Below 90 percent.
Oxygen can also be used at home if the oxygen level in your blood is too low for long periods. Long-term oxygen therapy should be used for at least 15 hours a day with as few interruptions as possible. Regular use can reduce the risk of death from low oxygen levels. 1 The level of oxygen in airplanes is about the same as the oxygen level at an elevation of 8000 ft (2400 m) . 2  This drop in oxygen can really affect people with COPD. 3  If you normally use oxygen or have borderline-low oxygen levels in your blood, you may need oxygen when you fly Healthy people usually have a blood oxygen level above 95 percent. Added oxygen may be needed for people whose oxygen level falls below 90 percent and home oxygen therapy may be recommended to regain normal levels. When oxygen saturation drops below 80 percent, severe hypoxemia occurs. In order to live, every cell in your body needs oxygen to properly function. Oxygen is a critical nutrient and the body can die within six minutes without it. Any level below 88 percent is dangerous and incapacitating to your body. High oxygen levels are important for overall good health By Miranda Hitti. WebMD Health News. April 27, 2005 -- A small U.K. study reports that as a result of changes in cabin pressure on commercial airplanes, oxygen levels drop to a degree that is potentially dangerous for people with heart and lung problems.  Hypoxemia, or low blood oxygen, describes a lower than normal level of oxygen in your blood. In order to function properly, your body needs a certain level of oxygen circulating in the blood to cells and tissues. When this level of oxygen falls below a certain amount, hypoxemia occurs and you may experience shortness of breath. Your doctor determines whether you have hypoxemia by measuring your blood oxygen level — the amount of oxygen traveling in your arteries. Oxygen saturation is a term referring to the fraction of oxygen-saturated hemoglobin relative to total hemoglobin (unsaturated + saturated) in the blood. The human body requires and regulates a very precise and specific balance of oxygen in the blood. Normal blood oxygen levels in humans are considered 95-100 percent. If the level is below 90 percent, it is considered low resulting in hypoxemia. 267 Followers. A. Many things can cause low oxygen levels in the blood. To name a few: Lung injury (collapsed lung); lung disease (COPD, lung cancer); pulmonary embolism; heart disease (congestive heart failure); anemias -- and that's just a handful I can think of off the top of my head. 
http://www.webmd.com/lung/copd/oxygen-treatment-for-chronic-obstructive-pulmonary-disease-copd http://www.ehow.com/about_5514172_dangerous-blood-oxygen-levels.html http://www.webmd.com/heart-disease/news/20050427/oxygen-level-on-flights-may-hit-danger-mark http://www.mayoclinic.org/symptoms/hypoxemia/basics/definition/SYM-20050930 https://en.wikipedia.org/wiki/Oxygen_saturation_(medicine) http://answers.webmd.com/answers/5003781/what-causes-low-oxygen-level
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