A polysomnography, which measures an individual's pulse oximetry (oxygen saturation), AHI rating of <5 is considered normal, 5-15 is mild and 15-30 is moderate, while anything over 30 is severe.
After the identification of the sleep disorder sleep apnea in the 1970s, the breathing functions, respiratory airflow, and respiratory effort indicators were added along with peripheral pulse oximetry. Patients are monitored with EEG leads, pulse oximetry, temperature and pressure sensors to detect nasal and oral airflow, respiratory impedance plethysmography or similar resistance belts around the chest and abdomen to detect motion, an ECG lead, and EMG sensors to detect muscle contraction in the chin, chest, and legs. An AHI of less than 5 is considered normal. An AHI of 5-15 is mild; 15-30 is moderate and more than 30 events per hour characterizes severe sleep apnea. 
