Ann: Fortescue and CSIRO Hydrogen Technology Partnership, page-79

There is a lot less to worry about with hydrogen as fuel than with carbon.

Carbon involves extracting carbon from the Earth's crust, where it has been for over 200 million years and releasing it into the atmosphere.  Natural processes that consume atmospheric carbon are very slow at the best of times, and the best of times for natural carbon capture and sequestration have not been repeated in the last 200 million years.

Hydrogen fuel on the other hand represents a cycle involving raw materials that are already abundant in the environment.

1. We start with water, which can be found in large quantities just lying about on the ground.
2. We use electricity to split the water into hydrogen and oxygen, capturing the hydrogen and releasing the oxygen.
3. We use some nitrogen from the atmosphere, which is 78% nitrogen, to combine with our hydrogen to make ammonia.
4. We take the ammonia to a refuelling facility where we break it up, releasing the nitrogen back into the atmosphere, keeping the hydrogen.
5. Some piece of equipment gets the hydrogen, which it "burns" using oxygen from the atmosphere, producing water, which is released.

It's a beautiful, clean system where the raw materials consumed are exactly the same as the waste products produced!  Not only that, the raw materials (and therefore the waste products) are completely benign.

The biggest risk is an ammonia release, which would be seriously nasty,