Researchers found that the brush-like layer pushes out sticky mucus and the foreign bodies it contains.
Human lungs rely on sticky mucus to expel foreign matter, including toxic and infectious agents during a cold or an allergy that causes a running nose and a wet cough, they said.
The study by Brian Button and colleagues from the University of North Carolina, Chapel Hill, helps to explain how human airways clear such mucus out of the lungs.
The findings may give researchers a better understanding of what goes wrong in many human lung diseases, such as cystic fibrosis (CF), chronic obstructive pulmonary disease (COPD) and asthma, the journal Science reported.
"The air we breathe isn't exactly clean, and we take in many dangerous elements with every breath," said Michael Rubinstein, study co-author, according to a university statement.
"We need a mechanism to remove all the junk we breathe in, and the way it's done is with a very sticky gel called mucus that catches these particles and removes them with the help of tiny cilia, (hairlike projections)," said Rubinstein.
"The cilia are constantly beating, even while we sleep," he said.
"In a coordinated fashion, they push mucus containing foreign objects out of the lungs, and we either swallow it or spit it out. These cilia even beat for a few hours after we die.
"If they stopped, we'd be flooded with mucus that provides a fertile breeding ground for bacteria," added Rubinstein.
Until now, most researchers have subscribed to a "gel-on-liquid" model of mucus clearance, in which a watery "periciliary" layer acts as a lubricant and separates mucus from epithelial cells that line human airways.
But this old explanation fails to explain how mucus remains in its own distinct layer.
