People cough up blood when something damages the blood vessels inside the lungs or airways, allowing blood to leak into the respiratory tract. The most common causes are respiratory infections, bronchitis, and bronchiectasis, though lung cancer, blood clots, and tuberculosis can also be responsible. In 20% to 50% of cases, no specific cause is ever identified.
Where the Blood Comes From
Your lungs have two separate blood supplies. The pulmonary arteries carry blood at low pressure to exchange oxygen and carbon dioxide. A smaller set of vessels, the bronchial arteries, branch off the body’s main artery (the aorta) and supply the airways themselves with oxygen-rich blood at much higher pressure. Less than 5% of the lung’s blood flows through these bronchial arteries, but they are the source of most bleeding when someone coughs up blood.
Because bronchial arteries operate at high pressure, damage to them can produce significant bleeding. Infections, chronic inflammation, or tumors can erode into these vessels over time. Less commonly, the low-pressure pulmonary arteries bleed when they’re damaged by trauma, a blood clot, or a tumor growing into the vessel wall.
The Most Common Causes
Respiratory infections are the single most frequent reason. In outpatient studies, infections account for roughly 19% of cases. Bronchitis, pneumonia, and other acute lung infections inflame the airways enough to rupture small blood vessels, producing blood-streaked mucus that typically resolves as the infection clears.
Bronchiectasis, a condition where the airways become permanently widened and scarred, leads to mucus buildup that’s often streaked with blood. It raises the risk of repeated infections, each of which can trigger more bleeding. COPD, including emphysema, is another common culprit, particularly in people with a long smoking history.
Lung cancer accounts for about 4% of outpatient cases but jumps to nearly 18% among hospitalized patients, reflecting how the severity of bleeding tends to be greater. Smokers over 40 with a history of 30 or more pack-years are at highest risk and generally need more extensive testing. Tuberculosis, while less common in developed countries (about 0.2% of outpatient cases), remains a major cause worldwide. A pulmonary embolism, where a blood clot lodges in a lung artery, causes roughly 1 to 3% of cases depending on the setting.
Less Common but Serious Causes
Certain autoimmune diseases attack the blood vessels in the lungs directly. A group of conditions called ANCA-associated vasculitis causes inflammation of blood vessels throughout the body, and in the lungs this can trigger diffuse alveolar hemorrhage, where bleeding occurs across large areas of lung tissue rather than from a single spot. In one study of 80 patients with this type of hemorrhage, over 96% had coughing up blood as a symptom.
Other rare causes include a narrowed mitral valve in the heart (which raises blood pressure in the pulmonary veins), chest injuries, cocaine use, inhaled foreign objects, and parasitic infections. Even blood-thinning medications or clotting disorders can cause bleeding into the airways. Among hospitalized patients, anticoagulation-related bleeding accounts for about 13.5% of cases.
How to Tell It’s Coming From the Lungs
Not all blood that comes up when you cough originates in the lungs. Blood from the stomach (vomited up during a cough) or from the nose and throat dripping down can look similar. A few features help distinguish them.
Blood from the lungs is typically bright red and frothy, mixed with sputum, and alkaline. Blood from the stomach tends to be darker, sometimes resembling coffee grounds, may contain food particles, and is acidic. If you’re coughing up blood and unsure of the source, the color and texture are the most useful clues before any testing is done.
When Coughing Up Blood Is an Emergency
A small amount of blood-streaked mucus during a bad chest cold is common and often resolves on its own. But the amount and context matter enormously. Doctors no longer define an emergency purely by a set volume of blood. Instead, any amount of coughed-up blood that causes difficulty breathing, drops in oxygen levels, or signs of circulatory problems (lightheadedness, rapid heart rate, low blood pressure) is treated as a life-threatening situation.
Other warning signs that warrant urgent evaluation include blood that keeps coming back over several days, large clots, blood without an obvious infection to explain it, unexplained weight loss, or a significant smoking history. Even a single episode in someone over 40 with risk factors for lung cancer should prompt medical workup.
How Doctors Find the Source
A chest X-ray is the standard first step. It can reveal infections, masses, or areas of fluid that point toward a cause. If the X-ray looks normal but you have risk factors for cancer (age 40 or older, heavy smoking history), a CT scan is typically the next step. CT imaging provides much more detail and can detect small tumors, abnormal blood vessels, or clots in the pulmonary arteries.
Bronchoscopy, where a thin camera is passed through the nose or mouth into the airways, is reserved for cases where imaging doesn’t give a clear answer or when the bleeding is active and needs to be located precisely. In fast or heavy bleeding, a rigid bronchoscope can keep the airway open while doctors identify and manage the source.
When all three tests (X-ray, CT, and bronchoscopy) come back without a clear cause, the condition is called cryptogenic hemoptysis. These patients have a low risk of an underlying cancer and are typically monitored over three years. If bleeding returns during that time, more advanced imaging of the bronchial arteries is usually the next move.
What the Cause Means for Outlook
For most people, especially those with infection-related bleeding, the outlook is excellent. The bleeding stops once the infection is treated. Chronic conditions like bronchiectasis or COPD may cause occasional blood-streaked sputum over months or years without posing an immediate danger, though they need ongoing management.
The prognosis is more serious when the underlying cause is cancer, a pulmonary embolism, or an autoimmune condition causing widespread lung hemorrhage. Massive or rapid bleeding, regardless of cause, carries significant risk because the main danger isn’t blood loss itself but the airway flooding with blood, which blocks oxygen exchange. Quick airway control and identifying the bleeding vessel are the priorities in those situations.