What is the approximate composition of Earth's atmosphere?

What is the approximate composition of Earth's atmosphere?

• A. 78% nitrogen, 21% oxygen, 1% other gases
• B. 70% nitrogen, 29% oxygen, 1% other gases
• C. 60% nitrogen, 40% oxygen, 0% other gases
• D. 50% nitrogen, 50% oxygen

Answer: (A)

The air we breathe at Earth's surface is dominated by two gases, with a small amount of everything else combined. About 78 percent is nitrogen, and about 21 percent is oxygen, with the remaining roughly 1 percent made up of all the other trace gases (argon, CO2, neon, helium, methane, krypton, hydrogen, and others). This combination is a standard reference for sea-level, dry air. Understanding this matters in aerospace physiology because the partial pressures of these gases determine how much oxygen is available for respiration and how the gases behave in the lungs and bloodstream, especially as altitude changes. The large dominance of nitrogen and the substantial but smaller amount of oxygen, plus the tiny fraction of everything else, is what makes this composition the best representation of Earth's atmosphere. The other options differ by tens of percent for the major components (for example, oxygen around 29% or 40%), which is not how the atmosphere is composed at sea level.

The air we breathe at Earth's surface is dominated by two gases, with a small amount of everything else combined. About 78 percent is nitrogen, and about 21 percent is oxygen, with the remaining roughly 1 percent made up of all the other trace gases (argon, CO2, neon, helium, methane, krypton, hydrogen, and others). This combination is a standard reference for sea-level, dry air.

Understanding this matters in aerospace physiology because the partial pressures of these gases determine how much oxygen is available for respiration and how the gases behave in the lungs and bloodstream, especially as altitude changes. The large dominance of nitrogen and the substantial but smaller amount of oxygen, plus the tiny fraction of everything else, is what makes this composition the best representation of Earth's atmosphere.

The other options differ by tens of percent for the major components (for example, oxygen around 29% or 40%), which is not how the atmosphere is composed at sea level.