A Gamow bag is a portable hyperbaric chamber. The 15-pound device consists of a foot pump and a sealed compartment. Once someone is inside the chamber, the compartment is filled full of air, which increases oxygen and simulates conditions at lower altitudes. After about two hours, the conditions inside the bag are about the same as those about 3,000 to 6,000 feet (914.4-1,828.8 m) lower than the bag.
The person's body will acclimate to those conditions, and the acclimatization will persist for up to 12 hours, which allows the person time to make a descent.