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Subchapter 20. Tunnel Safety Orders
Article 12. Ventilation and Dust Control
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§8437. Ventilation and Air Quality.

(a) Fresh air shall be provided in adequate quantities to all underground work areas. The supply shall at least be sufficient t o prevent dangerous or harmful accumulations of dusts, fumes, vapors or gases, and shall not be less than 200 cubic feet per minute for each person underground. The lineal velocity of the air flow in the tunnel bore shall not be less than 60 feet per minute in those tunnels where blasting or rock drilling is conducted or where there are other conditions that are likely to produce dusts, fumes, vapors or gases in harmful quantities.

EXCEPTION: The lineal velocity of air flow in tunnels or chambers more than 30 feet in diameter may be reduced to 30 feet per minute provided that air quality is maintained.

(b) The main ventilation system shall be so arranged that the air flow can be reversed from the surface.

(c) The main ventilation systems shall be arranged to exhaust return air to the outside atmosphere except where permitted by the Division in writing to reverse the airflow.

(d) After blasting, employees shall be required to wait at least 15 minutes before returning to the blast site and/or until air quality meets the requirements of subsection (a) and (g) of this section and Section 8424(a). A longer period may be required in the event of a misfire or in case of a fire.

(e) Employees shall not be permitted, except in extreme emergencies to work in a place where the oxygen content of the air is less than 19.5 percent by volume, unless they are wearing approved self-contained oxygen breathing apparatus.

(f) Employees shall not be allowed to work in tunnels where the oxygen content is more than 22 percent by volume.

(g) Where flammable gas or air contaminants have been encountered, adequate ventilation shall be maintained to keep the gas, vapor, or dust concentrations within safe limits as provided by these safety orders and the requirements of Section 5155 of the General Industry Safety Orders.

(1) Whenever 20 percent or more of the lower explosive limit for methane or other flammable gases is detected in any underground work area(s) or in the air return:

(A) All employees, except those necessary to eliminate the hazard, shall be immediately withdrawn to a safe location above ground; and

(B) Electrical power, except for acceptable pumping and ventilation equipment, shall be cut off to the area endangered by the flammable gas until the concentration of such gas is reduced to less than 20 percent of the lower explosive limit.

(h) Bulkheads and forced ventilation in one direction shall be provided after holing through to prevent harmful accumulations of dusts, fumes, vapors, gases, and concrete curing compounds from exceeding allowable concentrations.

(i) Auxiliary ventilation shall be used to provide the required airflow to all work areas of the tunnel.

(j) Fans used for all ventilation systems shall be so arranged to prevent recirculation of contaminated air.

(k) The exhaust from any internal combustion engine shall be so directed that it will be prevented from contaminating any inlet air source going to any underground or below surface work area.

(l) Whenever the ventilation system has been shut down with all employees out of the underground area, only competent persons authorized to test for air contaminants shall be allowed underground until the ventilation has been restored and all affected areas have been tested for air contaminants and declared safe.

NOTE: See Section 8470(c) of these safety orders for diesel engine ventilation requirements.

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


1. Amendment of Article 12 heading, section heading and section and new Note filed 3-5-96; operative 4-4-96 (Register 96, No. 10).

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