Why did the US Navy avoid hydrogen in its rigid airships?

What other lifting gas could airships use, and why did the US Navy avoid it?

Hydrogen was the alternative, but the US Navy preferred helium because hydrogen carried a much greater fire risk.

Hydrogen was the alternative lifting gas, but the US Navy preferred not to use it. Helium reduced the fire risk that made hydrogen-filled airships dangerous. A possible connection between ZR-2's design assumptions and its later use is raised only tentatively. The Navy's preference is clear, while the specific consequence for ZR-2 remains uncertain.