What is the approximate percentage composition of oxygen, nitrogen, and other gases at 18,000 feet MSL?

At an elevation of 18,000 feet mean sea level (MSL), the approximate percentage composition of gases in the atmosphere remains largely consistent with that at sea level. The atmosphere is composed of approximately 21 percent oxygen, 78 percent nitrogen, and about 1 percent of other gases, such as carbon dioxide, argon, and trace gases. This ratio is reflective of the standard atmospheric composition and is critical for understanding the respiratory requirements of individuals at high altitudes.

As altitude increases, the total atmospheric pressure decreases, leading to a reduction in the partial pressure of oxygen. However, the percentage composition of these gases does not change significantly—it remains constant, which is crucial for aviators to know for proper oxygen management and understanding hypoxia risks at altitude.

The other options suggest variations in the proportions of oxygen, nitrogen, and other gases that do not accurately reflect standard atmospheric composition, especially at the specified altitude of 18,000 feet. Knowing that the composition stays relatively stable while the effective availability of oxygen diminishes due to lower pressure helps clarify why maintaining awareness of these values is essential for flight safety and physiological considerations.