Will hydrogen powered cars upset the balance of oxygen in the atmosphere?

We put this question to Professor Stephen Bennington from the Rutherford Appleton Laboratory...

It shouldn't do because it's recycling it. So I mean you're taking water, you split it up into hydrogen and oxygen, and then you recombine them again in your car. So, the net effect is zero. Water vapour is different perhaps because if you're using these things to power jets up in the high atmosphere and then you're depositing water vapour up there, and that's a greenhouse gas. So we have to think carefully about those problems. It's probably not a problem, but it could be.