Production of green phlegm is usually the result of the body fighting off a sinus or respiratory infection. In fact, it is often an indication that the body is successfully getting rid of the invading virus or bacteria; the green color comes from enzymes in the immune cells that are responding to the disease. The types of infections that cause greenish phlegm can arise for a variety of reasons. Lung diseases like bronchitis or pneumonia may be caused by bacteria, viruses, or fungi. Irritation of the sinuses from dry air or allergens can make them susceptible to attack from various infections. People with asthma are often prone to lung infections due to the irritation in their airways, as are people who smoke.
People suffering from respiratory infections often cough up phlegm as their bodies work to fight and remove the germs. This is very common with upper respiratory infections, which are often caused by viruses. It is also frequently seen with more severe infections such as pneumonia and bronchitis. People with lung cancer will also sometimes produce and expel greenish phlegm.
Green phlegm can also be produced in the sinuses and be expelled from the nose. When the sinuses become irritated and air and mucous cannot move freely through them, they become a breeding ground for viruses or bacteria, often leading to a sinus infection. People with sinus infections will often notice green mucous coming from their noses and also draining down the throat; it should not be swallowed if at all possible, however, as it contains high levels of infectious agents. Sinus irritation is frequently the result of environmental factors; excessive exposure to dry air or to irritants or allergens like smoke, dust, or pollen can all be to blame.
Asthma patients are often susceptible to the types of issues that lead to green phlegm. The condition often irritates and inflames the airways in their lungs, opening them up to attack by pathogens. As asthma is an ongoing issue, people with the condition often have to deal with repeated instances of these types of infections.
Another common cause of the types of infections that create green phlegm is smoking. Inhalation of cigarette smoke irritates the airways in the lungs and leaves small particles in the lungs. This type of damage and buildup weakens them and makes it easier for infection to set in.