I don't usually take much notice of car ads on TV. But this one had me leaping out of my chair. Toyota, said the closing caption, is working towards making a car that will "clean the air" as it drives.

What could this mean? I knew they made the pioneering Prius hybrid car, which has among the lowest carbon dioxide emissions around. But cleaning the air is a bit different. Makes it sound like the more you drive the happier the planet will be.

It turns out that Toyota is not the only car-maker to have this magic trick up its sleeve. BMW too. At the Detroit Auto Show over the summer, BMW launched a hydrogen-fuelled "eco supercar that cleans the air as it drives". In research carried out by the prestigious Argonne National Laboratory in the US, its emissions were so low that they were "undetectable by standard emissions tests", the company claims.

What's this nonsense about? Well this is a classic case of highly selective use of statistics. You see, what the world is mainly worried about in car emissions is the carbon dioxide — the stuff that is warming the planet. Buy a Toyota and your emissions will range from 104g/km for a Prius up to 270g/km for a top-of-the-range Land Cruiser.

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.

Is this greenwash? Well, not in the sense that you could be conned into buying a BMW hydrogen car under the impressions that the more you drove it the cleaner the atmosphere would be. BMW has no plans for mass production of its hydrogen car. It plans, apparently, to hand a few out to high-profile opinion formers. So for now, all this is about trying to burnish the company's image, rather than sell you anything specific.

That impression was reinforced when I finally got to the small print of what Toyota is saying about its own plans to clean the air. They don't actually have an atmosphere-cleaning car for sale either. "We are," the small print says, "moving forward toward our ultimate idea: a car that cleans the air as it drives. Is this absurd? Maybe ..."

They said it. But it's obviously not too absurd to be put in rather large letters across our TV screens. The better to convince us that Toyota are – in some vague, aspirational, even "absurd" sense — good for the planet. Greenwash.

But why single out these two companies? Many car makers are finding new and ingenious ways to green the image of their products without actually breaking any advertising rules.

"Why does a 4x4 have to cost the earth?" asks Ford in the glossy ads for its new Kuga. I don't know, but the numbers bleeding off the top edge of the page reveal that the Kuga's official CO2 emissions are 169g a kilometre – which is way above the 130g target being set by the European Union for an average car.

But at least Ford are doing better than Land Rover. As one reader of this column observed of that company's "Our Planet" campaign to persuade us to buy its vehicles: "If that isn't greenwash, I'm a painted coconut."

Land Rover's claim to a "fragile Earth commitment" seems to boil down to the fact that "conservationists and environmentalists" drive them (maybe so); that the company is improving its technology (who isn't?); that it has two wind turbines at its Dagenham factory (good show); and that it is investing in carbon dioxide offset projects (which it is). But the problem remains what comes out of the exhaust pipe.

The company boasts that since it was formed 60 years ago, "our vehicles have always had a unique relationship with Our Planet". Well yes, but with officially declared CO2 emissions that range from 194g to a staggering 376g per kilometre that "unique relationship" seems to be that they are among the most polluting vehicles on the planet.