What are the average concentration limits a worker must not breathe in the atmosphere over an 8 hour period?

• A. 0.5 mg/m3 for respirable coal dust and 0.05 mg/m3 for free silica
• B. 1.5 mg/m3 for respirable coal dust and 0.05 mg/m3 for free silica
• C. 2.0 mg/m3 for respirable coal dust and 0.10 mg/m3 for silica
• D. 1.0 mg/m3 respirable coal dust and 0.02 mg/m3 silica

Answer: (B)

The main idea is the 8-hour time-weighted average exposure limit. In coal mine ventilation practice, workers shouldn’t have their average inhaled dust exposure over a full 8-hour shift exceed the permitted levels. The standard limits are 1.5 mg/m3 for respirable coal dust and 0.05 mg/m3 for free crystalline silica. Silica is held to a much lower limit because it’s more toxic and strongly linked to silicosis, so the protective threshold is tighter than for coal dust. In practice, you’d monitor dust in the worker’s breathing zone, apply engineering controls like ventilation and dust suppression, use PPE if needed, and aim to keep the 8-hour average below these values. The other numerical options don’t match the regulatory maxima for these substances, which is why this set is the correct standard.