Only bi-product on the buses is 100% pure water.
Yes, but so is gasoline. At least hydrogen is lighter than air, so leaks will dissipate. Gasoline vapor, and liquid for that matter, is heavier than air and will concentrate in low spots.
Watch the film clips of the Hindenberg fire and crash. Notice the flames are all above the structure. Many folks actually survived that inferno, because they were below the majority of the flames.