During normal respiration, air travels through the nose, down the trachea, and into smaller and smaller airways called bronchi. The bronchi divide into bronchioles and finally into tiny grape-like clusters of thin, fragile sacs called alveoli. The alveoli are where oxygen is exchanged for carbon dioxide in the blood. People who work in mines or quarries, or who work with substances such as concrete or glass, are at increased risk for inhaling small particles of silica dust. Silica is a common, naturally occurring crystal and is the principal component of sand. The tiny silica particles travel through the smallest airways into the alveolar sacs. Inside the alveoli, the silica particles are engulfed by immune cells called macrophages, which are designed to destroy foreign organisms in the body with special enzymes. Unfortunately, instead of the macrophages destroying the silica, the silica proves lethal to these immune cells, which release enzymes as they die. When the enzymes are released, they act as an irritant in the lungs and initiate an inflammatory process. When this happens, special cells called fibroblasts move in and begin to deposit fibrous tissue around the particles, forming nodules of scar tissue in the lungs. The buildup of scar tissue eventually reduces the exchange of oxygen in the lungs and causes shortness of breath that progressively worsens. Other symptoms of silicosis can include chest pain and a harsh, dry cough that may produce blood.