If you are exposed to high levels of asbestos dust over a long period of time, some of the airborne fibers can become lodged within your alveoli — the tiny sacs inside your lungs where oxygen is exchanged for carbon dioxide in your blood. The asbestos fibers irritate and scar lung tissue, causing the lungs to become stiff. This makes it difficult to breathe.
As asbestosis progresses, more and more lung tissue becomes scarred. Eventually, your lung tissue becomes so stiff that it can't contract and expand normally.
Smoking cigarettes appears to increase the retention of asbestos fibers in the lungs, and often results in a faster progression of the disease.