Misconceptions abound about mucus and phlegm and about what their consistency and color mean.

"This is probably one of the most common discussions that I have to have with patients as an ear, nose, and throat specialist," says Michael S. Ellis, MD, a clinical professor of otolaryngology at Tulane Medical Center in New Orleans.

"I call it the New Orleans nose. People in New Orleans think they have allergies causing their mucus problems, but that's wrong."

These "mucus problems" aren't just plaguing New Orleans residents. People across the country are fighting runny noses, clogged noses, postnasal drip, and everything in between.

Respiratory infections, the common cold, sinus infections, allergies, smoking, and even your environment all trigger changes in your mucus that can leave you frustrated and wandering the pharmacy aisles looking for the best solution.

"Mucus is like a gelatin, a sticky substance the function of which is to lubricate and also to filter," says Dr. Ellis. It's made by cells in membranes that run from your nose to your lungs. While you swallow most of it without noticing, what’s left behind keeps your airways moist so that they work properly.

However, Ellis explains, what most people think of as mucus in their nose and sinuses is a delicate balance of both mucus and watery secretions — fluid made by the cells of the nose and sinuses.

Usually these two substances work together smoothly, and you probably don't even notice them.

"If you get an allergy or a cold and your nose is running like a faucet, you're making an excess of watery secretions," Ellis says.

At the other extreme, thick mucus — the kind that leads to postnasal drip and congestion or gets crusty — is usually the result of being excessively dry.

Ellis says 90 percent of the patients he sees are actually experiencing problems with thick mucus, not a runny nose due to the common cold.

Phlegm is a slightly different substance. It's a form of mucus produced by the lower airways — not by the nose and sinuses — in response to inflammation.

You may not notice phlegm unless you cough it up as a symptom of bronchitis or pneumonia.

As is the case with mucus, phlegm that has a color such as green or yellow may indicate infection. And anytime you see blood in the phlegm you cough up, you should seek medical attention.

Mucus, explains Ellis, helps protect the lungs by capturing dirt and dust as you inhale. The dirt, dust, and debris are passed out of your system.

While a persistent yellow or green color, possibly accompanied by an unpleasant odor, is often a sign of infection, color does not always indicate a health problem, says Ellis.

The fact that your mucus may be discolored by what is in the air is a good thing.

"If you're in an environment where there is a lot of dust or smoke or smog, the function of the nose is to protect the lungs," he stresses.

Thick mucus, which may seem to have greater volume and may create problems such as postnasal drip, results from a number of situations and is usually an indication of being over-dry.

Here are some of the factors that contribute to that feeling of thick, clogging mucus:

A dry indoor environment, which may be due to air-conditioning or heating
Not drinking enough water and other fluids, or drinking beverages such as coffee, tea, and alcohol that may lead to losing fluids
Medications that are drying
Smoking

Using a small facial steamer or humidifier, taking a hot steamy shower, or even breathing through a warm wet washcloth can give you some relief from thick, crusty mucus.

Your doctor will evaluate the character of your mucus as well as the presence (or absence) of other allergy or cold symptoms. Getting the right diagnosis means you can get the right treatment.

Unfortunately, many people misdiagnose themselves, thinking, for example, that a large quantity of thick mucus indicates an allergy (which actually is signaled by a lot of watery secretions accompanied by allergy symptoms). They then mistakenly take antihistamines, which end up drying out the nose and making thick mucus worse.

"That's why it's important to make the right diagnosis," says Ellis, who also warns against self-medicating with decongestants like Sudafed, which are also drying and will further thicken your mucus.

With a bacterial sinus infection you may be able to take a doctor-prescribed antibiotic to treat the cause, but with viral colds the best you can do is correctly treat symptoms, so you want to be sure you have the right match.

Here are some of the mucus problems Ellis sees, along with his recommended treatments:

Postnasal drip (thick mucus) Take an expectorant (this thins mucus and phlegm), drink lots of fluids, breathe through a warm washcloth, and use steam to make the mucus more liquid.
Congestion (blocked nose) This is mostly due to swollen nasal membranes, so shrinking those membranes is the goal. Use an oral decongestant or decongestant nasal spray (for up to three days only), breathe through a warm wet washcloth, and use steam to ease congestion.
Runny nose (with sneezing and/or itching) Take an antihistamine.

Any option is fine, says Ellis. Since the mucus may be infected, make sure you follow basic hygiene after blowing your nose: Carefully dispose of the facial tissue and wash your hands. Also, avoid spitting out your mucus in public.

Mucus is so important that your body makes about a quart of the stuff on a normal day. Learn how to live with it when it is out of balance, and you'll be more comfortable until the flow goes back to normal.

Everything You Ever Wanted to Know About Phlegm and Mucus