Division 2.1—Flammable Gases. This division includes gases such as acetylene, hydrogen, and propane. They remain a gas at 68°F or less and 14.7 pound-force per square inch (psi), and a boiling point of 68°F or less at 14.7 psi.

According to the NFPA (2016), such gases have the following properties:

• Are ignitable at 14.7 psi when mixed with air of 13 percent or less by volume
  
•  Have a flammable range at 14.7 psi with air of at least 12 percent, regardless of the lower limit