The sinuses produce an astonishing one liter of mucus every 24 hours to flush the dirt and germs we breathe from our rhinal passageways. The mucus runs down into the esophagus and then to the stomach, where it is digested. But when mucus thickens, it cannot effectively clear the irritants we inhale with every breath. Bacteria build up, and a sinus infection sets in. The first is usually a bacterial infection, precipitated by a common cold.
Chronic sinusitis is often the result of untreated or repeated acute infections.
Neither is pleasant and avoiding them is not always easy. Your physician will be able to test for any food allergies you may have. Some common ones, found in most Asian diets, are soy, corn and yeast. If you do have sensitive sinuses, don't swim without nose plugs. And if you have a cold, don't swim at all.