Mo. Rev. Stat. § 293.120
Air safety requirements.
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.
(L. 1959 S.B. 188 § 13)