Sinusitis is the most common chronic illness in the United States and, if left untreated, it can become a serious illness.

The disease affects an estimated 14 percent of Americans, according to an article in the current issue of Redbook, a Hearst magazine, by Dr. Richard N. Podell and Beth Weinhouse, and many sufferers don't know they have it, dismissing their symptoms as a lingering cold.This inflammation of the sinus cavities can lead to asthma, bronchitis and an inflammation of the brain.

The sinuses are holes in the skull between the facial bones. There are four large sinuses - two inside the cheekbones and two above the eyes, as well as smaller ones located between the larger ones.

The sinuses are lined with membranes that secrete antibody-containing mucus, which protects the respiratory passages from the onslaught of irritants in the air we breathe. The nose and sinuses also provide the resonance in our voice.

Sinusitis is an inflammation of the membranes lining the sinuses that may or may not be accompanied by a bacterial infection.

There are many causes of sinusitis - allergies to dust, pollen and pet dander, indoor air pollutants such as cigarette smoke, rug shampoo and formaldehyde, and outdoor air pollutants - all can induce inflammation.

Excessive dryness in homes and offices from dry-air heating and air-conditioning systems can also inflame the sinuses, as can a cold.

Once the sinuses are inflamed, they are more vulnerable to bacterial infection. If the nasal congestion from a cold gets worse instead of clearing up after three to five days, it may be because the sinus membranes, inflamed and weakened by the cold virus, have been attacked by a bacterial infection.

Once infected by bacteria, the sinus membranes are more sensitive to allergens and irritants and become even more inflamed, setting in motion the vicious cycle of chronic sinusitis.

The classic symptoms of sinusitis are an itchy, stuffy nose - sometimes with green or yellow nasal discharge - and postnasal drip. Often sufferers feel as if they have a perpetual cold, with pressure or pain in the cheekbones, forehead or between the eyes.

There are many other complaints that can be signs of sinusitis, including migraines, chronic throat-clearing, coughing, raspy voice, sore throat, snoring, bad breath, puffy eyes, stuffy ears, fatigue, irritability and depression.

More than two of these symptoms, especially if they have been present for more than 10 days, should make you suspect sinusitis. See your doctor for diagnosis and treatment.

If a bacterial infection is present, standard antibiotics are usually prescribed.

Patients should be counseled about avoiding sinusitis triggers such as cigarette smoke, dry air, dust, mold, pet dander or pollen.

Among the remedies your doctor may prescribe are decongestants, over-the-counter nasal sprays, prescription inhalers, expectorants, antihistamines, humidifiers and salt-water sprays.

The best way to prevent sinusitis from recurring is to be vigilant about following a few preventive measures. Keep sinus membranes moist by raising low household humidity with a humidifier. Reduce indoor air pollution by keeping the house smoke free. At the office, request a smoke-free working space. Improve ventilation by opening windows when-ever possible.