Chronic obstructive pulmonary disease (COPD) is a type of obstructive lung disease characterized by long-term breathing problems and poor airflow. The main symptoms include shortness of breath and cough with sputum production. COPD is a progressive disease meaning it typically worsens over time. Eventually everyday activities such as walking or getting dressed become difficult. Chronic bronchitis and emphysema are older terms used for different types of COPD. The term "chronic bronchitis" is still used to define a productive cough that is present for at least three months each year for two years. Tobacco smoking is the most common cause of COPD with factors such as air pollution and genetics playing a smaller role. In the developing world one of the common sources of air pollution is poorly vented heating and cooking fires. Long-term exposure to these irritants causes an inflammatory response in the lungs resulting in narrowing of the small airways and breakdown of lung tissue. The diagnosis is based on poor airflow as measured by lung function tests. In contrast to asthma the airflow reduction does not improve much with the use of a bronchodilator.