Oxygen concentrators are used to replenish the body with oxygen at a safe level. The machine pulls in the surrounding air and filters everything else out. So, the user is left with up to 98% pure oxygen through a nasal cannula or approved oxygen mask. Oxygen concentrators are also used in conjunction with CPAP, Bi-Level or ASV machines to help maintain the right level of oxygen. You can find Portable Oxygen concentrators here, as well. They make traveling with oxygen a lot easier.