According to the revised mines safety regulations, what is the threshold limit value of coal dust?

The threshold limit value of coal dust is determined to minimize the health risks associated with coal mining operations. The correct range of 20-40 mppcf (million particles per cubic foot) indicates the maximum allowable concentration of coal dust in the mine atmosphere to protect miners from respiratory diseases, including pneumoconiosis. This range is based on extensive research that examines the long-term effects of coal dust exposure, providing a balance between operational practicality and health safety.

Choosing a limit of 20-40 mppcf reflects an understanding of the acceptable environment for workers and aligns with regulatory standards established to ensure their safety. This threshold aims to minimize the risk of coal dust-related health issues while allowing for effective mining operations. In contrast, the other options present higher limits, which would not adequately safeguard miners' health given the known risks associated with increased coal dust exposure.