portable oxygen concentrator
For the longest time compressed oxygen has been the standard solution to patients suffering from lack of oxygen. They were either frozen then stored in containers that resembled thermos bottles; or compressed and stored in cylinders under great amounts of pressure. They were usually heavy and cumbersome. Supply can be a problem as they must be ordered ahead of time. And safe storage can be a problem.
Oxygen concentrators are machines which provide supplemental oxygen by compressing room air to 4 atmospheres of pressure and trapping the nitrogen out of the compressed air. This concentrates the oxygen in the compressed air. This machine can be costly, but air is literally free for the supply. A portable oxygen concentrator will turn out to be less expensive for long term use. They are also useful to bring along on travels. The disadvantage is that they can be noisy when in use.