Here's a very good discussion of legal and safety-engineering considerations of the use of hydrogen as an energy carrier, that is, as the fuel for hydrogen fuel cells.
The comparison photos of a fuel fire in a hydrogen car and in a gasoline car are impressive. Combustion of the hydrogen occurs above the vehicle, because hydrogen is very light, relative to air. The hydrogen vehicle suffers no damage. The gasoline car is destroyed by the fire, because gasoline flows downward, beneath the vehicle.
http://www.fuelcells.org/info/HydrogenandtheLaw.pdf
As to damage to the atmosphere from hydrogen, there was no reference in the Washington Post article mentioned a few weeks ago by Linda Scarth to either the mechanism by which the damage is caused, or to quantitative data as to how much hydrogen would have to leak in order for there to be significant damage to the environment. Such information is necessary to make a determination of the danger of hydrogen to the environment.
[http://www.washingtonpost.com/wp-dyn/content/article/2006/02/14/AR2006021401771.html]
Tom