NO stands for nitric oxide, a gas that is produced by the cells that line the inner wall of the airways. When the airways are inflamed, they release more NO than normal.

The amount of NO in the air breathed out (exhaled) shows how much inflammation there is in the lungs. In other words, a high level of NO means inflammation is present.

Normal cells release a small amount of NO.	Cells in an inflamed airway produce more NO.