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(a) (5-1a) The exposure to airborne contaminants of a person working in a mine shall not exceed, on the basis of a time-weighted average, the threshold limit values adopted by the American Conference of Governmental Industrial Hygienists, as set forth and explained in the most recent edition of the Conference's publication entitled "Threshold Limit Values of Airborne Contaminants." Excursions above the listed threshold limit values shall not be of a greater magnitude than is characterized as permissible by the Conference. This paragraph (a) does not apply to airborne contaminants given a "C" designation by the Conference--for example, nitrogen dioxide.
(b) (5-1b) Employees shall be withdrawn from areas in which there is a concentration of an airborne contaminant given a "C" designation by the Conference which exceeds the threshold limit value (ceiling "C" limit) listed for that contaminant.
(c) General Industry Safety Orders for control of Dust, Fumes, Mists, Vapors, and Gases shall be minimum standards for mines.
(d) (5-2) Dust, gas, mist, and fume surveys shall be conducted as frequently as necessary to determine the adequacy of control measures.
(e) (5-5) Respirators shall not be substituted for environmental control measures. However, where environmental controls have not been developed or when necessary by nature of the work involved (for example, welding, sandblasting, lead burning), a person may work for reasonable periods of time in concentrations of airborne contaminants which exceed ceiling "C" limits or the limit of permissible excursions referred to in (a) and (b), if such person wears a respiratory protective device approved by the Bureau of Mines as protection against the particular hazards involved.
(f) (20-5) Carbon tetrachloride shall not be used unless under strict environmental controls.
(g) (20-9) Dusts suspected of being explosive shall be tested for explosibility. If tests prove positive, appropriate control measures shall be taken.