If it sounds wrong, it probably is wrong so you should check snopes first.
A Prius polluting more than a Hummer is based on bad research that’s so bad it’s an outright lie.
Some of the nonsense in the paper was that a Prius would be completely dead at 109,000 miles while a Hummer would last for 379,000 miles. Another was charging the R&D to develop the Prius as environmental damage while ignoring the Hummer R&D. And last was the claim that the nickel in the Prius was an environmental nightmare despite it being recyclable and that the Hummer uses more nickel in its chassis than the Prius uses in its battery!
But Rush Limbaugh talked about the paper as if it was gospel so the lie lives on forever.