Green sputum is often caused by a bacterial infection, though it also may appear several days in to a viral infection. If the green sputum is present from the first onset of symptoms, it is more likely to be the result of bacteria. The two main causes of green sputum are respiratory infections and post nasal drip from sinus infections. Pneumonia, bronchitis and sinusitis are all common causes of green sputum.

Sputum itself is typically made up of mucus secretions from the respiratory system, in addition to drainage from the sinus. Many respiratory infections can result in the inflammation of the mucus membranes, which produces excess mucus. This excess mucus is often expelled from the lungs through coughing, and can become mixed with post nasal drip to become sputum that may be swallowed or spit out. A green color often indicates a bacterial infection within the respiratory system or the sinus, though in some cases the cause may be viral or environmental.

Bronchitis is an inflammation of the mucus membranes within the lungs, and may have viral, bacterial or environmental causes. A small portion of the cases of acute bronchitis are caused by bacteria and will typically produce green phlegm. The chronic form, however, will often have environmental causes and can result in coughing up green sputum several months a year for years at a time. Root causes of chronic bronchitis and the green phlegm it produces can be anything from cigarette smoke to excessive atmospheric pollution.

Another common cause of green sputum is sinusitis. This infection of the sinus will typically result in some form of post nasal drip. A bacterial infection of the sinus will usually cause green discharge, though viral sinusitis may also result in green phlegm. Sinus infections can cause the sinus passageways to become clogged, allowing the mucus to become stagnant. Like bronchitis, sinusitis has both acute and chronic forms. Chronic sinusitis may have environmental or physiological causes, and internal problems with the nasal passages may be at fault.

Any infection or irritation to the upper or lower respiratory system can ultimately result in green sputum. In cases where the onset of illness are accompanied by clear sputum, taking nasal decongestants may help prevent the formation of green phlegm by promoting proper drainage. When a cough is producing green discharge, it is important to avoid taking any medication that suppresses the cough. This may interfere with the natural ability of the body to discharge irritants, and may result in the development of more severe illnesses such as pneumonia.