California Code of Regulations
Title 8 - Industrial Relations
Division 1 - Department of Industrial Relations
Chapter 4 - Division of Industrial Safety
Subchapter 7 - General Industry Safety Orders
Group 20 - Flammable Liquids, Gases and Vapors
Article 135 - General
Section 5416 - Flammable Vapors

(a) Ventilation shall be sufficient so that under normal operating conditions concentrations of flammable vapors or gases in buildings, rooms or similarly enclosed places shall not exceed 25 percent of the lower explosive limit for such vapors except that in pits, sumps, or the other locations which are not normally entered, except in cases of emergency, such ventilation will not be required. In such locations, the provisions of paragraph (c) shall be complied with.

(b) No source of ignition shall be permitted in or near a pit or sump in a location near which flammable liquids are regularly and frequently, or have recently been, used, handled or stored in other than closed containers unless tests have been made which indicate that the concentration of flammable vapor is less than 25 percent of the lower explosive limit.

(c) No source of ignition shall be permitted in any location, indoors or outdoors, where the concentration of the flammable gases or vapors exceeds or may reasonably be expected to exceed 25 percent of the lower explosive limit. Tests shall be made to ascertain that this limit is not exceeded before a source of ignition is introduced into such location, and such tests shall be repeated frequently (or a continuous indicator used) as long as conditions giving rise to such concentrations of flammable vapors or gases continue and a source of ignition is present. If electronic or thermal testing equipment is used, it must be approved for use in such flammable conditions as required by section 2540.2

(d) Smoking shall be forbidden in any location where flammable vapors in concentrations greater than 25 percent of the lower explosive limit may reasonably be expected.