What grade of breathing air must compressed breathing air at least meet?

Compressed breathing air must meet the standards set for Grade D air, as established by the Occupational Safety and Health Administration (OSHA). Grade D air is essential for respiratory safety and is defined by specific quality standards, including limits on contaminants such as carbon monoxide, carbon dioxide, and oil mist. This grade of air is intended for use in respiratory protection and ensures that the air being breathed is free from harmful levels of these contaminants, making it safe for individuals to use in environments where they might be exposed to hazardous conditions.

In contrast, other grades such as Grade A, B, or C do not meet the stringent requirements for breathing air, which is why they are not appropriate for this purpose. Grade B might still contain impurities that can be detrimental to health, while Grade C would have even higher allowable limits of contaminants. Thus, Grade D is the correct classification for any compressed air used in situations requiring respiratory protection.