Blood in phlegm usually means that a small blood vessel in your airways has broken, most often from coughing hard during a chest infection like bronchitis. In the majority of cases it’s not dangerous, but it can occasionally signal something more serious that needs medical attention. What matters most is how much blood you’re seeing, how often it happens, and whether you have other symptoms alongside it.
The Most Common Causes
Acute bronchitis is the single most frequent reason people notice blood-streaked phlegm. When your airways are inflamed and you’re coughing repeatedly, the delicate lining of your bronchial tubes can tear slightly, leaving pink or red streaks in your mucus. This type of bleeding is almost always minor and resolves on its own once the infection clears.
Beyond bronchitis, the other leading causes depend partly on where you live. In a study of 606 patients across five Italian hospitals, the top three causes were lung cancer (19%), pneumonia or lung abscess (19%), and bronchiectasis (15%), a condition where the airways become permanently widened and prone to repeated infections. In countries where tuberculosis is more prevalent, TB is one of the most common causes instead.
Other possibilities include blood clots in the lung (pulmonary embolism), certain autoimmune conditions that inflame blood vessels, and chronic obstructive pulmonary disease (COPD). People taking blood-thinning medications also face a higher risk. In one study of lung cancer patients who started blood thinners, about 8% developed blood in their phlegm within six months.
What the Color and Texture Tell You
Not all bloody phlegm looks the same, and the appearance gives clues about where the blood is coming from. Bright red, slightly foamy blood typically comes from the lungs or airways. The foamy quality comes from air mixing with the blood as you cough it up. Rust-colored or brownish phlegm often points to an older bleed, where the blood has had time to break down before you cough it out, and is common with pneumonia.
It’s also important to distinguish blood that’s truly coming from your lungs versus other sources. Blood from the stomach tends to be darker (sometimes resembling coffee grounds), may contain bits of food, and usually comes up with vomiting rather than coughing. Bleeding from the back of your nose or throat can also drip down and mix with phlegm, mimicking a lung problem. If you’ve had nosebleeds recently or notice blood when you spit without coughing, the source may be your nasal passages or gums rather than your lungs.
When It’s an Emergency
A few streaks of blood in your phlegm during a bad cold or chest infection is common and rarely dangerous on its own. But certain combinations of symptoms need immediate medical attention. Seek emergency care if you’re coughing up more than a few teaspoons of blood, or if the bleeding comes with:
- Chest pain
- Severe shortness of breath
- Dizziness or lightheadedness
- Fever
- Blood in your urine or stool (which suggests a broader bleeding problem)
Heavy bleeding from the lungs, while rare, can become life-threatening. Although there’s no universally agreed-upon cutoff, many experts consider coughing up 100 milliliters of blood (roughly a third of a cup) in 24 hours to be potentially dangerous. At that volume, the main risk isn’t blood loss itself. It’s that blood can flood the airways and interfere with breathing. About 90% of severe lung bleeding originates from the bronchial arteries, which carry blood at higher pressure than the surrounding lung tissue.
How Doctors Find the Cause
If you see your doctor about blood in your phlegm, a chest X-ray is the standard first step. It can reveal pneumonia, masses, or other obvious abnormalities, but it has limited sensitivity. Many causes of bleeding won’t show up on a plain X-ray. If the X-ray looks normal but the bleeding continues or raises concern, a CT scan is the next move. CT scans are better at pinpointing both where the bleeding is coming from and what’s causing it.
In some cases, doctors use bronchoscopy, a thin camera threaded through the nose or mouth into the airways, to look directly at the lining of the bronchial tubes. This is especially useful for spotting growths or lesions inside the airway, and it doubles as a treatment tool because doctors can sometimes stop active bleeding during the same procedure. CT and bronchoscopy catch roughly the same percentage of bleeding sites, but CT is better at identifying the underlying disease. When both are used together, the detection rate goes up.
Who’s at Higher Risk
Smokers face elevated risk because smoking damages the airway lining over time and is the primary driver of lung cancer, one of the more serious causes. If you’re over 40, have smoked for years, and notice blood in your phlegm for the first time, that combination warrants prompt evaluation rather than a wait-and-see approach.
People with chronic lung conditions like bronchiectasis or COPD are more likely to experience recurring episodes because their airways are already inflamed or structurally damaged. Those on anticoagulant medications (blood thinners) should also pay close attention, since these drugs make any bleeding harder to stop, including bleeding in the airways. Even a minor source of irritation that might produce no visible blood in someone else can cause noticeable bleeding in someone on these medications.
What to Do if It Happens Once
A single episode of blood-tinged phlegm during a respiratory infection, with no other worrying symptoms, is usually nothing to panic about. Keep an eye on it over the next few days. If the blood goes away as your cough improves, the most likely explanation is that forceful coughing irritated your airway lining.
However, if the bleeding recurs without an obvious cause like a cold, if the volume increases, or if you notice unexplained weight loss, persistent hoarseness, or night sweats alongside it, those are signs that something beyond a simple infection may be going on. In those situations, getting a chest X-ray sooner rather than later gives you a clear starting point and either reassurance or a path toward the right diagnosis.