Hydrogen balloons explosion

An accident occurred before the start of a scientific demonstration at a school. Part of the school demonstration was about hydrogen-oxygen balloons.

To prepare for the demonstration, the demonstrator had transported 15 balloons (pre-filled with a hydrogen-oxygen gas mixture) in a large, black polyethylene garbage bag. During the demonstration set-up he opened the bag to remove a single balloon. The entire bag of balloons detonated immediately. Since the incident occurred an hour prior to the program, no one else was in the surroundings. The consequences of the explosion were limited only to a small box which caught fire, despite the presence of the other chemicals in place for other demonstrations.

Due to permeation from the balloons, hydrogen accumulated in the sealed bag's headspace, where it mixed with air. Both the ballons and the bag could generate enough static electricity (even just by touching it) to ignite the mixture.

The demonstrator suggests that the slow leakage of hydrogen from the balloons which allowed for the accumulation of hydrogen and oxygen in the bag's headspace.

balloons

It can be assumed that this demosnrtation was a one-off action.

A person suffered painful second-degree burns to his right forearm and had to be taken to the hospital.

This was incident related to a scientific demonstration in presence of school pupils (probably for some sort of educational Lab).
Already the transportation of balloons filled only with hydrogen requires a careful assessment of the risks and measures aiming at reducing it. Storing and transporting balloons with a hydrogen-oxygen mixture envisage higher risks. A good practice would be to avoid transportation at all, because of the intrinsic difficulty to guarantee the required level of mitigating measures, such as ventilation able to exclude formation of flammable mixture. Enveloping the ballons inside a bag worked against these safety principles.
If an experiment requires the preparation of a hydrogen-oxygen mixture, this should be done in a laboratory equipped with the required preventive and mitigating safety measures, using the prescribed PPE, and used immediately, avoiding short- or long-term storage or transportation.

This case represented an additional hazard, due to the presence of a public unaware of the underlying risks. It asked for additional safe precautions to protect them from hazards and to avoid possible actions with increased risks.