You might think of mucus – the runny, goopy, or gummy stuff you sneeze out, sniff in, and cough up – as something pesky you never seem to have a tissue for. It might not be sexy, but mucus is one of your body’s biggest defenders.

This slippery, sometimes sticky fluid comes from the mucus membranes that line your respiratory tract – your nose, mouth, larynx, trachea, and lungs. Mucus provides moisture for the air we breathe and lubricates your respiratory tract. “Mucus serves a protective function in capturing debris, allergens, dust, and pathogens such as viruses and bacteria, which can then be shuttled out of the body,” says Ken Yanagisawa, MD, president of the American Academy of Otolaryngology-Head and Neck Surgery.

You’ve likely noticed mucus shows up in a range of colors and textures when you’re sick, sniffly, or congested, or have a condition like a sinus infection. Though mucus color isn’t enough to always diagnose illness or a certain condition, it can point you in the right direction.

And even if you don’t notice it, mucus is always there.

Thin and clear. It’s the sniffly stuff that appears with allergies. Mostly water, yet packed with dissolved salts, proteins, and antibodies, clear mucus also can mean your body’s in healthy mode. Your nasal tissues constantly churn it out. Most of it slyly slips down the back of your throat and dissolves in your stomach without you knowing it.

White. Your nose might be stopped up. The swollen tissues in your nasal passages slow down mucus flow, while moisture loss thickens it and makes it cloudier. This type of mucus ripens conditions for a cold or a sinus infection.

Yellow.  A cold or infection may be building up steam. Your mucus can take on a yellow tint when your white blood cells race to the scene of an infection to try and fend it off.

Green. Dead white blood cells can make your mucus green and gunky. Your immune system’s on high alert. Call your doctor if it doesn’t improve in 10 to 12 days, or if you have a fever. If you have sinusitis, which is a bacterial infection, your doctor likely will order antibiotics to clear it up.

Brown. You might have gotten a big whiff of something like dirt. Brownish mucus could also be tinged with old blood.

It’s rare, but in chronic lung diseases like bronchiectasis or cystic fibrosis, bacteria in your lungs can cause inflammation and bleeding that turn you mucus a dark brown.

What’s key to remember is that yellow, green, or brown mucus all can signal bacterial or viral infections.

Red. A red or pinkish tinge might mean a little harmless blood from, say, blowing your nose often or something riling up your nasal lining. In some cases, “Red mucus – or bloody scabs – can be due to blood from nosebleeds, or bleeding from a polyp or even a tumor,” Yanagisawa says.

Black. You might be breathing in debris that’s clumping up in your nose. Lots of dust or cigarette smoke can blacken mucus, too. And though rare, it also can be a sign of a fungal infection. If you have a weakened immune system, check in with your doctor.

Knowing what mucus colors can mean can help, but it all comes down to what’s normal for your body. So pay attention, Yanagisawa advises. “Any discoloration or alteration in consistency which is abnormal for your body should be monitored closely, and if persistent, should warrant a call or visit with your doctor.”

The key to dealing with mucus is to ID its source. For example:

Allergies or colds. This thin, clear, watery mucus can be treated with antihistamines. Be mindful if you have a prostate disorder, though. Nasal steroid sprays can give relief, too. If you have glaucoma, you’ll need to be cautious with both of these treatments.

Over-the-counter (OTC) decongestants can help too, but take care using them, says Yanagisawa. They can trigger heart palpitations or speed up your heart rate. Meanwhile, nasal decongestant sprays like oxymetazoline (Afrin) can open up your nasal passages and help you breathe better for a while, but they don’t get rid of mucus. If you use a nasal decongestant for more than 3 days in a row, the congestion can come back and trigger a cycle that’s hard to break. Also:

“For children, there are a number of nasal aspirators which can be utilized to suction out nasal secretions and mucus safely and effectively,” Yanagisaw notes.

Nose, sinus, or throat infections. These typically trigger green or yellow mucus. “This is best treated with antibiotics if bacteria are the culprit,” he says.

If the mucus comes from gastroesophageal reflux disease (GERD), the best treatment is meds such as a histamine-2 receptor blocker, such as famotidine. A proton pump inhibitor such as omeprazole may also work. Cutting out certain foods, including chocolate, spicy foods, citrus, and drinks with caffeine or red wine can help, too.

“Mucus is normal and of vital importance in the proper functioning of our respiratory tract,” Yanagisaw says.

To keep things moving right:

Photo Credit: Maria Fjeldgaard-Sundstrøm / EyeEm/ Getty Images

Ken Yanagisawa, MD, FACS; managing partner, Southern New England Ear Nose Throat & Facial Plastic Surgery Group, LLP,  New Haven, CT; president, American Academy of Otolaryngology-Head and Neck Surgery, Alexandria, VA.

Cleveland Clinic: “What the Color of Your Snot Really Means.”

Penn Medicine: “Yuck! My Snot Is Green: A Look at What Your Mucus Says About Your Health.”

Texas A&M: “You Asked: What Does the Color of My Mucus Mean?”

Ohio State University: “What does the color of phlegm mean?”

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