Hydrogen is the one to watch out for with a leak.
It is flammable over a wide range of concentrations, 4% to 75%.
It will detonate at 18% to 54% by volume.
It will detonate without being confined.
It will leak out of the tightest seams and connections.
It is mitigated somewhat by being so light. It dissipates fairly quickly.
Corection - It will detonate at 18% to 59% by volume.
Hydrogen also requires very little energy for ignition. It’s easy to ignite, and is flammable over a wide range of concentrations.