Blood in your phlegm is usually caused by minor irritation or infection in your airways, not something dangerous. Bronchitis is the single most common reason, especially during or after a cold. That said, blood-streaked phlegm can occasionally signal something more serious, so the amount of blood, how long it lasts, and what other symptoms you have all matter.
The Most Common Causes
When you have a chest infection like bronchitis or pneumonia, repeated coughing inflames the lining of your airways. The tiny blood vessels in that lining can crack under the strain, releasing small streaks of blood into your mucus. This is the explanation behind the vast majority of cases. It looks alarming, but the bleeding is superficial and typically stops on its own as the infection clears.
Bronchiectasis, a condition where the airways become permanently widened and prone to mucus buildup, is another frequent cause. People with bronchiectasis often produce thick, dark brown or blood-tinged phlegm because of chronic inflammation in the lungs. Cystic fibrosis produces a similar pattern.
Dry air is an overlooked culprit. Low humidity, whether from winter heating or an arid climate, dries out the delicate membranes in your nose and throat. Those membranes can crack, rupturing small blood vessels. Blood from the back of your nasal passages drips into your throat and mixes with phlegm, making it look like the bleeding is coming from your lungs when it’s actually coming from your nose. This is especially common in winter months.
Heavy or forceful coughing on its own, even without an infection, can burst a small vessel. Smokers are particularly vulnerable because chronic irritation from smoke keeps the airway lining inflamed and fragile.
What the Color Tells You
Not all bloody phlegm looks the same, and the appearance offers clues about where the blood is coming from. Bright red, slightly frothy sputum typically means the blood originated in your lungs or lower airways. The frothiness comes from air mixing with blood in the respiratory tract. Pink, foamy phlegm can point to fluid buildup in the lungs, which sometimes happens with heart problems.
Dark red or brown blood that looks more like coffee grounds, or phlegm with bits of food in it, usually means the blood came from your stomach or esophagus rather than your lungs. Blood from the digestive tract tends to be darker because stomach acid changes its color. The distinction matters because the causes and treatments are completely different. If you’re unsure, your doctor can check the acidity of the sample to confirm the source.
Bleeding from the back of your nose or throat can also masquerade as lung bleeding. If you’ve had a nosebleed recently, or if your nose feels dry and raw, that’s a likely explanation. A quick look at the inside of your nose and mouth is often enough to rule this out.
When Blood in Phlegm Is More Serious
Lung cancer can cause blood-streaked phlegm, particularly in current or former smokers. If you smoke and notice blood in your mucus, that alone is worth getting checked. Other warning signs that point toward cancer include unexplained weight loss, a cough that won’t go away, and increasing shortness of breath.
A pulmonary embolism, which is a blood clot that travels to the lungs, can also produce bloody or blood-streaked mucus. This is a medical emergency. The combination of bloody phlegm with sudden shortness of breath, sharp chest pain that worsens when you breathe in, a rapid heartbeat, dizziness, or swelling in one leg should prompt an immediate trip to the emergency room. Risk factors include recent surgery, long periods of immobility (bed rest, long flights), heart disease, cancer, and severe COVID-19.
Volume is an important indicator of severity. A few streaks of blood mixed into your phlegm during a cold is very different from coughing up a tablespoon or more of pure blood. Researchers classify coughing up more than about 50 milliliters of blood in 24 hours (roughly 3 tablespoons) as moderate hemoptysis, and more than 200 milliliters (close to a cup) as severe. Severe hemoptysis is rare but life-threatening and requires emergency care.
What to Expect at the Doctor’s Office
If your bloody phlegm doesn’t clear up within a few days, keeps coming back, or is accompanied by other symptoms, your doctor will likely start with a chest X-ray. This is a good first step for spotting infections, masses, or fluid in the lungs, but it has limited sensitivity. A normal X-ray doesn’t always mean everything is fine.
If the X-ray doesn’t reveal a clear cause, the next step is usually a CT scan of the chest with contrast dye injected into a vein. CT scans are better at pinpointing both the location and the underlying cause of the bleeding. In some cases, your doctor may also recommend a bronchoscopy, where a thin, flexible camera is passed through your nose or mouth into your airways to look directly at the source. CT scans and bronchoscopy are similarly effective at locating where the bleeding is coming from, but CT is better at identifying why it’s happening. Sometimes both are needed.
What You Can Do Right Now
If you’re dealing with a minor amount of blood in your phlegm during a cold or bout of bronchitis, the most helpful thing is to let the infection run its course while keeping your airways moist. A humidifier in your bedroom, staying well hydrated, and avoiding irritants like cigarette smoke all reduce the strain on inflamed airways. Suppressing a harsh, dry cough with an over-the-counter cough suppressant can also help by giving irritated blood vessels a chance to heal.
Keep track of how much blood you’re seeing and whether it’s increasing. A single episode of pink-tinged mucus during a bad cough is rarely cause for concern. Blood that shows up repeatedly over several days, blood without any obvious cause like an infection or dry air, or blood in your phlegm combined with chest pain, weight loss, or breathlessness warrants a visit to your doctor sooner rather than later.