Hydrogen may have emissions benefits compared to gasoline (the DOE says that total greenhouse gas emissions for a hydrogen-burning mid-size SUV are, at worst, half that for gasoline), but ProPublica points out that there are challenges as well, saying hydrogen "is highly flammable, and can ignite more easily than other fuels. Hydrogen is also colorless and odorless, making it difficult to readily detect leaks."
ProPublica looked through data that was submitted voluntarily by people at hydrogen industrial, government and academic facilities to a database supported by the US Department of Energy called H2Incidents and found 37 "events" - things like leaks and fires – from the last few years at hydrogen fueling stations or with hydrogen delivery trucks. In total, there were seven fires, but nothing serious was reported. The danger, ProPublica says, lies in the upcoming expansion of hydrogen vehicles. The Pipeline and Hazardous Materials Safety Administration (PHMSA) is the federal agency responsible for regulating hydrogen transport, but, "the agency has been chronically understaffed and underfunded." And the rules aren't even all that tight. A PHMSA spokesman admitted to ProPublica that the rules don't actually force the agency to conduct inspections. And in California, where most of the early hydrogen vehicles are expected to be sold, the State Fire Marshal only offers a one-hour class (PDF) on hydrogen safety for first responders, and it's optional. We shall see if this changes once more H2 cars hit the streets in the coming years. Read more on hydrogen safety over at ProPublica.