Debunking various greenwash claims can be complicated but even so this is something of a howler.
What the ad copywriters are talking about is not carbon dioxide, but carbon monoxide. This is nasty stuff in large quantities, to be sure. But it isn\’t warming the planet. What BMW\’s hydrogen car does "in tests" is emit gas through its tailpipe that contains less carbon monoxide than the surrounding air. In fact, somewhere deep in the system the engine breaks down carbon monoxide. Well done, guys. But it is not cleaning the air in any sense that you or I might understand it. Sure, hydrogen cars don\’t emit much carbon dioxide either, but very large amounts are emitted where the hydrogen is made.
Umm, hydrogen powered cars don\’t emit any CO2 at all. The "pollutant" is going to be dihydrogen monoxide, that terribly scary material that kills hundreds of thousands every year.