An ‘oxygen concentrator’ is a device that draws in outside air and purifies it for people with medical conditions that require oxygen therapy. These conditions include respiratory diseases that prevent the lungs from absorbing enough oxygen, such as chronic obstructive pulmonary disease (COPD) and asthma, as well as other conditions that affect breathing like sleep apnea and cystic fibrosis. Oxygen concentrators require a doctor’s prescription, and cannot be purchased over the counter.