MSHA - Code of Federal Regulations - 30 CFR 75.321
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MSHA - Title 30 CFR
30 CFR § 75.321
Air quality.

(a) The air in areas where persons work or travel, except as specified in paragraph (a)(2) of this section, shall contain at least 19.5 percent oxygen and not more than 0.5 percent carbon dioxide, and the volume and velocity of the air current in these areas shall be sufficient to dilute, render harmless, and carry away flammable, explosive, noxious, and harmful gases, dusts, smoke, and fumes.

(2) The air in areas of bleeder entries and worked-out areas where persons work or travel shall contain at least 19.5 percent oxygen, and carbon dioxide levels shall not exceed 0.5 percent time weighted average and 3.0 percent short term exposure limit.

(b) Notwithstanding the provisions of §75.322, for the purpose of preventing explosions from gases other than methane, the following gases shall not be permitted to accumulate in excess of the concentrations listed below:

(1) Carbon monoxide (CO)--2.5 percent

(2) Hydrogen (H(sub)2)--.80 percent

(3) Hydrogen sulfide (H(sub)2S)--.80 percent

(4) Acetylene (C(sub)2H(sub)2)--.40 percent

(5) Propane (C(sub)3H(sub)8)--.40 percent

(6) MAPP (methyl-acetylene-propylene-propodiene)--.30 percent