact numbers for children's allergies, and the new report isn't precise. It uses annual surveys of thousands of adults interviewed in person. The report compares answers from 1997-99 to those from 2009-11.

Parents were asked if - in the previous year - their child had any kind of food or digestive allergy, any eczema or skin allergy, or any kind of respiratory allergy like hay fever.

Researchers did not ask if a doctor had made the diagnosis or check medical records. So some parents may have been stating a personal opinion, and not necessarily a correct one.

But experts also said they believe there is a real increase going on, too.

One of the more popular theories is "the hygiene hypothesis," which says that exposure to germs and parasites in early childhood somehow prevents the body from developing certain allergies.

The hypothesis argues there is a downside to America's culture of disinfection and overuse of antibiotics.

The argument has been bolstered by a range of laboratory and observational studies, including some that have found lower rates of eczema and food allergies in foreign-born children in this country.