Coughing up blood, called hemoptysis, is most often caused by respiratory infections like bronchitis and pneumonia. These two conditions alone account for roughly a third of all cases. But the list of possible causes is long, ranging from a simple chest cold irritating your airways to serious conditions like lung cancer or blood clots in the lungs. The amount of blood matters: a few streaks in your mucus after a bad coughing fit is very different from coughing up spoonfuls of bright red blood.
Respiratory Infections Are the Most Common Cause
A large study of 606 patients across five Italian hospitals found that pneumonia and lung abscesses caused 18.6% of cases, while acute bronchitis accounted for another 13.7%. Together, that’s roughly one in three people who cough up blood. The mechanism is straightforward: inflammation and forceful coughing damage the tiny blood vessels lining your airways. You might see pink-tinged mucus or small streaks of blood mixed in with phlegm. This type of bleeding usually resolves once the infection clears.
Tuberculosis, though less common in many countries, is still responsible for about 5% of hemoptysis cases worldwide. It tends to cause more persistent or recurring episodes of bloody sputum compared to a typical chest infection.
Chronic Lung Conditions
Bronchiectasis, a condition where the airways become permanently widened and damaged, was the third most common cause in that same study at 14.9% of cases. The damaged airways develop enlarged blood vessels (bronchial arteries) that are fragile and prone to bleeding. Research shows that the wider these arteries grow, the more frequently bleeding episodes occur. People with cystic fibrosis are particularly susceptible because it causes progressive bronchiectasis over time.
Chronic obstructive pulmonary disease (COPD) and severe asthma can also produce blood-streaked sputum, especially during flare-ups when coughing is intense enough to rupture small blood vessels in inflamed airways.
Lung Cancer
About 20% of hemoptysis cases are caused by lung cancer, making it one of the more common causes, particularly in smokers and people over 40. Around 15% of people with tumors growing inside the airways experience hemoptysis as one of their symptoms. In 3% of lung cancer cases, the bleeding becomes massive and life-threatening.
The bleeding happens when a tumor erodes into blood vessels as it grows. What makes lung cancer hemoptysis concerning is that it can appear as the first noticeable symptom, sometimes before any other warning signs like weight loss, persistent cough, or chest pain. Recurring episodes of unexplained bloody sputum, especially in current or former smokers, should always be evaluated promptly.
Blood Clots in the Lungs
A pulmonary embolism, where a blood clot lodges in the lung’s blood vessels, causes hemoptysis in about 12% of cases. The bleeding occurs because blocked blood flow damages lung tissue. It typically comes with other symptoms: sudden shortness of breath, sharp chest pain that worsens when you breathe in, rapid heartbeat, and sometimes lightheadedness.
People with chronic heart failure are more prone to this type of bleeding because prolonged fluid buildup in the lungs makes the tissue more vulnerable. Younger patients (under 48) and those with autoimmune diseases also have higher risk of bleeding when a clot is present.
Blood-Thinning Medications
If you take anticoagulants (blood thinners) for conditions like atrial fibrillation or a history of blood clots, these medications can increase your risk of hemoptysis. This applies to older medications like warfarin and newer ones like rivaroxaban, apixaban, and dabigatran. The drugs don’t cause the bleeding directly. Instead, they make it harder for your body to stop bleeding that starts from another cause, turning what might have been invisible spotting into noticeable bloody sputum. If you’re on blood thinners and start coughing up blood, that warrants a call to your prescriber, not a decision to stop taking the medication on your own.
Less Common Causes
Several other conditions can produce hemoptysis:
- Chest trauma: A blow to the chest, rib fractures, or even very forceful coughing can tear small blood vessels in the lungs.
- Autoimmune diseases: Conditions like granulomatosis with polyangiitis or Goodpasture syndrome can cause bleeding deep in the lung tissue, called diffuse alveolar hemorrhage.
- Heart problems: Severe mitral valve disease or heart failure can raise pressure in lung blood vessels enough to cause bleeding.
- Foreign body aspiration: Particularly in children, inhaling a small object can irritate or damage airways and trigger bleeding. In kids, respiratory infections and inhaled foreign bodies are the most common causes, unlike adults where cancer and chronic lung disease dominate.
How to Tell It’s Coming From Your Lungs
One of the first things to sort out is whether the blood is actually coming from your lungs, your stomach, or even your mouth and nose. Blood from the lungs is typically bright red or pink, often frothy (because it’s mixed with air), and comes up with coughing. Blood from the stomach tends to look like dark coffee grounds, has an acidic quality, and comes up with vomiting or retching. It may be mixed with food particles. Nosebleeds that drip down the back of your throat can also mimic hemoptysis, as can bleeding gums.
If you’re not sure about the source, pay attention to what triggered it. Did you cough, or did you retch? Was the blood bright and bubbly, or dark and grainy? These details help pinpoint where the bleeding started.
What Happens During Evaluation
When you’re evaluated for hemoptysis, the standard workup typically starts with a chest X-ray. If that doesn’t reveal the cause, a CT scan of the chest is the next step, as it can identify tumors, blood clots, bronchiectasis, and areas of active bleeding with much greater detail. In some cases, a bronchoscopy is also performed, where a thin camera is passed through the nose or mouth into the airways to directly visualize the source of bleeding.
Your doctor will also ask about the volume and duration of bleeding, your smoking history, recent infections, medication use, and any travel that might raise the possibility of tuberculosis.
When the Amount of Blood Matters
A few streaks of blood in your mucus after a week of bronchitis is common and usually not dangerous. But the volume of bleeding is a critical factor in how urgently you need care. There’s no single agreed-upon threshold for “massive” hemoptysis. Definitions range from 100 milliliters to over 1,000 milliliters in 24 hours (for reference, 100 mL is less than half a cup). What’s clear from the data is that higher volumes are far more dangerous: patients bleeding more than 1,000 mL in 24 hours have in-hospital mortality rates as high as 58% to 78%.
In practical terms, if you’re coughing up more than a few tablespoons of blood, if the bleeding isn’t slowing down, or if you feel dizzy, short of breath, or faint alongside the bleeding, that’s an emergency. Even smaller amounts of blood that keep recurring over days or weeks deserve medical attention, because the cause still needs to be identified.