Pulmonary barotrauma refers to the injury to the pulmonary tissue caused by a relative positive or negative pressure inside the lungs.
Basic knowledge for Open Water Diver* (OWD*)
Since we are breathing compressed air and our regulator is giving us air according to the ambient pressure, the lungs equalize automatically. As long as we keep breathing normally, this is a continuous process. However, if you hold your breath on ascend, or ascend to fast without continuous breathing, the expanding air in your lungs get trapped. At the same time the ambient pressure is decreasing causing a relative overpressure in your lungs which will lead to tear in the pulmonary tissue. Depending on the severity, this can be life-threatening.
Airtrapping is a form of a pulmonary barotrauma: The airways must be open up to the last alveoli. Blockages due to mucus or tar (smoking) can cause airtrapping. Air from individual, closed alveoli can no longer escape. The result can be a tear in the thin wall of the affected alveoli and air can enter the bloodstream.
When diving, especially when ascending, never hold your breath. Keep on breathing normally and continuously. Do not exceed the maximum ascent rate of 10 m per minute. If you don't feel well, don't dive. Avoid smoking especially before a dive and go regularly for a medical examination.