O2 concentration of _%-__% in the air will cause nausea, vomiting, lethargy, ultimately leading to unconsciousness.

• A. 4%-8%
• B. 2%-6%
• C. 10%-14%
• D. 6%-10%

Answer: (D)

When oxygen in the air drops, the body experiences hypoxia because there isn’t enough O2 available to meet tissues’ needs, with the brain being especially sensitive. As O2 levels fall toward the lower end of the range, you see a progression from early effects like headaches and dizziness to more overt signs like nausea, vomiting, and increasing lethargy, eventually leading to unconsciousness as brain function can no longer be sustained. The range that aligns with these symptoms—nausea, vomiting, and lethargy progressing to unconsciousness—is about 6% to 10% O2 in inspired air. Normal ambient air is roughly 21% O2, so at about 10% the body is already significantly hypoxic, and at around 6% the risk of losing consciousness rises quickly.