What Makes Snot Turn Green?

We all do it, one time or another. You blow your nose for what must be the hundredth time that day. Before you throw out the tissue, you take a surreptitious glance inside. Rather than the usual clear or slight yellow, you see something different. Something green. Suddenly, your snot resembles the color palette of an overly ambitious landscape architect — lime, olive, even a speck of chartreuse.

Tossing the tissue in the wastebasket, you wonder: Why has your snot turned green?

You’ve been fighting your sickness with decongestants and hot tea. But your immune system has been fighting it with neutrophils, the body’s most common type of immune system cell and your first line of defense against infection and inflammation. In this case, the neutrophils have gathered in your nasal mucus to combat infection there.

“They do their job by essentially surrounding and gobbling up microorganisms, and they hold them inside the cell in little digestive sacs,” says Dr. Harry Malech, a neutrophil expert at the National Institute of Allergy and Infectious Diseases. To digest the microorganism — the virus or bacteria that’s making you sick — the neutrophils rely on enzymes, proteins “that can help to kill or chew up the microorganisms.”

By far, the most plentiful enzyme within a neutrophil is myeloperoxidase. Myeloperoxidase has a lot of iron in it, and in the environment provided by the surrounding snot, the iron turns a green color.

After they’ve been fighting infection for a while, neutrophils fall apart and spill their myeloperoxidase into your mucus, Malech says. “Once the myeloperoxidase is released, the green color for some reason becomes more prominent than when it’s in the cells,” giving your snot that verdant tinge.

This only happens when your nose is running for certain reasons, says Carol Shoshkes Reiss, a New York University microbiologist who studies the immune system. “You don’t see it with an allergy, and you wouldn’t see it with just irritation, like how some people sneeze when they see the sun,” she says. “But people often get that kind of response when they have an infection” in their sinuses or upper nasal passages.

Does this change in coloration mean you should call your doctor for an antibiotic?

“If all you see is leaky clear stuff, you probably don’t need an antibiotic,” Malech says. “But if it turns green, that’s an indication that you might have a bacterial infection and need antibiotics.” When your immune system is already weakened from battling a cold, it’s much easier for disease-causing bacteria to set up camp, leaving you with a secondary infection such as sinusitis.

Green snot alone isn’t enough to warrant antibiotics, Reiss cautions, because it’s sometimes caused by a virus rather than bacteria. Antibiotics don’t work against viruses, and widespread prescription can lead to drug resistance.

“It depends on the quality and quantity of it,” she says, and whether you have other symptoms like swollen lymph nodes, a fever, or a feeling of all-around rottenness — all things that might point to a bacterial infection.

Otherwise, stick with drinking hot tea and — don’t  be embarrassed — inspecting your tissues. If someone catches you at it, just say you’re checking on the myeloperoxidase content.