In an emergency descent, the oxygen masks should be donned at what concentration?

In an emergency descent, the oxygen masks should be donned at what concentration?

• A. 100%
• B. 50%
• C. 20%
• D. Variable

Answer: (A)

During an emergency descent, the goal is to prevent hypoxia as quickly as possible while you’re performing tasks to regain safe cabin conditions. The oxygen masks should deliver the maximum available oxygen, so you don 100 percent oxygen. This provides the highest arterial oxygen content and protects brain and heart function as you descend and troubleshoot, regardless of altitude or speed of descent. Lower concentrations—such as 50 percent or normal air (roughly 20 percent)—would not supply enough oxygen to offset the reduced ambient oxygen at altitude, and a variable mix introduces uncertain protection. Using the full 100 percent ensures immediate, reliable oxygen delivery when it’s most critical.

During an emergency descent, the goal is to prevent hypoxia as quickly as possible while you’re performing tasks to regain safe cabin conditions. The oxygen masks should deliver the maximum available oxygen, so you don 100 percent oxygen. This provides the highest arterial oxygen content and protects brain and heart function as you descend and troubleshoot, regardless of altitude or speed of descent. Lower concentrations—such as 50 percent or normal air (roughly 20 percent)—would not supply enough oxygen to offset the reduced ambient oxygen at altitude, and a variable mix introduces uncertain protection. Using the full 100 percent ensures immediate, reliable oxygen delivery when it’s most critical.