[Reeses]

any released hydrogen rises fast in air due to its low density

Hydrogen gas rises into the stratosphere where some of it reacts with ozone. Widespread use of leaky hydrogen is probably not a good idea. There must be safer alternatives to H2O that could be electrolyzed in a closed loop system.

[SFConservative]

The studies and models that claimed this in the early 2000s are loaded with qualifiers like may, might, could, are thought to, possible, expected. They assessed the impact of a mature, worldwide, fully hydrogen-based energy economy. My suspicion is that they were slanted to favor battery-based electric vehicles.

Hydrogen vented into the atmosphere in our current mostly fossil energy world is adsorbed on airborne particulates which aids its reaction with oxygen into water. Sporadic leakage from home fuel cell systems is negligible compared to the natural ongoing dissociation of water into hydrogen and oxygen by UV radiation, right there in the stratosphere.