There is another aspect of using hydrogen as a fuel for ICE vehicles is rarely discussed. We hear that the exhaust is just water vapor, but this isn't always the case. The combustion temperature is so high that nitrogen from the atmosphere (which is always part of the intake mixture) is converted to nitrous oxides, which are expelled during the exhaust phase of the cycle (nitrous oxides are also produced in regular ICEs). This is an undesirable pollutant.