Our blood is the vessel that carries nutrients to every cell of our body. The nutrients include both the inhaled and the absorbed ones; inhaled oxygen from the air, and the absorbed nutrients from the digested food in the gut. But what is a normal blood oxygen level for everyone?

For optimal functioning of the body, the nutrients in our blood should meet an appropriate scale. Too low or too many levels can cause problems. For example, too much sugar can cause diabetes, and too low sugar can cause fatigue or even loss of consciousness.

Similarly, blood oxygen levels should always remain optimal for the body to keep working at its best. Low oxygen level is called hypoxemia, while high levels of oxygen are called hyperoxia. Either of the conditions can cause disturbance in body functioning.

Read on to know details about normal blood oxygen levels, what conditions cause a disruption in normal levels, and how to correct or maintain the optimal oxygen levels.

What is your blood oxygen level?

The blood oxygen level is the amount of oxygen carried by the red blood cells of our vascular system.

Our body regulates and monitors the oxygen levels in the blood quite precisely. The red blood cells of our blood contain hemoglobin, a protein that specializes in binding oxygen inhaled via the lungs.

Any condition that disturbs either of the following disrupts the normal blood oxygen levels;

• functioning of lungs or
• the ability of hemoglobin to carry oxygen or other vascular disturbances

Blood oxygen levels are not a vital sign, unlike body temperature or pulse. So it is not regularly checked in hospital settings. However, if you suffer from some chronic condition involving lungs, heart, or blood, your blood oxygen levels may need monitoring.

A measure of blood oxygen levels indicates how well a treatment is working for you. It also shows if any adjustments in the treatment protocol are required.

Measurement of blood oxygen levels

What is a good blood oxygen level? Well, that depends on the measurement method. There are two ways to measure the oxygen saturation of the blood.

1. Arterial blood gas

Measuring arterial blood gas is an invasive method yet very accurate. Blood is drawn from an artery, often the one in the wrist, because it is superficial and easily accessed.

Arteries contain oxygenated blood, so their levels show the maximum oxygen carried by the blood.

Arterial blood can detect levels of other gases as well. It also shows the pH levels of the blood.

• Pulse oximeter

A pulse oximeter is a device to measure blood oxygen levels. The method is non-invasive and measures the oxygen saturation by sending infrared light into the small vessels, the capillaries in one of your fingers.

The pulse oximeter can measure the readings from your earlobe and toes. The intensity of the reflected light gives a measure of the oxygen saturation in the blood.

The unit of measurement of blood oxygen saturation is SpO2. A pulse oximeter has an error window of about 2%. This means the reading may be two readings higher or lower than the exact value.

Though pulse oximeter is slightly less accurate than the arterial blood gas measurements, it is convenient to perform. The device gives a continuous reading as long as it is attached to the patient's body, removing the requirement of taking measurements now and then. The device is easy to use at home as well.

Pulse oximeter requires clear nails to give a reading. This means you need to remove any nail polish from the nails. Also, cold extremities may give a false reading, so taking care of the ambient temperature is important.

Smokers usually have inaccurate pulse oximeter readings. Their blood contains high carbon monoxide levels, and a pulse oximeter cannot differentiate between this gas and oxygen. Smokers usually have a falsely high pulse oximeter reading. In their case, arterial blood gas is the only way to measure blood oxygen saturation.

Other Blood oxygen levels measuring devices

Apple has launched a watch with a built-in sensor to measure blood oxygen levels. There is a hype created around the use of such and other health devices.

However, these health devices should be used with caution since they are consumer devices with different integrated functions. Their readings should not be used for self-diagnosis. Instead, the values should be taken as a warning for one's health status.

The Apple Watch is equipped with an oximeter that uses a built-in blood oxygen app. The app can be used to trigger on-demand or periodic check-ups during 24 hours manually.

The blood oxygen sensor in an Apple watch comes with four LED clusters and four photodiodes. These are placed into the back of the watch case. These are green, red, and infrared LEDs.  The light shines onto the blood vessels in the wrist. The photodiodes capture and measure the amount of light reflected. The app works to detect the color of your blood as a sign of blood oxygen measure.

The measurements should not be considered accurate since the readings are affected by a low oxygen environment or body temperature, increasing or decreasing the local blood supply.

What are the different blood oxygen measurements?

Doctors use the term oxygen saturation level to describe the blood oxygen levels in clinical settings.

The arterial blood gas is indicated as ‘PaO2’ while an ‘O2 sat’ or ‘SpO2’ indicates readings from a pulse oximeter.

• Normal blood oxygen

The normal reading of arterial blood oxygen level is about 80-100 millimeters of mercury (mm Hg).

The normal reading for blood oxygen levels via pulse oximeter reads about 95-100%.

A disease of the lungs causes a disturbance of these normal levels. In chronic conditions, the normal levels of oxygen saturation do not apply. In such cases, a doctor can tell better what the normal range for an individual case is.

For example patients with a severe case of chronic obstructive pulmonary disorder, the pulse oximeter may show values between 88-92%. This is normal for a COPD patient.

• Hypoxemia

Blood oxygen levels below the normal are called hypoxemia or hypoxia.

Hypoxemia corresponds to a PaO2 reading below 80 mm Hg or a pulse ox (SpO2) below 95 percent.  

If you suffer from a chronic lung disease, it helps to know what denotes hypoxemia for you.

• Hyperoxia

Increased blood oxygen levels are rare except in cases where a patient is put on supplemental oxygen. The condition is usually indicated via the arterial blood gas method.

Blood oxygen level chart

Causes of low blood oxygen levels

Our body has a very exclusive regulatory protocol to maintain blood oxygen levels.

