Each time we swallow, small amounts of air enter the stomach. This gas in the stomach is usually passed into the small intestine where part of it is absorbed. In some people, part of the gas is belched out instead of being passed from the stomach into the intestine. This happens for several reasons. People under a lot of stress often swallow large amounts of air.
Some people swallow air frequently because they have post-nasal drip, chew gum, or smoke.
Rapid eating or poorly fitting dentures also may cause too much air to be swallowed. Also, drinking beverages that contain carbonated water may increase gas in the digestive tract. These drinks contain carbon dioxide, which can produce large amounts of gas when warmed in the stomach. Some people experience frequent belching. This might occur after a person has swallowed air without realizing it.