When a person is admitted to the hospital, part of the routine admission process is to determine if the patient requires oxygen. The easiest way to measure that is through a pulse oximeter. It measures the percentage of oxygen concentration in your respiratory system. Most of us have an oxygen saturation percentage in the high 90's or even 100%. When a patient's oxygen saturation drops to 88% or below, they are said to require oxygen to bring their saturation level up to the 90's.
Pulse oximeters come in various sizes, qualities and prices. Hospitals use professional grade oximeters, but there are oximeters that can be purchased for home use that are very reasonably priced and yet can be very accurate.