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. People who work in mines or quarries, or who work with materials such as insulation or fireproofing, are at increased risk for inhaling small particles of asbestos dust. The tiny asbestos particles travel through the smallest airways into the alveolar sacs. Inside the alveoli the asbestos 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 asbestos, the asbestos proves to be 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 build-up of scar tissue eventually reduces the exchange of oxygen in the lungs and causes shortness of breath that progressively worsens. Other symptoms of asbestosis can include chest pain and a harsh, dry cough that may produce blood.