In the 1930s Zeppelins used hydrogen as the lighter-than-air gas to fill the envelope and give the ship buoyancy.
What kind of gas is used today?
Is it still as flammable as the Hydrogen from airships of the 1930s?
There are only 2 lifting gasses that are lighter than air and practical for airships;
helium: this is a noble gas which means it's inert. Safest option all around except that US has a monopoly on the production of it. That is also the reason Zeppelins didn't use it because of an embargo the US had at the time against the Germans.
hydrogen: even more buoyant than helium but flammable. It is however easier to get by running a current through water or dropping some metals in an acid.
The third option for providing buoyancy is heated air, this requires a heat source to counteract the loss of heat over time.