Emphysema is a chronic lung disease. It is often caused by exposure to toxic chemicals or long-term exposure to tobacco smoke. Emphysema is characterized by loss of elasticity of the lung tissue; destruction of structures supporting the alveoli; and destruction of capillaries feeding the alveoli. The result is that the small airways collapse during expiration, leading to an obstructive form of lung disease (airflow is impeded and air is generally "trapped" in the lungs in obstructive lung diseases). Symptoms are: shortness of breath on exertion - typically when climbing stairs or inclines (and later at rest), hyperventilation and an expanded chest. As emphysema progresses, clubbing of the fingers may be observed, a feature of longstanding hypoxia.