The color of the mucus that is coughed up from the lower airways (sputum) may vary in color from clear to yellow to green.

The granules are small enough to be able to enter the lower airway.

It has subsequently been shown that the lower airways contribute most of the exhaled NO, and that contamination from the sinuses is minimal.

Sputum is mucus that is coughed up from the lower airways.

Breathing in these irritants may make the respiratory system less effective at removing organisms from the lower airways.

Smoking or breathing secondhand smoke makes the respiratory system less effective at removing organisms from the lower airways.

Proximal divisions (those closest to the top of the tree, such as the bronchi) mainly function to transmit air to the lower airways.

The mechanism, by which they persist in the lower airways in healthy state, is unknown.

It can be broadly classified into being either in the upper airway or lower airway.

From there the mucous, the trapped particles from the lower airways can be coughed up (this is called sputum).