Between which oxygen percentages will a person become unconscious?

• A. 8% to 12%
• B. 4% to 6%
• C. 15% to 18%
• D. 20% to 22%

Answer: (A)

When oxygen in the air drops to about 8–12%, the brain isn’t getting enough oxygen to function, so consciousness is lost quickly. Normal air is about 21% oxygen, so 8–12% is a severe hypoxic level that commonly marks the onset of unconsciousness. Higher levels like 15–18% may impair performance but usually keep a person conscious, while 20–22% is above normal and would not cause unconsciousness; 4–6% is far lower and would lead to unconsciousness and other dangers even sooner. So the typical onset window for losing consciousness is in the 8–12% range, keeping in mind individual differences.