Clean air standards referred to as "Grade D" apply to compressed air for use in __________.

Correct Answer: Option D — filling open-circuit breathing systems

Grade D breathing air standards, as referenced in 29 CFR 1910.134 and maritime safety guidelines, are mandatory for filling open-circuit self-contained breathing apparatus (SCBA) cylinders. This ensures the air is free of contaminants like carbon monoxide, oil mist, and moisture, protecting personnel from respiratory hazards during emergency response operations.