Emphysema is the presence of gas in a tissue where there normally isn't any. The gas can be located under the skin, but emphysema is most frequently encountered in the lungs: this is called pulmonary emphysema. Pulmonary emphysema is a condition caused by the expansion and destruction of the terminal bronchioles (the smallest) and the alveoli of the lungs. Smoking, exposure to fumes, or certain chronic diseases of the lungs may be the cause. Emphysema causes more and more pronounced breathing difficulties. There are several types of pulmonary emphysema, such as bullous emphysema, which is characterized by the presence of bubbles of varying sizes and numbers in the lungs, which reduces the volume of gas exchange between the alveoli and the blood.