Blood in your phlegm is usually caused by irritated or inflamed airways, most commonly from bronchitis or a chest infection. While it can look alarming, the majority of cases trace back to a minor, treatable cause. That said, roughly 1 in 5 people evaluated for this symptom in one large study were found to have a lung malignancy, so it’s worth understanding what’s behind it and when to take it seriously.
The Most Common Causes
Bronchitis is the single most frequent reason people see blood streaks in their phlegm. When your airways are inflamed, whether from a cold, flu, or ongoing irritation like smoking, the lining of the bronchial tubes swells. That swelling pushes tiny blood vessels close to the surface. Repeated, forceful coughing can then rupture those vessels, mixing small amounts of bright red blood into your mucus.
Beyond bronchitis, other common causes include:
- Pneumonia: a lung infection that inflames the air sacs, sometimes producing rust-colored or blood-tinged sputum
- Bronchiectasis: a condition where the airways become permanently widened and accumulate mucus that can be streaked with blood
- COPD: chronic lung diseases like emphysema that damage airways over time
Less common but more serious causes include lung cancer, tuberculosis, a blood clot in the lung (pulmonary embolism), and cystic fibrosis. Chest trauma, inhaling a foreign object, and even cocaine use can also cause bleeding in the airways.
Why Inflammation Leads to Bleeding
The lining of your airways is thin and packed with small blood vessels. When an infection or irritant triggers inflammation, that lining swells and becomes fragile. The superficial blood vessels sit closer to the surface than usual, and the tissue around them weakens. A hard cough creates sudden pressure against those fragile walls, and the vessels rupture. The blood mixes with mucus on its way up, which is why you typically see streaks or flecks rather than pure blood.
This is the same basic mechanism whether the cause is a week-long chest cold or a chronic condition like bronchiectasis. The difference is that with a short-term infection, the inflammation heals and the bleeding stops. With chronic conditions, the cycle can repeat.
Is the Blood Coming From Your Lungs?
Not all blood you cough up actually comes from your lungs. Blood can drip into your throat from a nosebleed, irritated tonsils, or even your gums, and then get coughed out with saliva or mucus. This is sometimes called pseudohemoptysis.
A few features help distinguish the source. Blood from your lungs is typically bright red and may look slightly foamy because it’s mixed with air. If the blood looks dark brown or resembles coffee grounds, and especially if you feel nauseous, it’s more likely coming from your stomach or esophagus. A sensation of postnasal drip or visible bleeding from your nose without coughing suggests the blood is coming from your nasal passages rather than your lungs.
Medications That Increase the Risk
If you take blood thinners, they can make airway bleeding more likely or harder to stop. Traditional blood thinners like warfarin are well known for this, but newer anticoagulants carry risk too. Reports have linked these medications to bleeding within the lungs, sometimes made worse when combined with common anti-inflammatory painkillers like ibuprofen or naproxen. If you’re on a blood thinner and notice blood in your phlegm, that combination is worth mentioning to your doctor promptly.
How Doctors Find the Source
When you report blood in your phlegm, a doctor will typically start by asking how much blood you’ve seen, how long it’s been happening, and whether you smoke or have other lung conditions. A chest X-ray is usually the first imaging test, looking for signs of infection, masses, or fluid. If the X-ray doesn’t give a clear answer, a CT scan provides a much more detailed view of the lungs and airways. In some cases, a doctor may use a thin, flexible camera threaded into the airways to look directly at the source of bleeding.
A sample of your sputum may also be sent to a lab to check for bacteria, viruses, or other infectious causes driving the bleeding.
How Likely Is It to Be Cancer?
This is the question most people are really asking. In a study of 606 patients who were evaluated for coughing up blood, about 20% were diagnosed with a lung malignancy. That means 4 out of 5 cases had a non-cancerous explanation. The risk is higher if you’re over 40, have a history of smoking, or if the bleeding is persistent or heavy.
Even among patients whose initial scans and procedures came back clean, a small percentage (about 7%) were later diagnosed with lung cancer during follow-up. This is one reason doctors may recommend repeat imaging several months after an unexplained episode, particularly for people with risk factors.
What Counts as an Emergency
A few streaks of blood in your mucus during a bad cough is common and rarely dangerous on its own. But the situation becomes urgent when the volume of blood increases significantly. Clinically, coughing up more than about 200 milliliters of blood (roughly three-quarters of a cup) in 24 hours is considered massive and potentially life-threatening. At that point, the danger isn’t just the bleeding itself but the risk of blood flooding the airways and blocking breathing.
Other signs that warrant immediate medical attention: coughing up blood with no obvious cause like a cold or bronchitis, blood that keeps coming back over days or weeks, blood accompanied by chest pain or shortness of breath, unexplained weight loss alongside the bleeding, or a choking sensation as bright red blood comes up.
What to Track Before Your Appointment
If you’re planning to see a doctor about blood in your phlegm, paying attention to a few details beforehand will help them narrow down the cause faster. Note how often it happens: once, daily, only with hard coughing? Estimate the amount. A few streaks mixed into mucus is very different from a tablespoon of pure blood. Note the color: bright red, dark red, pinkish, or brown. And pay attention to whether you have other symptoms like fever, night sweats, weight loss, or shortness of breath, all of which help point toward or away from specific diagnoses.