FiO2 on Room Air
The Fraction of Inspired Oxygen (FiO2) on room air is 20.9% (0.209), which is the standard concentration of oxygen in ambient air at sea level 1, 2.
Understanding FiO2 on Room Air
FiO2 represents the concentration or percentage of oxygen in the gas mixture that a person breathes. It is expressed as either:
- A decimal from 0.21 to 1.0
- A percentage from 21% to 100%
Key Points About Room Air FiO2
- The American Heart Association defines room air FiO2 as 20.9% ± 0.03% at 0% humidity 1
- This value may be slightly affected by humidity levels 1
- For calibration purposes, room air should read 20.93% ± 0.03% O2 at 0% humidity 1
- In clinical practice, room air FiO2 is typically rounded to 21% (0.21) 2
Clinical Significance
Understanding the baseline FiO2 of room air is critical for:
Calibration of respiratory equipment:
- Modern cardiopulmonary exercise testing (CPX) systems and ventilators must be calibrated with room air reading 20.9% ± 0.03% O2 1
- Proper calibration ensures accurate measurement of oxygen consumption and delivery
Oxygen therapy administration:
- Supplemental oxygen increases FiO2 above the room air baseline of 20.9%
- Oxygen delivery systems provide FiO2 ranging from 21% (room air) to 100% (pure oxygen) 2
Altitude considerations:
- At higher altitudes, while the FiO2 remains constant at 20.9%, the partial pressure of oxygen decreases
- Commercial aircraft maintain cabin pressures equivalent to 6,000-8,000 ft altitude, which reduces the partial pressure of inspired oxygen 2
Measurement and Verification
For accurate measurement of FiO2:
- Well-ventilated laboratory spaces ensure representative fraction of inspired O2 1
- A small fan can help maintain proper air circulation 1
- Calibration gases should be used as provided by system manufacturers 1
Common Misconceptions
- FiO2 should not be confused with the partial pressure of oxygen (PaO2)
- While FiO2 remains constant at different altitudes (20.9%), the partial pressure of oxygen decreases with increasing altitude
- The precise FiO2 may vary slightly with humidity levels, but for clinical purposes is considered 21% 1
Understanding the baseline FiO2 of room air (20.9%) is fundamental for proper respiratory assessment, oxygen therapy administration, and equipment calibration in clinical practice.