Doctors and other healthcare professionals use a small device called a pulse oximeter to measure oxygen saturation levels.
These levels indicate the percentage of oxygen the hemoglobin in your blood is carrying compared with the maximum amount it could carry. The results help determine if there is a need for medical treatment, such as supplemental oxygen.
This article covers how doctors look at blood oxygen and what the different levels mean.
There are variables that affect how doctors interpret levels of blood oxygen, such as whether or not someone has a chronic lung condition.
Generally, however, the levels that medical professionals consider normal, borderline, and low are as follows:
It is not possible for blood oxygen saturation levels to exceed 100%. However, researchers explain that people who receive concentrated oxygen may be at risk of oxygen toxicity or oxygen poisoning. This includes:
Oxygen toxicity can occur when someone receives highly concentrated oxygen for a short amount of time or a lower concentration but for a longer time.
Pulse oximetry to measure blood oxygen saturation
A pulse oximeter clips or tapes to your fingertip and shines two lights — a red beam and an infrared beam — through your skin. The device uses the lights to analyze the color and movement of blood cells in your capillaries to measure oxygen saturation levels.
Among other things, it can also:
- monitor how well you are responding to treatment if you have a condition that affects your lungs
- help analyze how your heart performs during a diagnostic stress test
- help identify sleep apnea
You can buy pulse oximeters over the counter to measure your own blood oxygen levels. However, these may not be as accurate as medical grade devices. If you monitor your levels at home, follow the instructions for use carefully, and contact your doctor right away if you have any concerns about your readings.
Factors that can affect oximeter readings
All pulse oximeters have limitations. In general, the lower your blood oxygen level, the less accurate the oximeter will be.
Oximeter measurements are estimates, and several factors can affect readings, including:
- poor circulation
- skin temperature
- nail polish or artificial nails
Dark skin tones and oximeter readings
Due to how different skin tones absorb light, pulse oximeters can overestimate the oxygen levels in people with dark skin.
In a 2020 letter to the New England Journal of Medicine, the authors describe how they compared blood oxygen saturation as measured by a pulse oximeter with measurements from an arterial blood gas test in large groups of white and Black participants. Those in the Black cohort were
three times as likely to have low blood oxygen levels in the arterial blood gas test after a pulse oximeter showed normal oxygen saturation readings.
Older research in the journal Anesthesia & Analgesia found that measurement discrepancies between light and dark skin can be more pronounced with an adhesive-style pulse oximeter rather than a clip-on device.
Because doctors use pulse oximetry so widely as a diagnostic tool, the Food and Drug Administration (FDA) recommends further research into how skin color affects the accuracy of pulse oximetry readings.
Oxygen levels and COVID-19
Many people became aware of the importance of blood oxygen saturation levels during the COVID-19 pandemic, when low oxygen saturation became a known indicator of serious illness.
A 2021 study in the journal Influenza and Other Respiratory Viruses found that people with COVID-19 who presented with oxygen levels below 92% were up to four times more likely to have a severe progression that became fatal.
However, the same study notes that many people with low oxygen saturation do not necessarily have respiratory symptoms, such as difficulty breathing. The Centers for Disease Control and Prevention (CDC) advises doctors to begin oxygen therapy in anyone with COVID-19 who has a blood oxygen saturation level below 90%, even if they are not showing any breathing symptoms.
If you measure your oxygen level at home and get a reading of 91–94% or lower, call your doctor, particularly if you may have had exposure to COVID-19.
If you are having difficulty breathing or experiencing other symptoms — such as chest pain, chest pressure, or dizziness — call 911.
You may not have any symptoms when you have oxygen levels that are slightly below normal, but if your levels drop significantly, symptoms can include:
- shortness of breath or difficulty breathing
- a worsening cough
- chest pain or a feeling of tightness
- a fast heartbeat
- a bluish tint to your nails or skin
Only a healthcare professional can diagnose a medical condition such as hypoxemia, so it is important to call a doctor if you think your oxygen levels might be low.
Several conditions can cause low blood oxygen saturation, including:
If your oxygen levels are low, a doctor can decide if you need supplemental oxygen. Medical professionals can give you supplemental oxygen, or oxygen therapy, in several ways.
If you have a chronic condition that means that you need extra oxygen at home, you can receive:
- Compressed oxygen gas: This is almost pure oxygen that is stored in tanks under pressure. When activated, the tank delivers oxygen to you through a cannula (plastic tubing that fits in your nostrils) or a face mask.
- Liquid oxygen: This is almost pure oxygen in liquid form. It can be stored in a small device but may evaporate.
- Oxygen concentrators: These are stationary devices that supply oxygen from room air. This means that they do not run out.
In a hospital or medical setting, you may receive supplemental oxygen as compressed gas or more intensive supplemental oxygen, such as:
- Invasive mechanical ventilation: A mechanical ventilator “breathes” for a person in respiratory failure, acting like bellows to push air in and out of their lungs.
- Hyperbaric oxygen: This uses pressurized oxygen delivered in a special chamber. It can speed up healing from wounds or carbon monoxide poisoning.
A pulse oximeter reading indicates how much oxygen is getting to your body through your bloodstream compared with the maximum amount it could carry. It is a basic indicator of health, similar to vital signs.
A normal blood oxygen saturation level is in the range of 95–100%. If it falls below 91%, it requires immediate medical attention. Doctors consider a reading of 91–94% as borderline.
Doctors and other healthcare professionals use pulse oximeters to measure blood oxygen saturation. These devices are painless and easy to use. However, there are concerns about their accuracy, particularly in people with dark skin.
COVID-19 has brought attention to blood oxygen levels, as a low blood oxygen saturation level can indicate severe disease that may become fatal. Talk with your doctor right away if you measure your blood oxygen saturation at home and have any concerns about your reading or if you experience mild respiratory symptoms.
Call 911 for any serious symptoms, such as difficulty breathing, chest pain, or a loss of consciousness.