Blood in your phlegm is usually caused by irritation or minor damage to the airways during a respiratory infection like bronchitis or pneumonia. The inflamed lining of your airways contains tiny blood vessels that can break when you cough hard or repeatedly, sending small streaks or spots of blood into your mucus. While this is understandably alarming, the most common causes are treatable and temporary.
That said, blood in phlegm has a wide range of possible explanations, from a simple chest cold to conditions that need prompt attention. The color, amount, and duration of bleeding all offer clues about what’s going on.
How Blood Gets Into Your Phlegm
Your airways are lined with a network of small blood vessels that supply oxygen and nutrients to lung tissue. When an infection, irritant, or disease inflames that lining, those vessels become swollen and fragile. Forceful or repeated coughing can then rupture them, mixing blood into the mucus you cough up. In more serious conditions, the damage goes deeper, affecting the tiny air sacs in the lungs themselves or larger blood vessels in the chest.
The Most Common Causes
Acute bronchitis is by far the most frequent reason people notice blood-streaked phlegm. The combination of airway inflammation and hard coughing is enough to break small capillaries. Pneumonia can do the same thing, often producing rust-colored or brownish phlegm as the infection generates thicker, more concentrated mucus.
Bronchiectasis, a condition where the airways become permanently widened and scarred (often from repeated infections), is another common culprit. People with this condition may notice blood in their phlegm on and off for months or years, particularly during flare-ups.
Chronic bronchitis in smokers frequently causes mild, recurring blood-tinged sputum, especially when an acute infection develops on top of the ongoing irritation. Tuberculosis, though less common in many countries, remains a significant cause worldwide and typically brings night sweats, weight loss, and a persistent cough alongside the bleeding.
What the Color Tells You
The appearance of the blood offers useful information. Bright red or pink blood that looks frothy or bubbly is genuinely coming from your lungs or airways. It’s often mixed with mucus and may appear as streaks through otherwise clear or yellowish phlegm.
Dark red or brown blood that looks more like coffee grounds is more likely coming from your stomach or esophagus, not your lungs. This type is usually not frothy and may contain food particles or come with nausea. The distinction matters because the causes and treatments are completely different.
Rust-colored phlegm points toward pneumonia, where blood has been sitting in the lungs long enough to oxidize and darken. Pink, foamy phlegm can signal fluid buildup in the lungs, sometimes related to heart problems.
Sometimes the Blood Isn’t From Your Lungs
Not every case of “coughing up blood” actually involves the lungs. Bleeding gums, a nosebleed that drains down the back of your throat, or irritation in your throat can all produce blood that you notice when you cough or spit. If you have little or no actual cough, if the blood appears mainly when you clear your throat in the morning, or if you’ve recently had a nosebleed, the source is likely your upper airway rather than your lungs.
Stomach and esophageal bleeding can also be mistaken for lung-related blood. The key differences: blood from the stomach tends to be darker, isn’t frothy, and often comes with nausea or vomiting.
When It Could Be Something Serious
In a study of patients presenting with blood in their phlegm, lung cancer was found in about 7.3% of cases and pulmonary embolism (a blood clot in the lungs) in about 2.6%. These are not the most likely explanations, but they’re serious enough that certain warning signs deserve attention.
Factors that raise the risk of a more serious cause include:
- Smoking history: the single strongest risk factor for both lung cancer and chronic lung disease that causes bleeding
- Blood thinner use: anticoagulant or antiplatelet medications make bleeding from any cause more likely and harder to stop
- Unexplained weight loss or persistent fever: these “B symptoms” can point toward cancer or chronic infection
- Recent immobilization, surgery, or long flights: these increase the risk of blood clots that can travel to the lungs
- A personal or family history of clotting disorders or venous blood clots
- Known liver disease, kidney disease, or low platelet counts: all of which impair your body’s ability to stop bleeding
Fungal lung infections (particularly aspergillosis) and nontuberculous mycobacterial infections are also risk factors for recurring episodes of blood in the phlegm.
How Much Blood Is Too Much
A few streaks of blood in your mucus during a bad cough is common and usually resolves on its own. The threshold for a life-threatening amount is generally considered to be around 100 milliliters (roughly a third of a cup) within 24 hours. At that volume, the danger isn’t just blood loss. It’s the risk of blood flooding the airways and interfering with breathing.
Any amount of blood that makes it hard to breathe, that fills your mouth rapidly, or that comes with dizziness, a racing heart, or chest pain needs emergency evaluation. The same applies if you’re coughing up blood and are on blood thinners, since the bleeding may not stop on its own.
How Long It Typically Lasts
With a standard respiratory infection like bronchitis or pneumonia, blood in the phlegm usually clears up as the infection resolves, generally within one to three weeks. Even in cases where no specific cause is identified, the majority of patients see the bleeding stop within six months.
Smokers with chronic bronchitis may notice mild, sporadic blood-tinged sputum over a period of years, particularly during acute flare-ups. Slow-growing lung tumors called bronchial adenomas can cause occasional bleeding over many years before being diagnosed, which is one reason persistent or recurring blood in phlegm warrants investigation even when the amount seems small.
What Happens During Evaluation
If your doctor decides to investigate, the process typically starts with a chest X-ray. If the X-ray is normal but the bleeding continues or risk factors are present, a CT scan of the chest is usually the next step. CT imaging is highly effective at identifying the source and cause of bleeding, and in many cases it provides enough information to make a diagnosis without further testing.
When the CT scan reveals an abnormality or when someone is actively bleeding, a bronchoscopy may follow. This involves a thin, flexible camera passed through the nose or mouth into the airways. It allows direct visualization of the bleeding site and can collect tissue samples if a growth or unusual area is found. For patients with risk factors for lung cancer, bronchoscopy is often necessary to confirm or rule out a diagnosis even when imaging shows a clear abnormality.