Pulse oximetry became an everyday vocabulary term during the COVID-19 pandemic, when millions of people bought an oximeter to monitor themselves at home. But beyond that juncture, oximetry is a fundamental medical tool that doctors, nurses, firefighters and mountaineers have been using for decades.
This article explains exactly what the oximeter measures, how the technology works, what the normal values are and when the reading should prompt you to consult a doctor.
If you are at altitude - in the mountains or in a city like Mexico City (2,240 m) or Guadalajara (1,556 m) - the normal values are different from those at sea level. The Oxymeter calculator allows you to know the oxygen available in your environment according to your altitude.
What exactly does the pulse oximeter measure?
A pulse oximeter measures two parameters:
- SpO₂ (Peripheral oxygen saturation): the percentage of hemoglobin in the peripheral capillary blood that carries oxygen. A value of 98% means that 98% of the hemoglobin molecules are "loaded" with O₂.
- Heart rate (pulse): the number of beats per minute, detected by the same optical sensor as it captures the pulse in the capillaries of the finger.
What the oximeter does not measure: absolute blood oxygen content (which also depends on total hemoglobin), CO₂, blood pH, nor ventilation. That is why it is a screening tool, not a definitive diagnostic tool.
How the technology works: photoplethysmography
The physical principle behind the oximeter is photoplethysmography (PPG):
Two LEDs - one red light (660 nm) and one infrared light (940 nm) - illuminate the finger tissue from one side 2. 2. A photodetector on the opposite side measures how much light passes through the tissue. 3. Oxygenated hemoglobin (HbO₂) and deoxygenated hemoglobin (Hb) absorb the two colors differently: HbO₂ absorbs more infrared; deoxygenated Hb absorbs more red 4. The quotient between the absorptions at the two wavelengths allows SpO₂ to be calculated with a margin of error of ±2-3%.
The device also detects heart beats by observing how the light absorption varies with each beat - hence the term "pulsed".
Normal SpO₂ values per context.
At sea level
| SpO₂ | Interpretation | | |---|---| | | 97-100% | Normal - excellent oxygenation | | | 95-96% | Normal - lower acceptable limit | | | 93-94% | Mild hypoxemia - monitor, evaluate with physician | | | 90-92% | Moderate Hypoxemia - consult physician | | | < 90% | Severe Hypoxemia - urgent medical attention | | | < 85% | Emergency - call 911 | |
| At altitude (effect of barometric pressure)
In high altitude cities such as Mexico City (2,240 m), Puebla (2,135 m) or Toluca (2,680 m), available oxygen is lower than at sea level. A SpO₂ between 92% and 96% is perfectly normal for an acclimatized inhabitant of these cities.
| City / Altitude | Available O₂ | Normal SpO₂ acclimatized |
|---|---|---|
| Mexico City (2,240 m) | 75% | 93-97% |
| Puebla (2,135 m) | 77% | |
| Toluca / Metepec (2,680 m) | 72% | 91-96% |
| Xalapa (1,440 m) | 83% | 95-98% |
| Monterrey (540 m) | 94% | 96-99% |
*Values calculated with the ICAO Standard Atmosphere model.
How to use the oximeter correctly
An incorrect reading can lead you to wrong decisions. Follow these steps:
- Sit quietly for at least 2-3 minutes before measuring.
- Warm your hands if they are cold: cold causes vasoconstriction which gives falsely low readings.
- Insert index or middle finger without nail polish (dark polish blocks the sensor).
- **Stand still and breathe normally - do not hold your breath.
- Wait for stabilization (10-30 seconds) before reading the value.
- Record both values: SpO₂ and heart rate.
Factors that alter the reading
- Cold / vasoconstriction:** readings 3-7 points lower than actual value.
- Dark nail polish:** blocks the light from the sensor
- Movement:** generates artifacts and unstable readings
- Heavily pigmented skin:** may reduce accuracy in low-end devices
- Severe anemia:** the oximeter may give "normal" values even if there is low total hemoglobin.
Pulse oximetry in specific contexts
At home for monitoring respiratory disease
Pulse oximeter is useful in home monitoring of people with:
- COPD (Chronic Obstructive Pulmonary Disease)
- Severe asthma
- Heart failure
- Sleep apnea (nocturnal monitoring)
- Post-COVID with residual respiratory symptoms
In these cases, the physician usually establishes the SpO₂ threshold below which to call or go to the emergency department.
In high altitude
At altitude, SpO₂ drops physiologically because air pressure is lower. For mountaineers, the oximeter is a key tool for monitoring acclimatization. At Pico de Orizaba (5,636 m) or Iztaccihuatl (5,286 m), expected SpO₂ values in acclimatized individuals are 75-85%.
In newborns and children
Normal SpO₂ values in children are similar to those in adults (≥ 95% at sea level). In newborns during the first hours of life, transient values of 90-95% may be normal while pulmonary transition is completed. If in doubt, consult a pediatrician.
When to call the doctor
- SpO₂ < 94% persistently at rest at sea level.
- SpO₂ dropping below 90% at any time.
- SpO₂ < 85%: emergency - call 911.
- Drop of more than 5 points from previous measurements without explanation (fever, recent strenuous exertion)
- Discordance between how you feel and what the oximeter shows.
Frequently Asked Questions
What does a pulse oximeter measure?
It measures SpO₂ (peripheral oxygen saturation) and heart rate. The SpO₂ indicates what percentage of the hemoglobin in blood is carrying oxygen.
What oximetry value is normal?
At sea level: 95-100%. In high altitude cities like Mexico City (2,240 m): 93-97% is normal for acclimatized inhabitants. Below 94% at sea level, consult a physician.
Is the pulse oximeter accurate?
CE IIa or FDA 510k certified models are accurate to ±2-3%. Cold, movement and dark enamel can alter the reading. Always read at rest with a warm finger.
When should I go to the emergency room for an oximeter reading?
SpO₂ less than 90% persistently, or less than 85% at any time: seek emergency care (911 in Mexico). At altitude, consult the Oxymeter calculator table of values by altitude.
Do you live or travel to an altitude city in Mexico? The Oxymeter calculator shows you how much oxygen is available in the air according to your altitude - and what SpO₂ values are normal in your environment.