And as the passengers of the Hindenburg found out, hydrogen in a sufficiently closed space, leaking, can also become an explosive mix.
There was a very interesting TV program about the Hindenburg disaster
on PBS not too long ago. A retired NASA engineer whose speciality
was hydrogen fuel for the space rocket program decided to investigate
the accident as the the documentary film evidence did not jive with
his expert knowledge of what should have happened - hydrogen flames
should have no colour and the flash sequence looked odd.
Anyway the conclusion: The Hindenburg had been delayed by an
electrical storm and by the time she tried to dock at Lakehurst NJ
the air was filled with static. In close-up the fabric that formed
the aerodynamic shell it was composed of thousands of small panels
sewn onto the Aluminium girder lattice. This effectively created
thousands of capacitor cells. A spark likely jumped the metal
girders at the tail fin end and ignited the fabric. This fabric was
painted with dope (highly flammable cellulose acetate) impregnated
with aluminium powder, another highly explosive substance.
A lab test of a fragment of the original Hindenburg fabric supplied
by a collector indeed burst into flames when ignited by a spark. And
that was what gave that spectacular bright flames and sparks in the
film record of that event.