Under normal circumstances, concentration of oxygen in inspired air is about percent?

Appeared in: AIIMS Patna NO - 2020

Explanation

Under normal conditions, the air we inspire is atmospheric air.
Atmospheric air is a stable mixture of gases, with oxygen consistently making up approximately 21% of its volume at sea level.
The most abundant gas in the atmosphere is nitrogen (about 78%), followed by oxygen.
In respiratory care, this 21% concentration is referred to as the Fraction of Inspired Oxygen (FiO2) of 0.21, which is the baseline for room air.

Option A: 100% is an artificially high concentration of oxygen not found in normal atmospheric air.
Option B: An oxygen concentration of 6% is critically low and incompatible with human life for any sustained period.
Option C: 29% is an arbitrary and incorrect value for the oxygen concentration in the atmosphere.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Composition of Inspired Air as background academic context rather than a clinical decision trigger.
Understanding that room air is 21% oxygen is fundamental in nursing and respiratory care. This value, expressed as a Fraction of Inspired Oxygen (FiO2) of 0.21, is the baseline for administering all supplemental oxygen therapy.
Nurses use this baseline to titrate oxygen delivery via devices like nasal cannulas or masks to achieve desired blood oxygen saturation levels (SpO2) in patients with hypoxemia or respiratory distress.
Knowledge of this value is critical for patient safety to avoid both hypoxia (too little oxygen) and hyperoxia/oxygen toxicity (too much oxygen).

How to Approach the Question

Identify the core of the question: it asks for a basic, fundamental fact about the composition of the air we breathe.
Recognize this as a factual recall question based on foundational scientific knowledge.
Access your memory of the main components of Earth's atmosphere.
Recall that air is primarily nitrogen (about 4/5th) and oxygen (about 1/5th).
Calculate 1/5th as a percentage, which is 20%. The closest and most accurate answer is 21%.
Evaluate the given options. 21 is a direct match to this known fact. The other options are either too high (100), too low (6), or simply incorrect (29).

Concept Tested & Keywords

QCnNKCh85FDr2RrZoGV6Om

E6 Nelson Textbook of Pediatrics(2024) — Volume 2 p. 298-300

Practise the full AIIMS Patna NO - 2020

Attempt every question from this paper in a timed mock, then review the full solution for each one.