Cal. Lab. Code § 7985

All such testing device or devices shall be U.S. Bureau of Mines approved or acceptable to other authorities recognized by the division and shall automatically sound an alarm and activate flashing red signals visible to employees underground whenever the concentration of gases or vapors reaches or exceeds permissible levels. Permissible levels may be established lower than the limits set in division rules, regulations, or general orders whenever a division inspector considers such action necessary to make the operation safe for employees.