What is the 8-hour average exposure limit for respirable silica in the atmosphere?

• A. 0.05 mg/m3
• B. 0.5 mg/m3
• C. 0.01 mg/m3
• D. 1.0 mg/m3

Answer: (A)

An 8-hour time-weighted average is used to protect workers from the cumulative, long-term effects of silica exposure by converting an entire shift into a single representative value. For respirable crystalline silica, the health-based limit adopted here is 0.05 mg per cubic meter of air as the average over an 8-hour shift. This very low threshold reflects the risk of silicosis and other silica-related diseases with repeated or prolonged exposure, so the daily exposure must stay at or below this level to keep risk acceptably low.

In practice, meeting this limit means implementing dust controls, good ventilation, dust suppression, and (if needed) respiratory protection to ensure the average concentration across the shift doesn’t exceed 0.05 mg/m3. The alternatives—being much higher or even much lower than this value—do not align with the standard used, which is why 0.05 mg/m3 is the correct limit.