Oxygen concentration in the air space shall not be more than __________ free oxygen by volume to prevent coal dust explosion.

• A. 10%
• B. 12%
• C. 14%
• D. 16%

Answer: (B)

Oxygen is the stuff that feeds combustion, so keeping its concentration in the mine air low is a key way to prevent coal dust explosions. When the air contains less oxygen, the ability of a coal dust cloud to ignite and sustain a flame is greatly reduced, even if a spark is present. The safety standard sets not more than about twelve percent free oxygen by volume in the air space to minimize the risk of a dust explosion forming and propagating. Since normal air is around twenty-one percent oxygen, aiming for about twelve percent creates an environment that’s much less conducive to explosive combustion of coal dust.

The other numbers don’t align with that protective threshold. Higher values (like fourteen or sixteen percent) leave more oxygen for the reaction and can still permit an explosion under the right dust conditions, while a much lower value (ten percent) is stricter than the standard requirement but doesn’t reflect the commonly adopted limit used for safety planning. So, twelve percent is the value that best matches the intended protection against coal dust explosions.