CA Lab Code Section 7979

In tunnels or underground mines classified extrahazardous, sufficient air shall be supplied to maintain an atmosphere of all of the following conditions:

(a)

Not less than 19 percent oxygen.

(b)

Not more than 0.5 percent carbon dioxide.

(c)

Not more than 5 parts per million nitrogen dioxide.

(d)

No petroleum vapors or other toxic gases in concentrations exceeding the threshold limit values established annually by the American Conference of Governmental Industrial Hygienists.