Hydrogen is not a fuel. Hydrogen is an energy transfer mechanism.
How is hydrogen produced? What are the effluents from that process?
Mine it.
I'll have a go. Hydrogen is typically gathered from splitting methane, where the by-product is carbon dioxide. So if you have a hydrogen car, you can drive around claiming that you are only emitting water and not adding to "carbon pollution," even though its just a natural gas powered car.