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Nasal passage is a channel for airflow through the nose. Its walls  are coated with respiratory mucous membranes, which contain innumerable tiny hair-like cells that move waves of mucus toward the throat. Dust, bacteria, and other particles inhaled from the air are trapped by the mucus in the nose, carried back, swallowed, and dropped into the gastric juices so that any potential harm they might do is nullified.