The most common cause of blood in phlegm is acute bronchitis, a temporary infection of the airways that irritates and inflames the bronchial lining enough to cause small amounts of bleeding. Other frequent causes include pneumonia, bronchiectasis (permanent widening of the airways), and lung cancer. In most cases, a small streak of blood in your mucus after a bad cough is not an emergency, but the symptom always deserves attention because the range of possible causes spans from completely benign to life-threatening.
How Blood Gets Into Your Phlegm
Your lungs have two separate blood supplies. One is a low-pressure system that handles oxygen exchange. The other is a high-pressure system fed by the bronchial arteries, which supply the walls of your airways. When something damages or inflames the airway lining, those high-pressure bronchial vessels can leak blood directly into the mucus you cough up. Despite supplying only about 2% of the lung’s total blood flow, the bronchial arteries are responsible for 90% of serious airway bleeding events, precisely because they operate at higher pressure.
Minor irritation, like the repeated coughing of a chest cold, can rupture tiny surface vessels and produce a few red streaks. Deeper damage from infection, tumor growth, or chronically dilated airways can erode into larger vessels and produce much more blood.
Common Causes
In a study of 606 patients who coughed up blood, the three most frequent diagnoses were cancer (19%), pneumonia or lung abscess (19%), and bronchiectasis (15%). At referral hospitals, bronchiectasis, lung cancer, bronchitis, and pneumonia together account for more than 70% of cases. Here’s what each looks like in practice.
Acute Bronchitis
This is the single most common cause of blood-streaked phlegm in otherwise healthy people. A viral chest infection inflames your airways, and forceful, repeated coughing irritates the lining enough to produce small amounts of blood mixed into yellow or green mucus. The bleeding typically stops on its own as the infection clears, usually within one to three weeks.
Pneumonia
Bacterial or viral lung infections can produce rust-colored or blood-tinged sputum, often alongside fever, chest pain with breathing, and significant fatigue. The infection damages lung tissue directly, and the inflammation increases blood flow to the affected area, making bleeding more likely.
Bronchiectasis
In this condition, sections of your airways become permanently widened and scarred, usually from past infections or chronic inflammation. The damaged walls develop extra blood vessels that are fragile and prone to bleeding. People with bronchiectasis often cough up blood repeatedly over months or years, sometimes in larger amounts. Cystic fibrosis is one well-known cause, but many cases develop after severe childhood pneumonia or tuberculosis.
Lung Cancer
A tumor growing in or near an airway can erode into blood vessels as it expands. Blood in phlegm from lung cancer tends to persist or worsen over weeks rather than resolving like an infection would. It often appears alongside unexplained weight loss, a new cough that won’t go away, or worsening shortness of breath. Smoking history significantly raises the likelihood that blood in phlegm points to cancer.
Causes Outside the Lungs
Not all blood you spit up comes from your lungs. Nosebleeds, bleeding gums, or irritation in the throat can mix blood into saliva or post-nasal drip, making it look like the blood came from deeper in your chest. This is sometimes called pseudohemoptysis. The key difference: blood from the nose or mouth tends to appear when you spit or clear your throat, while true lung bleeding comes up with a cough and is often mixed into the phlegm itself rather than sitting on top of it.
Severe acid reflux can also cause blood-tinged spit, and vomiting blood from the stomach can be confused with coughing it up from the lungs. If you’re not sure where the blood is coming from, that distinction matters for getting the right evaluation.
Blood Thinners and Medication Risks
If you take blood-thinning medications like warfarin or newer anticoagulants, your risk of airway bleeding is higher. Even medications considered relatively safe can occasionally cause bleeding into the lung tissue, especially if you already have a lung condition that weakens the delicate membranes where air and blood meet. NSAIDs like ibuprofen can amplify the bleeding risk when combined with prescription blood thinners. If you’re on anticoagulants and notice blood in your phlegm, bring it up with your prescriber promptly.
Blood Clot in the Lung
A pulmonary embolism, where a blood clot travels to the lungs, can cause blood-streaked mucus along with sudden shortness of breath, sharp chest pain that worsens when you breathe in, rapid heartbeat, and lightheadedness. You might also notice swelling or pain in one leg, which is where the clot often originates. This combination of symptoms is a medical emergency. The chest pain can feel like a heart attack, and fainting can occur if the clot is large enough to drop your blood pressure suddenly.
Tuberculosis and Fungal Infections
In many parts of the world, tuberculosis remains one of the leading causes of blood in phlegm. TB slowly destroys lung tissue and creates cavities that are surrounded by fragile, overgrown blood vessels. Fungal infections can behave similarly, colonizing damaged areas of lung and eroding into nearby vessels. Both conditions can cause recurrent, sometimes heavy bleeding and are among the more common causes of life-threatening episodes.
How Doctors Find the Source
A chest X-ray is the standard first step. It’s fast, inexpensive, and can reveal pneumonia, masses, or other obvious problems. If the X-ray doesn’t explain the bleeding, CT scanning with contrast dye is the preferred next test. CT can detect smaller tumors, bronchiectasis, blood clots, and other abnormalities that a plain X-ray misses. In some cases, a doctor may also use a bronchoscopy, threading a thin camera into your airways to look directly at the source of bleeding.
The sequence depends on how much blood you’re producing and how quickly. For small amounts of blood-streaked phlegm with an obvious recent chest infection, your doctor may simply monitor you. For larger volumes, persistent bleeding, or risk factors like smoking or older age, imaging typically happens quickly.
When the Amount of Blood Matters
A few streaks of blood in your mucus during a cough is very different from coughing up spoonfuls or cups of blood. Large-volume bleeding is rare but dangerous. The biggest immediate risk isn’t blood loss but rather blood flooding the airways and blocking oxygen exchange. The most common causes of massive bleeding are bronchiectasis, tuberculosis, fungal infections, and lung cancer.
For active, heavy bleeding, treatment can involve blocking the bleeding vessel through a procedure called bronchial artery embolization, where a catheter is threaded to the source and the vessel is sealed off. This works initially in 70% to 99% of cases, though bleeding can return in some patients. A medication that helps blood clot, delivered as a nebulized mist, has shown promise in stopping bleeding within 30 minutes in about 73% of patients. Surgery to remove the bleeding section of lung is a last resort due to high complication rates.
Patterns Worth Paying Attention To
A single episode of blood-streaked phlegm during a chest cold, with no other concerning symptoms, is usually benign. Patterns that warrant faster evaluation include blood in your phlegm lasting more than a week, blood appearing without an obvious infection, increasing volume of blood over time, or blood accompanied by weight loss, persistent chest pain, or worsening breathlessness. Smoking history, age over 40, and prior lung disease all shift the odds toward causes that need prompt investigation.