Bronchitis is an inflammation of the bronchi of the lungs (medium to large respiratory tract). Symptoms include expectoration, wheezing, shortness of breath and chest discomfort. Bronchitis can be acute or chronic. Acute bronchitis is mostly caused by viral infection. Other factors include smoking, dust and air pollution. A small number of symptoms are directly caused by high levels of air pollutants or bacteria. Chronic bronchitis lasts for two years or more and has sputum cough for more than three months of a year. Chronic bronchitis is often accompanied by other symptoms, such as cough and mucus. In addition, the patient may also have difficulty in breathing and feel pain in some body parts (such as the chest or muscles).