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Environmental concerns

There are many concerns regarding the environmental effects of the manufacture of hydrogen. Hydrogen is made either by electrolysis of water, or by fossil fuel reforming. Reforming a fossil fuel leads to a higher emissions of carbon dioxide compared with direct use of the fossil fuel in an internal combustion engine. Similarly, if hydrogen is produced by electrolysis from fossil-fuel powered generators, increased carbon dioxide is emitted in comparison with direct use of the fossil fuel.

Using renewable energy source to generate hydrogen by electrolysis would require greater energy input than direct use of the renewable energy to operate electric vehicles, because of the extra conversion stages and losses in distribution. Hydrogen as transportation fuel, however, is mainly used for fuel cells that do not produce greenhouse gas emission, but water.

There have also been some concerns over possible problems related to hydrogen gas leakage.[83] Molecular hydrogen leaks slowly from most containment vessels. It has been hypothesized that if significant amounts of hydrogen gas (H2) escape, hydrogen gas may, because of ultraviolet radiation, form free radicals (H) in the stratosphere. These free radicals would then be able to act as catalysts for ozone depletion. A large enough increase in stratospheric hydrogen from leaked H2 could exacerbate the depletion process. However, the effect of these leakage problems may not be significant. The amount of hydrogen that leaks today is much lower (by a factor of 10–100) than the estimated 10–20% figure conjectured by some researchers; for example, in Germany, the leakage rate is only 0.1% (less than the natural gas leak rate of 0.7%). At most, such leakage would likely be no more than 1–2% even with widespread hydrogen use, using present technology.