California Code, Labor Code - LAB § 7978

In any extrahazardous tunnel or underground mine smoking by employees or open flame shall be prohibited. Welding or cutting with arc or flame underground in other than fresh air shall be done under the direct supervision of qualified persons who shall test for gas and vapors before welding or cutting starts and continuously during such an operation. No cutting or welding shall be permitted in atmospheres where any concentration of gas or vapor reaches 20 percent of the lower explosive limit or more while a probe hole is being drilled or when the tunnel face or material from a mine is being excavated.