Blood Oxygen and Oximeter Levels Explained

We've all had a bulky clippy thing placed on our finger when we see our doctor. That little gizmo is called a pulse oximeter or pulse ox. This electronic medical device measures the level of saturated oxygen your red blood cells are carrying. The preferred location for placement of the pulse ox is one of your fingers, usually the ring, index, or middle finger. Your healthcare worker can also attach the oximeter to your nose, foot, forehead, ears, or toes. Modern smartwatches are also able to detect your blood oxygen levels from your wrist.

Read on to learn what your blood oxygen levels mean.

How a Pulse Oximeter Works

A pulse oximeter estimates the amount of oxygen in your blood. Once attached to a part of the body, the pulse ox emits a type of light that passes through the body, usually a finger. A sensor on the back side of the finger detects and measures how much light passes through the finger. The device uses that measurement to calculate the level of oxygen saturation in the blood.

Using pulse oximetry offers advantages over other, more traditional methods of measuring blood oxygen levels. Drawing a blood sample to test blood oxygen levels usually involves a needle stick into an artery in the wrist or arm. This is a painful experience for the patient and requires about 15 minutes to analyze the blood sample. A pulse ox device is noninvasive and provides its readings almost immediately.

What Are Blood Oxygen Levels?

When you breathe in, oxygen goes from your lungs into your bloodstream. The amount of oxygen you have in your blood is your blood oxygen level. To function properly, your body needs a certain amount of oxygen, and low oxygen levels can lead to serious health issues. 

Once oxygen enters your body through your lungs, it passes into your bloodstream. From there, it goes to all cells in the body. The cells use the oxygen and create carbon dioxide as a waste product. Your bloodstream carries the carbon dioxide back to your lungs, where you breathe it out. Your body regulates the amount of oxygen in your blood because serious conditions and damage to body organs and systems can occur when the blood oxygen levels drop.

What Is a Good Blood Oxygen Level?

Your oxygen saturation levels indicate how well your lungs and heart are doing their jobs — namely, supporting your body's essential functions, such as repairing cells, supplying energy, and supporting your immune system. Blood oxygen levels show the percentage of oxygen actually carried by the hemoglobin in your blood compared with the amount of oxygen it could carry. The results guide the doctor in assessing your need for medical treatment, such as supplemental oxygen.

Doctors interpret blood oxygen levels based on an individual's medical condition, such as whether the person has a chronic lung condition. Generally, medical professionals divide blood oxygen readings into the following levels:

• Normal, or 95-100%
• Borderline, or 91-94%
• Low, or under 91%
• A reading below 85% requires immediate medical attention

In many kinds of situations, the amount of oxygen in your blood may be valuable information to know. Some of these situations include: 

• To assess how well lung medications are working
• During or after surgery or medical procedures that require sedation
• Assessment of a person's ability to tolerate increased activity levels
• As an adjunct to whether or not a person needs a ventilator to help with breathing, or to assess how well a ventilator is working
• To ascertain if a person has moments when they stop breathing during sleep, or sleep apnea

Oxygen levels fluctuate throughout the day, depending on the activity in which you're engaged. Exercise is a good example.

How Blood Oxygen Levels Change During Exercise

It's beneficial to measure your oxygen saturation with a pulse oximeter during exercise, but the numbers could also be misleading. At the start of your workout, your oxygen levels will fall because the change in activity level alters various properties of your blood. Your increased physical activity decreases the amount of oxygen that can bind to the hemoglobin in your red blood cells. 

Your body quickly adjusts, however, and your oxygen levels come back up. As you continue exercising, your body adapts to the various levels of oxygenation by increasing your breathing rate. This increases your blood oxygen levels. If you aren't getting enough oxygen during exercise, your body will let you know. Your breathing will become labored, and you won't be able to continue your exercise. 

Other Factors that Affect Your Blood Oxygen Levels

All devices carry with them the risk of inaccurate readings. According to the FDA's safety communication in 2021, although pulse oximetry is a useful tool for estimating blood oxygen levels, the devices have limits, and the risk of inaccurate readings should be considered under certain circumstances. Factors that can affect the accuracy of a pulse oximeter reading include:

• Skin pigmentation
• Poor circulation
• Skin temperature
• Skin thickness
• Tobacco use
• Fingernail polish

Certain physical conditions may interfere with your lungs' ability to adequately process the oxygen-containing air you inhale and the carbon dioxide-filled air you exhale. Some blood disorders and circulation problems may prevent your blood from obtaining oxygen from your lungs and transporting it throughout your body. 

Any of the following problems can lead to a decline in your oxygen saturation levels:

• Chronic obstructive pulmonary disease (COPD)
• Chronic bronchitis and emphysema
• Acute respiratory distress syndrome
• Anemia
• Heart disease
• Asthma
• Pulmonary embolism
• Collapsed lung
• Congenital heart defects

Any of the conditions can lead to a decline in your oxygen saturation levels.  

What to Do if You Have Low Blood Oxygen Levels

If your oxygen levels are only slightly low, you may not have any symptoms. If your oxygen levels drop significantly, you may exhibit any of the following:

• Shortness of breath
• Restlessness
• Headaches
• Dizziness
• Chest pain
• Rapid breathing
• High blood pressure
• Confusion
• Lack of coordination
• Visual disorders
• Rapid heartbeat
• Sense of euphoria

See your doctor if you experience any of these symptoms while exercising or while at rest.

It Doesn't Stop There

Even after you've completed your workout and resumed your normal routine, your body's metabolism is still burning more calories. This is known as excess post-exercise oxygen consumption (EPOC) and is the amount of oxygen your body needs to get back to its usual resting level of metabolic function. Learn more about how oxygen plays a role in your workouts, such as by calculating your VO2 max.