Hydrogen gas is much more explosive than either petrol or propane.

Explosive ranges ( % fuel in in air to produce an explosive mixture)

Petrol: Min 8%, Max 12%.

Propane: Min 10%, Max 15%. ( Not too sure about this one)

Hydrogen: Min 25%, Max 75%.

Those are the approximate limits for a mixture that can detonate. Outside of these, you can a have a mixture that may deflagrate, but not detonate. Can't remember if it's by weight or by volume, but either way, H2 has a much wider range than the others.

Oh yes, jpk. Check what comes out of a car exaust at the minute. CO2, a few trace gasses, and alot of water vapour.