Safety in working with pneumatics is often overlooked. Safety procedures are pretty easy to employ, but all too often neglected. The primary reason is that people often don't recognize how dangerous pneumatic equipment can be. The second failure is when people don't consider scenarios where things don't go as intended.
Any animated prop or device has the potential to cause injury. A pneumatic cylinder is like an automated stabbing machine. The rod is propelled out, potentially with great force. However, a small amount is all that is needed to put an eye out or to break a bone. A mere 9 pounds of force is all that is required to break a bone.
The single most important safety guideline, is to keep distance between a moving prop and the audience. It is easy to imagine a kid, or grown-up, looking down into the hiding space that a prop just popped out of, only to be hit in the face as the prop is triggered again. Furthermore, many props utilize a metal apparatus where moving parts can scissor with great force and sever fingers. Display your rig in such a way that people cannot get close to it.
Secondly, mechanical props suffer repetitive stress and can fail over time, or just come apart from vibration. Use restraints that will keep props from falling onto your audience or from launching pieces at them. Lightweight metal stranded cable, sometimes called aircraft cable, can be fastened to props and secured to prevent a accidental contact of props and audience members.
Pressurized air is at the root of pneumatic displays. The best practice is to use the lowest pressure possible to activate a prop. Air storage tanks should always have pressure relief valves to prevent over-pressurizing and the risk of explosion.
Finally, when testing a prop, there is a tendency to forget how dangerous an unexpected trigger of the prop can be. Avoid leaning over the path of a prop. Keep fingers clear of moving parts. When possible reduce the air pressure to minimal levels, to slow the prop if triggered and to minimize the force exerted.
This is not an exhaustive list of the potential hazards, simply a heads-up on some common safety considerations. As always consider the safety of your audience as a first priority and use common sense.