One of the first responders to microbial invaders is a type of white blood cell called a neutrophil. Neutrophils are full of myeloperoxidase2, an enzyme that contains green-colored heme, or iron. When super concentrated, these green neutrophils can make your mucus appear straight-up verdant. But when less concentrated, the mucus appears pale green—which, depending on how your eyes work, might look yellow to you instead.
You may also notice that your mucus is a deeper shade of yellow (or looks like it has gone from yellow to green) after several days of being sick, not blowing your nose for a while, or when you wake up in the morning. When mucus sits around in your nasal passages for prolonged periods, these inflammatory cells can build up and tint your mucus more intensely, Dr. Benninger explains. “The less you clear it out, the more it becomes discolored,” he says. So, when you’ve got yellow mucus, you should blow your nose often to clear out any trapped debris and keep things moving.
To get a bit more granular, we rounded up the most common (and specific) questions about yellow mucus so you can find the exact answers you need:
Is yellow mucus bacterial or viral?
This is a toughie, Dr. McCormick says. “It is difficult to rely on the color of mucus to determine viral versus bacterial infection,” he explains. “While most people think that discolored mucus is always associated with bacterial infections, this is not always the case. Some people with viral infections may have yellow mucus.”
Is yellow mucus a symptom of COVID-19?
COVID-19 hasn’t been around for too long (even though it feels like it), so this hasn’t been studied extensively yet. Neil Bhattacharyya, M.D., a comprehensive ear, nose, and throat specialist at Massachusetts Eye and Ear in Boston, tells SELF that yellow mucus “could be” a COVID-19 symptom “but it’s relatively rare.” Anecdotally, he points out that he generally hasn’t seen yellow mucus in many patients who have tested positive for the virus. In fact, “their mucus has been pretty clear,” he says.
Basically, it’s possible to have yellow mucus with COVID-19 but far from a guarantee. “Patients with COVID-19 might develop excessive or discolored mucus; however, not all patients diagnosed with COVID-19 will have these symptoms.”
Is yellow mucus a symptom of a sinus infection?
Yellow mucus may be a sign of a sinus infection, but you would likely also have other symptoms, Dr. McCormick says. This includes a signature pain or pressure in your face, a headache, a runny or stuffy nose, postnasal drip (when mucus runs down your throat), a sore throat or cough, and even bad breath that can’t be explained by your fragrant dinner, per the Centers for Disease Control and Prevention (CDC).
Does yellow mucus ever mean you are getting better after being sick?
“Generally not,” Dr. Bhattacharyya says. “If you have yellow mucus, it means you still have white blood cells that are fighting whatever is actively causing inflammation or infection.”
What do other mucus colors mean?
Your mucus can give you a total cornucopia of colors, with shades that go way beyond yellow. Here’s a breakdown of what specific shades of snot might allude to: