Every part of underground coal mine which is not naturally wet or which is not isolated by explosion-proof stopping shall be treated with the incombustible dust having not more than the following % of free silica

Solution (By Examveda Team)

Definition: Incombustible Dust
Incombustible dust refers to dust that will not burn or support combustion. It's used in underground mines, particularly coal mines, to suppress coal dust explosions.

Definition: Free Silica
Free silica is crystalline silica (SiO2) that exists in a form that is not chemically bound to other elements. It's a common component of many rocks and minerals, including coal. Breathing in free silica dust can cause serious lung diseases like silicosis.

Correct Answer: Option A (5%)
Regulations for underground coal mines strictly limit the percentage of free silica in incombustible dust used for dust suppression. This is because, while the dust itself is incombustible, the presence of free silica poses a significant health risk to miners. A limit of 5% free silica is a common standard to minimize the risk of silicosis while still providing effective dust suppression. Higher percentages significantly increase the risk of respiratory illnesses.

Why other options are incorrect:
Options B, C, and D (10%, 15%, and 20%) represent significantly higher percentages of free silica. These higher percentages would greatly increase the risk of silicosis and other respiratory illnesses for miners, making them unacceptable according to safety regulations. The lower the free silica content, the safer the dust is for use in underground coal mines.