It may not be obvious unless they snore at night, but the risks of mouth breathing go beyond the noise.
Mouth breathing may seem normal to some parents because there are many children who do it.
It can be caused by stuffy noses from allergies or enlarged tonsils and adenoids.
Drugs may help.
When they don't, ignoring the problem can have consequences for children.
Many of us don't know where the tonsils are.
They're the two pink lumps of tissue on either side of the throat opening.
But, Alexa Reich had very large tonsils that were blocking her breathing.
"She couldn't breathe through her nose and she would snore like an old man. And I thought this is not normal for a 4 year old," said Paola Reich, Alexa's mother.
It's not normal.
Alexa was mouth breathing.
She was always tired and got infection after infection.
Many doctors think kids tonsils will shrink with time, but they don't always.
They can block the nasal airways and cause mouth breathing.
"It's so normal, it's so common to see children with their mouths open, but they really should be able to close their mouth and breath most of the time. They don't do it out of habit, they do it because they can't breathe," said Dr. Linda Dahl, of Lenox Hill Hospital.
Mouth breathers can get sleep apnea, have behavior and learning problems, and even delayed speech.
Mouth breathing as a child can lead to an arched palate at the roof of the mouth, a deviated septum in the nose, and even more breathing trouble as an adult.
You can also have poor growth of the lower jaw and teeth.
Alecia Lopez still has sore throats linked to her enlarged tonsils.
"I do yoga and they say, 'breathe through your nose,' and it's very difficult for me," Lopez said.
Alexa didn't get better with medication, so she had her tonsils removed.
"She's more active and plays now, because she sleeps, before she was always tired," Paola Reich said.
She has more energy to play.
"I can play ball and toys, and yes I can breathe OK," Alexa Reich said.
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