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Subchapter 20. Tunnel Safety Orders
Article 8. Tunnel Classifications
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§8424. Airborne Contaminants.

(a) When the air in any part of a tunnel and/or shaft is known to contain or is suspected of containing dangerous airborne contaminants, it shall be tested by means acceptable to the Division before employees are allowed to work therein, in accordance with Section 5155 (Airborne Contaminant) of the General Industry Safety Orders. These tests shall be made by a person who has been certified by the Division.

(1) Records shall be kept of these tests.

(2) Records of gas tests and air flow measurements shall be maintained on the surface by the employer and available to the Division upon request.

(b) Tests for dangerous or explosive gas(es) shall be taken at such times and locations to accurately represent the conditions existing in all areas where employees may be exposed to the danger(s) of these gases and specifically:

(1) In tunnels classified as Potentially Gassy a manual flammable gas monitor shall be used as needed, but at least at the beginning of each shift and at least every four hours to ensure that the conditions prescribed in Section 8422(b)(2) are unchanged.

(2) In any tunnel classified as gassy, there shall be tests for gas or vapors continually during actual operations to ensure that the conditions prescribed in Section 8422(b)(3) are not exceeded. In addition, a manual electrical shut down control shall be provided near the heading. (See (Section 8425 for additional requirements for the operation of Gassy and Extrahazardous Tunnels).

(c) Whenever any of the following conditions have been encountered, all underground work shall cease, employees shall be removed, and re-entry except for rescue purposes shall be prohibited until the Division has been notified and has authorized re-entry in writing.

(1) An underground ignition of gas or vapor occurs.

(2) 20 percent of LEL of flammable gas or vapor is exceeded.

(3) A toxic or suffocating gas in concentrations dangerous to health or life is encountered.

(d) Whenever the presence of hydrogen sulfide is suspected or detected in an underground work area, a test shall be conducted in the affected area at least at the beginning and midpoint of each shift.

(e) Whenever hydrogen sulfide is detected in an amount exceeding 5 ppm, a continuous sampling and indicating hydrogen sulfide monitor shall be used. The monitor shall be designed, installed and maintained to provide a visual and audible alarm when the hydrogen sulfide concentration reaches 10 ppm to signal that additional measures, such as respirator use, increased ventilation, or evacuation might be necessary to maintain hydrogen sulfide exposures below the permissible exposure limit.

(f) When the competent person determines, on the basis of air monitoring results or other information, that air contaminants may be present in sufficient quantity to be dangerous to life, the employer shall:

(1) Instruct all affected personnel in the characteristics and means of identifying the hazard(s) and the required safeguards to protect them from the effects of said hazard(s);

(2) Prominently post a notice at all entrances to the underground jobsite to inform all entrants of the hazardous condition; and

(3) Ensure that the necessary precautions are taken.

(g) All gas testing devices shall be operated, tested, calibrated and serviced in accordance with the manufacturer's recommendations.

(h) Tests for oxygen content shall be made before tests for air contaminants.

NOTE: Authority cited: Sections 142.3 and 7997, Labor Code. Reference: Sections 142.3 and 7997, Labor Code.


1. Amendment of section heading, section and new Note filed 3-5-96; operative 4-4-96 (Register 96, No. 10).

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