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Alcohol or drug abuse
Chronic or short-term congestion of the nose and sinuses
Diabetes, type 1 or type 2, that is treated with insulin (usually)
Drugs that can cause drowsiness
Gastroesophageal reflux if severe
Habitual air swallowing
Heart disorders, such as coronary artery disease, heart failure, irregular heart rhythms, valve disorders, and congenital heart defects that allow blood to leak from the venous to the arterial system such as atrial septal defects
Inguinal hernia that has not been repaired
Impulsive behavior or being prone to accidents
Lung problems*, such as asthma, lung cysts, COPD/emphysema, or previous pneumothorax (collapsed lung)
Poor cardiovascular fitness
* Possible higher risk of pulmonary (lung) barotrauma.
† Higher risk of decompression sickness.