PAO2, the partial pressure of oxygen in the alveoli of your lungs, is calculated using the alveolar gas equation: PAO2 = FiO2 × (PB − PH2O) − (PaCO2 ÷ R). At sea level on room air, a normal PAO2 works out to roughly 100 mmHg. The calculation itself is straightforward once you know what each variable means and which standard values to plug in.
The Alveolar Gas Equation
The full formula is:
PAO2 = FiO2 × (PB − PH2O) − (PaCO2 ÷ R)
Here’s what each piece represents:
- FiO2 is the fraction of oxygen in the air you’re breathing. Room air is 0.21 (21% oxygen). Supplemental oxygen raises this value.
- PB is barometric (atmospheric) pressure. At sea level, this is approximately 760 mmHg. It drops at higher altitudes.
- PH2O is water vapor pressure inside the lungs. At normal body temperature (37°C), this is always 47 mmHg. It’s treated as a constant.
- PaCO2 is the partial pressure of carbon dioxide in arterial blood, obtained from an arterial blood gas (ABG). A normal value is around 40 mmHg.
- R is the respiratory quotient, the ratio of CO2 your body produces to the O2 it consumes. The standard value used is 0.8.
Step-by-Step Example at Sea Level
For a healthy person breathing room air at sea level with a normal PaCO2 of 40 mmHg:
First, subtract water vapor pressure from barometric pressure: 760 − 47 = 713 mmHg. This gives you the “dry” pressure available for gases in the lungs.
Next, multiply by the fraction of inspired oxygen: 713 × 0.21 = 149.7 mmHg. This is roughly how much oxygen pressure reaches the alveoli before gas exchange.
Then divide PaCO2 by the respiratory quotient: 40 ÷ 0.8 = 50 mmHg. This accounts for the oxygen displaced by carbon dioxide in the alveoli.
Finally, subtract: 149.7 − 50 = 99.7 mmHg. A PAO2 of about 100 mmHg is the textbook normal for a young, healthy adult at sea level.
What the Respiratory Quotient Actually Changes
Most calculations use 0.8 for R, but this value shifts depending on what fuel your body is burning. Fat metabolism produces an R of about 0.7, protein sits at 0.8, and pure carbohydrate metabolism pushes R to 1.0. The physiologic range spans 0.7 to 1.2.
In practice, this matters most in critical care. A patient receiving excessive carbohydrate calories through IV nutrition can develop an R above 1.0, increasing CO2 production and making it harder to wean off a ventilator. Conversely, an R below 0.7 suggests underfeeding, with the body turning to ketones for energy. For a standard calculation, 0.8 is the accepted default.
How Altitude Affects the Calculation
The only variable that changes with elevation is PB. At higher altitudes, atmospheric pressure drops, which lowers the amount of oxygen available in each breath. Denver (about 5,280 feet) has a barometric pressure near 630 mmHg instead of 760. Plugging that into the equation: 0.21 × (630 − 47) − (40 ÷ 0.8) = 122.4 − 50 = 72.4 mmHg. That’s a meaningfully lower PAO2, which is why people feel short of breath when they first arrive at altitude and why oxygen thresholds need to be interpreted differently in mountain cities.
PAO2 vs. PaO2: Why Both Matter
PAO2 (capital A) is the calculated oxygen level in the alveoli. PaO2 (lowercase a) is the measured oxygen level in arterial blood, taken directly from an ABG. In a perfect lung, these two numbers would be identical, but they never are. Oxygen always loses some pressure crossing from the alveoli into the bloodstream.
The gap between them is called the A-a gradient:
A-a gradient = PAO2 − PaO2
A normal A-a gradient increases with age. A conservative estimate for your expected normal is your age in years divided by 4, plus 4. So a 60-year-old would have a normal gradient of about 19 mmHg (60 ÷ 4 + 4 = 19). A gradient significantly above that expected value suggests a problem with gas exchange in the lungs, such as a blood clot, pneumonia, or fluid buildup, rather than simply breathing too slowly or too shallowly.
Normal PaO2 by Age
Normal arterial oxygen levels decline naturally with age. A healthy young adult at sea level typically has a PaO2 of 80 to 100 mmHg. By age 70, a PaO2 of 70 to 80 mmHg can be perfectly normal. A useful rule of thumb: at sea level, expected PaO2 in mmHg equals roughly 100 minus the number of years over 40. A 60-year-old’s expected PaO2 would be about 80 mmHg.
Measured PaO2 values are classified by severity:
- 80–100 mmHg: Normal oxygenation
- 60–79 mmHg: Mild hypoxemia
- 40–59 mmHg: Moderate hypoxemia
- Below 40 mmHg: Severe hypoxemia
- Above 100 mmHg: Hyperoxemia (too much oxygen, typically from supplemental O2)
The P/F Ratio: A Quick Severity Check
In clinical settings, the P/F ratio (PaO2 divided by FiO2) offers a fast way to gauge how well the lungs are transferring oxygen. A normal P/F ratio is above 300. The Berlin criteria use this ratio to classify acute respiratory distress syndrome (ARDS):
- Mild ARDS: P/F ratio between 200 and 300
- Moderate ARDS: P/F ratio between 100 and 200
- Severe ARDS: P/F ratio of 100 or below
This ratio is especially useful because it accounts for supplemental oxygen. A PaO2 of 80 mmHg sounds reasonable on its own, but if the patient is breathing 100% oxygen (FiO2 of 1.0), the P/F ratio is only 80, indicating severe lung impairment. Without the ratio, that context would be invisible.
Putting It All Together
The alveolar gas equation gives you the theoretical best-case oxygen level in the lungs. Comparing that number to the actual measured PaO2 tells you whether the lungs are exchanging gas efficiently. In practice, you need just one measured value (PaCO2 from an ABG) plus knowledge of two things: what the patient is breathing (FiO2) and where they are (altitude, which determines PB). Everything else is a constant or a standard assumption. The water vapor pressure is always 47, and the respiratory quotient is 0.8 unless you have a specific reason to think otherwise.