Phlegm is a thick type of mucus produced in the lower respiratory tract, including the lungs and airways. Its purpose is to trap inhaled foreign particles, bacteria, and irritants, preventing them from penetrating deeper into the lungs. When this defense mechanism clears the respiratory system, the expelled phlegm, also known as sputum, can take on various colors. Brown discoloration in this substance signals underlying processes that warrant attention to determine the cause.
The Role of Oxidized Blood
The most frequent explanation for brown, rust-colored, or coffee-ground-like spots in phlegm is the presence of old, oxidized blood. Fresh blood is bright red because the iron in hemoglobin is in a ferrous state, which readily binds oxygen. When blood remains in the airways, this iron is exposed to air and undergoes oxidation.
This oxidation process transforms the iron, converting hemoglobin into a brownish compound called methemoglobin. This chemical change is similar to how iron rusts. The resulting dark brown or rusty shade indicates the blood has dried or lingered in the respiratory passages before being expelled.
Minor bleeding causing this discoloration often results from forceful or prolonged coughing, which irritates the tiny blood vessels lining the airways. Irritations in the upper respiratory tract can also introduce small amounts of blood that oxidize as they mix with the phlegm. While a small amount of old blood may not be alarming, its presence indicates an injury or irritation somewhere along the respiratory tract.
Environmental and Inhaled Particulates
Brown phlegm can also result from the respiratory system trapping and eliminating non-biological, external materials. The body’s defense system catches foreign particulates inhaled deep into the lungs. If these particles are dark, they subsequently stain the phlegm a brown or black color as they are coughed up.
Heavy exposure to cigarette smoke is a common cause, staining the phlegm with inhaled tar and nicotine residues. Individuals exposed to high levels of air pollution, wildfire dust, or occupational dusts, such as coal or mineral dust, may also notice a brown or gray-brown discoloration. This discoloration is a physical sign that the lungs are actively clearing accumulated inhaled debris.
Chronic occupational exposure can lead to particulate-laden phlegm, known as pneumoconiosis, such as “black lung disease” from coal dust. In these instances, the brown spots are not biological but represent the physical matter the respiratory system is attempting to expel.
Brown Phlegm Associated with Infection
In some instances, brown or rust color indicates a specific infection, rather than old blood or inhaled irritants. Certain bacterial pneumonias produce a characteristic color that clinicians look for. For example, infection with Streptococcus pneumoniae often results in rust-colored sputum, a classic sign of the disease.
Pneumonia caused by Klebsiella pneumoniae produces a thick, gelatinous, dark-brown sputum often described as “currant jelly” sputum. This distinct color and texture are caused by the bacteria’s capsule, tissue necrosis, and inflammatory cellular debris, including dead white blood cells. The inflammatory response in the lung tissue causes damage, leading to this characteristic appearance.
Chronic conditions can also produce persistent brown phlegm, including bronchiectasis (permanent widening of the airways) or a lung abscess (a pus-filled cavity). In these cases, the color results from stagnant, purulent secretions and the breakdown of inflammatory cells and tissue within the damaged areas.
When to Consult a Healthcare Professional
While brown phlegm can be a temporary sign of clearing old debris, it can also signal a significant underlying health issue. It is prudent to monitor the symptom and seek medical advice if the brown phlegm is a new development or persists for more than a few days.
Consult a healthcare professional immediately if the brown phlegm is accompanied by concerning symptoms. Warning signs include a high fever or chills, suggesting an active infection. Sudden onset of shortness of breath, chest pain, or wheezing also indicate that an evaluation of lung function is necessary.
Other signs that warrant prompt medical attention are coughing up large volumes of phlegm, unexplained weight loss, or persistent fatigue. If the brown phlegm contains frank red blood or has a foul odor, it may indicate a severe infection, such as a lung abscess, or another condition that requires immediate diagnosis and treatment.