DOT Hazard Class 2 is composed of gases that are under pressure (see Figure 4.1). The hazard is the pressure, which can cause violent container failure when an accident occurs. Pressure containers can be dangerous under accident conditions; ambient temperature changes, flame impingement, and damage to containers can cause boiling liquid expanding vapor explosions (BLEVEs). Containers can also rocket under high pressure, creating an impact hazard when valves are knocked off. Class 2 is divided into three subclasses: 2.1 gases are flammable, 2.2 gases are nonflammable, and 2.3 gases are poisons. Each compressed-gas category presents its own special hazards, in addition to the hazard of being under pressure, in a specially designed and regulated pressure container. Container pressures range from 5 psi to as much as 6000 psi (see Figure 4.4). The higher the pressure, the more substantial the container must be constructed to contain the pressure; the higher the pressure, the greater the danger when the pressure is released or the container fails.