When transporting cylinders of compressed gas, they should be:

Compressed gas cylinders must be transported as designed — usually upright with valve protectors in place — and securely braced to prevent rolling or shifting. A loose cylinder can become a missile if it falls and the valve breaks (the cylinder becomes propelled by escaping pressurized gas); this has caused serious injuries and damage. Cylinders also must not exceed safe temperature limits during transport, must not be exposed to direct flame, and must be loaded so the valve protector caps are in place. Stacking horizontally is allowed only for certain cylinders and only with proper bracing. Mixing cylinders of incompatible gases (oxidizers with flammables, for example) is prohibited.