The carotid bodies present in the carotid arteries are responsible for sensing changes in blood pressure and blood oxygen saturation levels.

Any disturbance in oxygen levels in the arterial blood is picked up and sent as nerve signals via the carotid sinus nerve to the brain. In the brain, the signals are transformed to direct the breathing rate accordingly.

The breathing rate either increases or decreases in response to the signals from the carotid sensors.

In addition, there are backup support mechanisms as well.

An enzyme, heme oxygenase-2, is the primary blood oxygen sensor. In case of increasing blood oxygen levels, heme enzymes stimulate the production of carbon monoxide. This initiates a series of enzyme activation reactions that block an enzyme, the cystathionine-ϒ-lyase (CSE), with decreasing levels of hydrogen sulfide. When CSE is inactivated, it stops the carotid bodies from sending out nerve signals to increase breathing or air intake.

The reverse happens in the absence of heme oxygenase-2 activity. The hydrogen sulfide goes up, and so do the nerve signals from the carotid bodies.

The body promptly counteracts any oxygen shortage, such as stress, exercise, thin air of high altitude, lung disease as asthma, or sleep apnea. An inadequacy in this response can result in serious health repercussions.

As it is clear, the blood oxygen levels rarely go haywire except in case of disease. Some conditions resulting in hypoxia are;

• COPD like chronic bronchitis and emphysema
• Acute respiratory distress syndrome (ARDS)
• Asthma and allergic conditions
• Collapsed lung
• Anemia
• Congenital heart defects
• Heart disease
• Pneumonia
• Pneumothorax
• Lung cancer
• Pulmonary embolism
• Chemical or gas poisoning
• Low concentration of oxygen in the environment as at high altitudes, flying, accidental drowning, or smoke inhalation during fires
• Covid-19
• Narcotic overuse

Some of these conditions cause inadequate lung function with deficient oxygen inhalation and carbon dioxide removal. Other conditions affect the oxygen-carrying capacity of the blood, like low hemoglobin or anemia. 

Symptoms of hypoxia

Signs and symptoms of low blood oxygen or hypoxia include;

• shortness of breath
• chest pain
• confusion
• headache
• rapid heartbeat
• inability to communicate

Severe signs and symptoms include;

• shortness of breath even at rest that increases with physical activity
• decreased tolerance to physical activity
• shortness of breath while sleeping that may cause you to wake up
• a feeling of choking
• wheezing
• frequent cough
• bluish discoloration of the skin

Persistent low oxygen levels result in cyanosis, that is, the development of blue discoloration of your nail beds, skin, and mucus membranes.

Cyanosis is a medical emergency as it can lead to respiratory failure and death.

Children present with the following signs of hypoxia;

• Lethargy
• Irritability
• Inattentiveness and lack of focus
• Diaphragmatic breathing showed by a pose involving sitting and leaning forwards.
• Drooling and mouth breathing

Causes of high blood oxygen levels

Hyperoxemia is when the arterial oxygen partial pressure (PaO2) increases more than 120 mmHg.

Readings ranging between 120 and 200 mmHg are stated as moderate, while the ones exceeding 200 mmHg are considered severe.

Hyperoxemia is due to hyperoxia, which is an increase in oxygen in the blood. It usually occurs in patients that are on supplemental oxygen due to some disease. About 22% to 50% of mechanically ventilated patients in the ICU suffer from hyperoxia.

Some other causes for hyperoxia include ventilator-associated pneumonia.

There is a risk of hyperoxia in premature infants because their lungs are not fully functional. Any supplemental oxygen given to infants raises their risk of hyperoxia.

Symptoms of hyperoxia

Acute cases of critical illness require emergency interventions to avoid life-threatening issues in medical emergencies and ICU. Doctors prefer to use supplemental oxygen in hypoxic patients to prevent outcomes as tissue death.

It may happen that once the critical stage is over, the patient is still given the same amount of supplemental oxygen as that provided in the critical stage. Hyperoxia may also result from hyperbaric oxygen therapy and inhaling from an oxygen tank while scuba diving.

This can lead to detrimental health effects as the collapse of the lungs. Some symptoms of increased oxygen levels in blood include;

• Constriction of vessels in the brain
• Cell death, especially the neurons of the brain
• Coughing due to fluid accumulation in the lungs
• Mild throat irritation
• Chest pain
• Trouble breathing
• Muscle twitching in face and hands
• Headaches
• Dizziness
• Blurred vision
• Nausea
• Hiccups
• Tingling in hands and feet
• Ringing in ears (tinnitus)
• A feeling of unease or anxiety
• Confusion
• Convulsions  and seizures
• Increased heart rate
• Increased resistance to blood flow in peripheral blood vessels
• Cataract in the eyes on long-standing
• Cardiac arrest and death

How to maintain normal blood oxygen levels

Blood oxygen levels are maintained by intrinsic protocols that are effective and efficient at their function. Preserving overall health ensures the regulatory mechanisms keep working at their best.

Adopting moderate physical activity and quitting smoking help to keep the lungs and heart-healthy. This should suffice to maintain blood oxygen levels. Maintaining adequate personal hygiene to ward off infections and disease work as add-ons to bolster lung and heart health.

Either condition of hypoxia and hyperoxia are detrimental to health and contribute to increased mortality rate.

Note that wearing masks as a safeguard measure against COVID-19 does not impair the respiratory capability of our lungs. Facial masks do not disturb the blood oxygen levels in any way but contracting COVID-19 infection does put you at risk of hypoxia. So save yourself from infection of all kinds and any medical emergencies that may put you in an ICU.