Sinus infections generally are characterized by inflammation that blocks the nasal passages, leading to a buildup of mucus that can't drain, causing nasal congestion. Although allergies and cold viruses often are the cause of sinus infections, upper respiratory infections or allergens and pollutants in the air aren't always responsible. Sometimes the root of the problem is something you wouldn't normally consider.

Tobacco Smoke

Smoking is bad for your health in more ways than one. The smoke can irritate your nose, leading to inflammation. In fact, smoking may increase your risk of developing bacterial and viral infections by lowering your immunity and causing changes in your respiratory tract. Any type of pollutant also can damage the cilia – tiny, hairlike strands in the nasal cavity that move mucus out of the sinuses.

Air Travel

If you fly often, you may find yourself more susceptible to frequent sinus infections. The reduced air pressure inside the cabin of the airplane can cause a buildup of pressure in your head. The effect if these unequal pressure differences can lead to blocked sinuses. You can help keep your sinuses clear by using decongestant nasal drops before the plane takes off and lands.

OTC Decongestant Sprays

Rebound nasal congestion can occur if you use over-the-counter nasal sprays for more than 3 or 4 days. Although the spray may relieve nasal congestion from a cold at first, as the lining of your nose gets used to the medication, it can make your sinus symptoms worse.

The medication constricts the blood vessels in your nose, shrinking swollen tissues, which opens up the nasal passages. However, overuse of OTC nasal sprays can cause the nasal passages to swell again.

Chlorinated Water

If you're an avid swimmer, the chlorine in pools can irritate your sinuses by causing inflammation in the lining of the sinuses and nasal passages. When chlorine irritates your nose, mucus thickens and builds up in the sinus cavity, leading to infection. Over-exposure to chlorinated water can also cause swimmer's ear and irritated eyes.

You can help prevent sinus infections that come from swimming by wearing rubber ear plugs or nose clips to keep water from entering the sinuses through your ears or nose. Swimming for shorter periods and keeping your head above the water when you swim can help too.

Bacteria Gone Amiss

A cold that just won't go away may have developed into something more. While bacterial infections aren't a common culprit causing sinus infections, it is possible to develop bacterial sinusitis. Normally harmless bacteria can multiply in the mucus lining of the sinuses when congestion caused by a cold or other viral infection blocks the nasal passageways and prevents the sinuses from draining. If Streptococcus pneumoniae, Haemophilus influenza or other infection-causing bacteria are the source of your sinus infection, your doctor will prescribe antibiotics.

Fungal Infections

While not a common cause of sinus infections in healthy individuals, if you have a weakened immune system, fungi – particularly the Aspergillus species – can grow in your sinuses. Although fungal sinusitis can occur when you inhale a fungal organism that causes inflammation in the nasal passageways, an allergic reaction to the fungus also can trigger chronic sinusitis.

Along with nasal congestion, symptoms of a fungal sinus infection may include headache, fever, facial pain, thick nasal discharge, and changes in vision. Depending on the duration and severity of the infection, treatment may include taking anti-fungal medication or scraping the sinus. In some cases, you may need surgery to remove the fungi.