For respirable crystalline silica in construction, the permissible exposure limit is about 50 micrograms per cubic meter as an 8-hour average. At approximately what airborne level does the 'action level' trigger additional requirements such as exposure monitoring?

Giải thích

Correct: the action level is roughly half the PEL, about 25 micrograms per cubic meter as an 8-hour average; reaching it triggers duties such as exposure assessment and medical surveillance eligibility. Five micrograms is far too low and matches lead-related figures, not silica. One hundred and 250 micrograms both exceed the PEL itself, so they cannot be the action level.