The minimum oxygen concentration by volume required for entry is:

The minimum oxygen concentration by volume required for entry is:

• A. 20.9
• B. 19.5
• C. 16.0
• D. 21.0

Answer: (B)

Oxygen level is the key factor for safe entry into a confined space. The minimum concentration by volume that allows entry is 19.5%, because anything below this creates an oxygen-deficient atmosphere where people can quickly become dizzy, lose consciousness, or suffocate. Normal air is about 20.9% oxygen, so 19.5% is the lower safety threshold, not a full change from normal conditions but the point at which entry becomes unsafe without ventilation or precautions. The other values either reflect breathable levels above the minimum or are well below the threshold and would not be considered safe for entry. So 19.5% is the correct minimum.

Oxygen level is the key factor for safe entry into a confined space. The minimum concentration by volume that allows entry is 19.5%, because anything below this creates an oxygen-deficient atmosphere where people can quickly become dizzy, lose consciousness, or suffocate. Normal air is about 20.9% oxygen, so 19.5% is the lower safety threshold, not a full change from normal conditions but the point at which entry becomes unsafe without ventilation or precautions. The other values either reflect breathable levels above the minimum or are well below the threshold and would not be considered safe for entry. So 19.5% is the correct minimum.