You need a sinus like you need a hole in the head. In fact, that's all a sinus is.
The only place you've got sinuses is in your face. You've got a pair under your eyes in your cheeks: a pair just behind the bridge of your nose; a pair in your forehead above your eyes: if you put your hands flat over your face, they'll cover all your sinuses.
There are spaces left in the bones of the face when they form. All these spaces open into your nose, high up. They make the skull lighter; they help your voice resonate; and, importantly, provide a "crumple zone" that can protect your brain. When your face hits something (or something hits you) very hard, the sinuses break first, absorbing some of the impact and perhaps preventing lethal damage to the brain.
Sinuses are lined with the same kind of skin that's inside the nose and lungs. Like them, it has cells that line it, produce mucus (which is designed to catch dust and germs, like flypaper) and sweep the mucus to the sinus opening, which is about as big as a letter /o/.
People's sinus drainage varies. For some reason, people with long, narrow faces tend to have more sinus problems -- people with the "Prince Charles" type face. People who look like Nanook the Eskimo don't get their sinuses blocked as often, in my experience.
Sinuses get blocked when the mucus doesn't drain as fast as it's produced. Anything that causes your nose to run probably causes your sinuses to make more mucus, too. Allergies can do it, as can a cold or even chronic exposure to irritants like cigarette smoke or other chemicals. If your sinuses can't empty drain through the tiny hole to the nose, you've got a problem.
If allergies are your problem, there are now some new allergy treatments that can really help. Some of the new antihistamines don't make you sleepy, and there are topical steroid nasal sprays that can eliminate allergy symptoms in many people. Elastic strips that help hold your nose open help a lot, too.
Two things that commonly keep the sinus from draining well are tobacco smoke and dryness. Nicotine paralyzes the cells that sweep out the dust and germ-laden mucus, and dryness makes that mucus drier and stickier. When you're outside in a hot and dry place, the mucus in your nose turns dry, hard, and sticky. Your sinuses are designed to move moist, liquid mucus; not rocks.
If the opening to the sinus is blocked by one of these chunks of dried mucus, the problems get worse quickly. Cells that produce mucus keep on doing so, even when the sinus doesn't drain normally. This increases pressure, and the pressure causes pain. And, a pool of stagnant mucus in your sinus is ripe for infection. There's nothing more appetizing to a germ than a pool of stagnant mucus. Yum!
The real trouble starts once your sinus is infected. If the pressure gets high enough, it impairs the blood flow to the area. The body then can't get white blood cells there to fight the infection. The germs grow unopposed, and you can find yourself in a medical emergency. People can die from a bad sinus infection that doesn't get proper treatment.
Badly infected sinuses hurt. They're tender if you tap on the forehead or face over the sinus; the skin may be warm or red; and often you'll have a fever. This is a medical emergency: contact your doctor. When a sinus gets a bacterial infection, antibiotics can be life-saving.
Two things are necessary in treatment: drain the sinus, and kill the germs. Of the two, drainage is the most important. Remember the dried mucus that can plug the sinus opening? This may be contributing to the problem. To moisten it, you must drink lots of extra fluids (try to drink enough to make your urine clear) and get more moisture in the air you're breathing. Taking a hot shower with the doors and windows closed and the fan off can help; so can putting your face over a steaming hot pan of water with a towel over your head. If you moisten a piece of dried mucus that's blocking the tiny sinus opening, you might make it slippery enough to slip out.
Swollen mucus membranes can block the sinuses. If you nose is stuffy and it's hard to breath, the opening to your sinuses may be swollen causing problems draining. One of the few times that I recommend decongestants is when a sinus is blocked. If you do take them, take extra fluids! I also recommend a 12-hour nasal spray containing oxymetozaline to shrink swollen membranes.
It's important to use nasal sprays correctly. First, try not to use them for more than about 5 days, or you might get rebound congestion when you stop. When actually using the spray, you need to decongest your nose all the way up to the sinus opening. Since your nose has little "shelves" of bone (called turbinates) that make the air you breath swirl and drop its dust on the lining, you must decongest it a stage at a time.
Blow your nose well, and clean out whatever you can. Then, close the other nostril with a finger: breath in sharply; and spray the decongestant into each nostril; wait three minutes and repeat. This will hopefully open your nose all the way back to the sinus opening, and the decongestant may shrink the tissues around the hole itself, hopefully relieving the blockage. Remember that decongestants should not be used by people with high blood pressure or heart disease without checking with your doctor first.
Once your nose is open, it would be a good time to breath in the steam mentioned earlier. If one side of your face is more congested than the other, sleep with the bad side up since it's easier for a sinus to drain downhill. A hot water bottle or heating pad may helps, too, and ice bags can be a great help in stopping a sinus headache.
Remember: tobacco smoke paralyzes your sinus linings, as well as your lungs. So: if you’ve got a sinus problem, you need cigarettes like a hole in the head.