The oxygen level in your blood can be precisely measured using a blood test known as an arterial blood gas (ABG) study. Although this test is the most accurate of testing methods, it is invasive, meaning it uses a needle to draw blood out of an artery, usually from your wrist and is therefore less preferable to many patients. A less accurate, but more common way to estimate oxygen saturation levels is with a pulse oximeter, a small, non-invasive device that attaches to the end of your finger. When measured by ABGs, normal oxygen levels are approximately 75-100 millimeters of mercury (mm Hg). When measured by pulse oximetry, normal oxygen levels are 95-100 percent. In general, values below 55 mm Hg or 88% or less usually mean you need long-term oxygen therapy. If your oxygen levels drop only during sleep or exercise, you may need supplemental oxygen during these specific time periods but not all the time.
What Causes Oxygen Levels to Drop?
Many health conditions can cause oxygen levels to drop including:
The Benefits of Supplemental Oxygen
When normal oxygen levels are compromised, supplemental oxygen can restore them to a healthy level. Besides improving oxygen levels in your blood, there are many benefits of using supplemental oxygen. They include:
Some people with COPD experience lower than normal oxygen levels, even while using supplemental oxygen. If youâre using oxygen therapy at home, itâs important to have a pulse oximeter on hand and keep a record of your oxygen saturation levels along with your oxygen flow rate and activity level at the time your oxygen saturation is measured. This way, your doctor can analyze the results to determine if there needs to be an adjustment in your oxygen flow rate. Itâs also important to never increase or decrease your oxygen flow rate without first having a discussion with your doctor. Doing so can be hazardous to your health.
 Mayo Clinic. Hypoxemia (Low Blood Oxygen). Updated December 25, 2015.
 Christopher R. Swinburn, Hazel Mould, Timothy N. Stone, Paul A. Corris, and G. John Gibson âSymptomatic Benefit of Supplemental Oxygen in Hypoxemic Patients with Chronic Lung Diseaseâ, American Review of Respiratory Disease, Vol. 143, No. 5_pt_1 (1991), pp. 913-915.
 American Lung Association. âSupplemental Oxygenâ. Accessed August 16, 2015.
 Emtner, Margaret et. al. âBenefits of Supplemental Oxygen in Exercise Training in Nonhypoxemic Chronic Obstructive Pulmonary Disease Patientsâ.Â American Journal of Respiratory and Critical Care Medicine, Vol. 168, No. 9 (2003), pp. 1034-1042.
 Aasebo, U., et. al. âReversal of sexual impotence in male patients with COPD and hypoxemia with LTOTâ.Â JÂ SteroidÂ Biochem Mol Biol.Â 1993 Dec;46(6):799-803.