Mo. Rev. Stat. § 293.120: Air safety requirements.

Air in which men work or travel in mines shall be improved when:

(1) It contains less than nineteen and five-tenths percent oxygen;

(2) It contains more than one-hundredth percent carbon monoxide;

(3) It contains more than five-tenths percent carbon dioxide;

(4) It contains more than one percent methane; or

(5) Is contaminated with dust or with noxious or poisonous gases in excess of accepted limits as established by the director of the division of mine inspection.