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DAVID GREENE, HOST:

On a Monday, it's MORNING EDITION from NPR News. I'm David Greene.

Today in Your Health, a possible explanation for why girls test better than boys in elementary school. That in a moment. First, new research about how parents react when pacifiers fall out of their babies' mouths.

As NPR's Rob Stein reports, what they do with that dirty pacifier may affect their child's health in surprising ways.

ROB STEIN, BYLINE: Like lots of parents, Amanda Sauer pops a pacifier in her -year-old son Leo's mouth when he gets fussy.

AMANDA SAUER: He can't sleep without it, so at some point we're going to have to wean him. But we're not ready for that yet.

STEIN: Like a lot of kids, Leo sometimes drops the pacifier out of his mouth and onto the ground. The question is: What does his mom do when that happens?

SAUER: It depends on how desperate the situation is. It could be the three-second rule, the 10-second rule, or we put it away and try to find a drinking fountain or something to wash it. Unless it fell somewhere particularly gross and then we'll go home and, you know, boil some water.

STEIN: But there's a theory that says: That may not be the best way to go. That sterilizing that pacifier may actually have a big downside. To try to find out, Bill Hesselmar, of the University of Gothenburg in Sweden, and his colleagues, studied 184 babies who used pacifiers and their parents.

BILL HESSELMAR: We asked them how they cleaned the pacifier, if they rinsed them in water, and of course most of them did.

STEIN: But a lot of the parents did something else.

HESSELMAR: They sucked on it.

STEIN: That's right, they sucked on it.

HESSELMAR: They put it in their mouth, sucked on it, and then gave it back to the children. It's a quite common way to clean pacifiers.

STEIN: The researchers then kept checking back to see if there were any differences between the kids whose parents licked their pacifiers clean, and those who didn't. And in this week's issue of the journal Pediatrics, they report a striking difference.

HESSELMAR: Those parents who sucked on their child's pacifier and then gave it back to the infant, these infants had less eczema.

STEIN: A lot less eczema. And they were also less likely to develop another condition caused by allergic reactions: Asthma. Hesselmar says scientists think they may know why. It's because parents expose their kids to bacteria, harmless bacteria, we all carry around in our mouths.

HESSELMAR: We think that these bacteria that are transferred from the parents to the child, they stimulate the immune system.

STEIN: And stimulating the immune system basically teaches it how to do its job and to not overreact to stuff like pollen, dust, peanuts. The study adds to a growing body of evidence, that lots of kids these days may be growing up essentially too clean.

Elizabeth Matsui is a pediatric immunologist at Johns Hopkins.

DR. ELIZABETH MATSUI: We are much less likely to be exposed to organisms in water, to parasites for example. So the idea is there is much less for the immune system to fight off. So it starts reacting to things that perhaps it should be ignoring.

STEIN: For her part, Amanda Sauer is not sure the new research will make her start licking Leo's dirty pacifier.

SAUER: Probably not.

(LAUGHTER)

SAUER: But it's more for just the fact that I don't really want to put a pacifier in my mouth. But sometimes the dog cleans it off for us. So maybe that's just as good.

(LAUGHTER)

STEIN: Now, no one thinks this one study is enough to recommend all parents start licking their kids pacifiers. But there's no reason to think it can hurt, either.

Rob Stein, NPR News.

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