Why do people use hydrogen gas to fill up balloons?

They Do Not use Hydrogen gas to fill up balloons. People use Helium gas to fill up balloons (ordinary party balloons, and big working balloons such as the one used by Goodyear). Helium is Not Flammable. Hydrogen is Flammable and has as tendency to burst into Flame, similar to the German Blimp/Dirigible Hindenburg. Since the US Navy lost Two Helium Blimps in Bad weather; The Akron and the Shenandoah (with a great loss of lives), the US Navy almost never uses Blimps anymore.