Coal dust concentration in the air space shall not be more than ___________________ of air to prevent coal dust explosion.

• A. 20 g/m3
• B. 40 g/m3
• C. 60 g/m3
• D. 80 g/m3

Answer: (B)

In mine safety, preventing coal dust explosions hinges on keeping the dust in the air below the level at which it can ignite and propagate an explosion. That threshold is the lower explosive limit (LEL) for coal dust in air, which is about 40 g per cubic meter. So, the air space should not contain more than roughly 40 g/m3 of coal dust to avoid forming an explosive mixture. If the concentration climbs to or above this level, an ignition source could trigger an explosion, especially in the presence of oxygen and an adequate heat source. Real-world factors like particle size, moisture, and ventilation can shift the exact limit slightly, but 40 g/m3 is the standard target. The other values would either stay well below the limit or exceed it, increasing the explosion risk.