Mucus is not widely considered a topic for polite conversation. It's something to be discreetly blown into a tissue, folded up, and thrown away.

But the simple truth is that without mucus, you wouldn't be alive.

"Mucus is essential for the protection of your body," says Jeffrey Spiegel, an ear, nose, and throat surgeon at Boston University. "It's a protective barrier and it allows you to breathe comfortably. If you had no mucus, you'd be quite sorry you didn't."

Given how important mucus is — and how often colds and allergies cause mucus-related symptoms — it's worth learning a bit more about it.

1) You produce about 1.5 quarts of mucus a day — and swallow the vast majority

Most of us think of mucus as something that leaks from our nose, but the truth is that it also gets secreted in your trachea and other tubes that carry air through your lungs, where it's technically called phlegm. Wherever it's produced, mucus is a mix of water and proteins, and most of it gets pushed to the back of your throat by microscopic hairs called cilia.

Whether you're aware of it or not, you're constantly swallowing all this mucus, and it harmlessly ends up in your stomach. "You're swallowing, on average, twice a minute — even when you're sleeping at night," says Michael Ellis, an ear, nose, and throat doctor at Tulane University.

Ellis says that, on average, a person produces about 1.5 quarts of mucus per day, and contrary to what you might think, it doesn't vary by all that much. But that mucus gets diluted by a separate, watery secretion (called serous fluid), which can vary widely based on your health.

2) Mucus is basically the body's flypaper

Mucus has two main functions: it keeps the nasal cavity and the other airways inside your body moist, preventing them from drying out due to all the air that flows over them. (Relatedly, the serous fluid that mucus is mixed with also moistens the air itself before it enters the lungs.)

Mucus' other function, though, might surprise you. "Mucus is kind of like flypaper," Ellis says. "Debris that comes into the nose or throat sticks to it, and then you swallow it, so it doesn't get into your lungs."

Mucus, in other words, is nature's filter for your delicate lungs. The bacteria, dust and other tiny particles that you breathe in get stuck in mucus and pulled down into your stomach, where they're destroyed by enzymes.

3) There are two different things that cause runny noses

When a cold or allergies cause your nose to run, it's because they're triggering an inflammatory response in your nasal cavity and airways. Even though you always produce roughly the same amount of mucus, this dramatically increases the amount of the serous fluid it's diluted in.

We tend to experience this as an excess of watery, runny mucus, and it can be treate by taking an anti-histamine, which reduces the amount of water — leading to thicker, drier mucus.

Cold weather causes a runny nose in an entirely different way. In cold temperatures, your cilia (the microscopic hairs that sweep mucus to the back of your throat) stop sweeping back and forth as quickly, causing some of the mucus to drip down through your nose instead.

4) A stuffy nose isn't stuffed full of mucus — it's swollen

The inside of your nose is filled with structures called conchae, or turbinates. Their primary function is to warm the air you inhale to body temperature and add moisture until it's very humid — so that the air can enter your lungs without causing problems.

Stuffy noses occur when the conchae rapidly swell in size in response to cold, dry conditions, so there's more surface area for the air to flow over. Additionally, if you're fighting an infection, the conchae can swell further with blood, in order to bring more white blood cells to the site of the infection.

"We call it congestion when [noses] get swollen up, because it seems like we're having trouble getting air through," Ellis says. Most people think of this congestion as a result of too much mucus — but in reality, it's just swollen conchae.

This explains why many people are congested when they wake up in the morning (after breathing cold, dry air all night), especially because central air and heating systems dry out air significantly.

5) The best way to decongest your nose is with steam

Because cold, dry air is what most often causes your conchae to swell, the best remedy is to add hot, moist air. This is why taking a hot shower often opens up a clogged nose, and why hot washcloths and facial steamers are also effective treatments.

Nasal decongestants (such as pseudoephedrine and phenylephrine) can also help de-swell the conchae, but in some cases, there's a downside: they dry out the nasal cavity, by reducing the amount of serous fluid. So if you're also experiencing excessively thick, dry mucus, you're better off avoiding decongestants.

6) Thick mucus could mean you're dehydrated

A few different factors can reduce the production of serous fluid in your nose, leading to thick, dry mucus. This is often experienced as post-nasal drip — thick mucus at the back of your throat that's much more noticeable than the thinner mucus you swallow unconsciously.

One cause is dehydration: if your body doesn't have enough water, it'll cut back on the secretion of serous fluid. An excessively dry environment — often caused by central heat or air conditioning — can also cause the same problem, as can smoking cigarettes.

Instead of taking a decongestant to relieve post-nasal drip, Ellis recommends using an expectorant, which will increase the amount of serous fluid your mucus is diluted in.

7) Boogers are just dried mucus

Most of the mucus in your nose gets swept by your cilia to the back of your throat. But sometimes — especially in arid environments — some of the mucus near your nostrils (in an area formally called the nasal vestibule) begins to dry out first, becoming too viscous to be swept by cilia. If it sits there long enough, it dries even further, becoming the crusty accretion colloquially known as a booger.

Boogers, as it happens, are the subject of some scientific study. Several researchers have considered the question of why people pick their noses. One theory is that people simply derive pleasure from the act of "cleaning up," and while tissues aren't always available, your fingers are.

Whatever the reason, it's widespread. The authors of one small survey finding that 91 percent of adults admitted to picking from time to time. But that doesn't mean it's a good idea: Dutch researchers, among others, have found that nose-picking can spread infections.

8) The color of your mucus can tell you a lot

Mucus, in its natural state, is clear. But that doesn't mean that colored mucus is necessarily a bad thing.

Grey, whitish, or yellowish mucus could simply be the result of dust, pollen, or other particles you've inhaled from the air around you. On the other hand, these colors can be a sign of an infection, as they can be caused by an excess of white blood cells or pus. And darker colors — like pink, red, or brown — can be a sign of bleeding in your nasal cavity.

9) Afrin is powerfully addictive

The nasal decongestant spray Afrin (which has the active ingredient Oxymetazoline) works really, really well. Too well.

"It's not just habit-forming," Ellis says. "It's totally addictive, because the lining of the nose becomes completely dependent on it."

Afrin relieves congestion by cutting down on blood flow to the conchae, rapidly reducing swelling and opening up the nasal cavity. But soon after it wears off, it leads to rebound swelling, with the conchae getting even bigger than they were before. As a result, many people become totally dependent on Afrin, continuing to use it to fix congestion that it's causing in the first place.

"Once you start spraying, you can't stop," Ellis says. "If you go more than three or four days, the nose becomes so dependent on that it's almost like heroin." Afrin bottles do have a fine print warning telling people not to use the medicine for more than three days, but so many people miss it that there are Afrin addiction support groups online. Ellis, among others, thinks the spray should be a prescription-only medicine.