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They use metal tanks, from aluminum or steel fire extinguishers, to regular pressure cylinders. As cheap as PVC and Safer.
This is an old thread, as it happened in 2013. This one was only charged to 80 PSI. Three people hospitalized. Since then there was another at the Arkansaw Razorbacks game. It was charged and ready, 80 PSI, and just sitting in the bench on the sideline. Not sure if it got bumped, knocked over, or just went off.