Parents are reporting more skin and food allergies in their children, a big government survey found.Experts aren’t sure what’s 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 to rashes and reactions, and more likely to call it an allergy?“We don’t really have the answer,” said Dr. Lara Akinbami of the Centers for Disease Control and Prevention, the senior author of the new 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 increase, however, 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, like banishing peanuts at school parties or stocking emergency allergy medicine.Sally Schoessler started as school nurse in 1992 in New York state, and didn’t encounter a child with a food allergy for a few years. But 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.Food allergies tend to be most feared; severe cases may cause anaphylactic shock or even death from eating, say, a peanut. But many food allergies are milder and something children grow out of. Skin conditions like eczema, too, can be mild and temporary.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 1997-1999 to those from 2009-2011.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.The 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.“We see a lot of kids in clinic that really aren’t” allergic to the foods their parents worry about, said Dr. Morton Galina, a pediatric allergist at Atlanta’s Emory School of Medicine.For example, hives are sometimes blamed on a certain food when a virus was the actual cause, he added.But experts also said they believe there is a real — and unexplained — 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 that 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 the U.S.There could be other explanations, though. Big cities have higher childhood allergy rates, so maybe some air pollutant is the unrecognized trigger, said Dr. Peter Lio, a Northwestern University pediatric dermatologist who specializes in eczema.Some suspect the change has something to do with the evolution in how foods are grown and produced, like the crossbreeding of wheat or the use of antibiotics in cattle. But Lio said tests haven’t supported that.Emory’s Galina said the new CDC statistics may reflect a recent “sea change” in the recommendations for when young children should first eat certain foods.In families with a history of eczema or food allergies, parents were advised to wait for years before introducing their young children to foods tied to severe allergies, like peanuts, milks and eggs. But professional associations changed that advice a few years ago after research suggested that allergies were more likely in those kids when the foods were delayed.The old advice “was exactly the wrong thing to do,” and could have contributed to some of the increased cases, Galina said.The CDC report also found:• Food and respiratory allergies are more common in higher-income families than the poor,• Eczema and skin allergies are most common among the poor.• More black children have the skin problems, 17 percent, compared to 12 percent of white children and about 10 percent of Hispanic children.The mother of a 13-year-old girl, who is black, runs an eczema support group in suburban Washington, D.C. Renee Dantzler says roughly half the families in her group are African-American. Eczema is an itchy skin condition, which often occurs on the arms or behind the knees. The cause isn’t always clear.Her daughter, Jasmine, started getting rashes at 6 months and got much worse when she was 4.“Her whole body would flare. If she ate something, you would kind of hold your breath,” Dantzler said. “And she’s allergic to every grass and tree God made.”Her daughter took to wearing long sleeves and pants, even in hot weather, so people wouldn’t see her skin scarred — and whitened in spots — from scratching. She began to improve about four years ago with steroid creams and other treatments and has gradually become less self-conscious about her skin, Dantzler said.She’s now on a school track team, which means wearing shorts.“She’s the only one on the team with long socks,” her mom said.