Coughing up blood, known medically as hemoptysis, happens when blood vessels in your airways or lungs are damaged or inflamed. The most common causes in developed countries are acute bronchitis, bronchiectasis (chronic airway damage), and lung tumors. While a small streak of blood in your mucus after a bad cough is often linked to minor irritation, larger amounts or repeated episodes can signal something more serious that needs prompt evaluation.
The blood typically looks bright red and may appear frothy because it’s mixed with air from your lungs. This distinguishes it from vomiting blood, which tends to be darker red and may contain food particles. If you’re unsure whether blood came from your lungs or your stomach, that distinction matters for getting the right workup.
The Most Common Causes
Respiratory infections are the single most frequent reason people cough up blood. In outpatient settings, infections account for roughly 19% of cases, and that number rises to about 23% among hospitalized patients. Acute bronchitis is the most typical culprit: inflammation in the airways makes the lining fragile, and forceful coughing can rupture small blood vessels. Pneumonia and lung abscesses can do the same, often producing blood-streaked or rust-colored sputum alongside fever and chest pain.
Bronchiectasis, a condition where the airways become permanently widened and scarred, is another leading cause. The damaged airway walls develop abnormal blood vessels that bleed more easily. About 15% of hemoptysis cases in one large study of over 600 patients were traced to bronchiectasis.
Lung cancer accounted for roughly 4% of outpatient cases but jumped to nearly 18% among hospitalized patients in a 2022 analysis from the American Academy of Family Physicians. That difference reflects the fact that people with cancer-related bleeding often have larger or more persistent episodes that require admission. A history of heavy smoking, unexplained weight loss, or ongoing fatigue alongside bloody coughs raises the concern for a tumor.
Blood Thinners and Clotting Problems
If you take anticoagulant medication, that can contribute to or worsen bleeding in the lungs. Coagulation-related causes barely register in outpatient statistics (under 1%), but they account for about 13.5% of inpatient hemoptysis cases. This doesn’t mean blood thinners directly cause the bleeding. More often, they amplify bleeding from an existing source, like an inflamed airway or small lesion, that might have gone unnoticed otherwise.
Blood Clots in the Lungs
A pulmonary embolism, where a blood clot travels to the lungs, can cause you to cough up blood. About 10% of people with a pulmonary embolism develop bleeding because the clot blocks a branch of the pulmonary artery, killing a small patch of lung tissue. The classic combination is sudden sharp chest pain that worsens with breathing, shortness of breath, and blood-tinged sputum. Pulmonary embolism shows up in roughly 3% of hospitalized hemoptysis cases.
Less Common but Serious Causes
Autoimmune conditions can attack the tiny blood vessels in the lungs, causing what’s called diffuse alveolar hemorrhage, where blood leaks into the air sacs. The most common immune-related cause involves a group of conditions where the immune system targets small blood vessels throughout the body. These include granulomatosis with polyangiitis and microscopic polyangiitis. Another condition, anti-GBM disease (formerly Goodpasture syndrome), targets the lungs and kidneys simultaneously. These are rare but can be life-threatening without treatment.
Heart failure can also cause bloody sputum. When the heart can’t pump efficiently, pressure builds up in the blood vessels of the lungs, and fluid (sometimes tinged with blood) leaks into the airways. Cardiac causes represented about 4.6% of inpatient hemoptysis cases.
Fungal infections, particularly aspergillosis, can grow inside damaged lung cavities and erode into blood vessels. This is more common in people with prior tuberculosis, chronic lung disease, or weakened immune systems.
Causes in Children
When children cough up blood, the cause list looks different. Acute lower respiratory infections account for about 40% of cases. Foreign body aspiration, where a child inhales a small object that lodges in the airway, is another leading cause and happens most often in children under 4. Cystic fibrosis is a significant contributor as well: roughly 1% of people with cystic fibrosis experience mild hemoptysis each year, and about 5% will have a severe episode at some point, usually after age 18. Tuberculosis, by contrast, rarely causes bleeding in children (under 1% of pediatric TB cases).
When No Cause Is Found
In 20% to 50% of hemoptysis cases, no clear source is identified even after imaging and direct examination of the airways. These are classified as cryptogenic, meaning the bleeding likely came from a minor source that healed on its own or was too small to detect. A single episode of blood-streaked sputum during a bad cold, for instance, often falls into this category. If the bleeding doesn’t return and imaging looks normal, the prognosis is generally good.
Warning Signs That Need Urgent Attention
Not all bloody sputum carries the same urgency. Certain patterns raise the stakes significantly:
- Large volume of blood: Roughly half a cup (about 150 mL) in 24 hours is considered life-threatening and requires emergency care.
- Shortness of breath at rest or decreased breath sounds on one side of the chest.
- Unexplained weight loss, fatigue, or malaise lasting weeks, especially in someone who smokes or formerly smoked.
- Back pain accompanying the bleeding, which can suggest a vascular or structural problem.
- Recurrent episodes even if each one seems small.
How the Cause Is Found
Doctors typically start with a chest X-ray to look for obvious masses, fluid, or signs of infection. If the X-ray doesn’t explain the bleeding, or if the episode is severe, a CT scan provides much more detail and can reveal tumors, blood clots, bronchiectasis, or vascular abnormalities. In some cases, a bronchoscopy is used, where a thin camera is passed through the nose or mouth into the airways to find the exact bleeding site. The choice and order of these tests depends on how much blood you’ve lost and how quickly it’s happening.
How Severe Bleeding Is Treated
For minor hemoptysis caused by an infection, treating the underlying infection with appropriate medications is usually enough, and the bleeding stops as the inflammation resolves.
When bleeding is heavy or keeps coming back, a procedure called bronchial artery embolization is considered the first-line treatment. A radiologist threads a thin catheter through an artery to the bleeding site and injects material that blocks the vessel. The technical success rate is about 97%, and clinical success (meaning the bleeding actually stops and stays stopped in the short term) is around 93%. The procedure carries a very low risk of major complications, at roughly 0.1%.
The main limitation is recurrence. About 25% of patients experience bleeding again after the procedure. Recurrence rates are higher in people with cancer compared to those with benign conditions, and combining the procedure with ongoing medication significantly improves outcomes. In one comparison, the recurrence rate at 30 days was about 12% with the combination approach versus 45% with medication alone.