Motorized off-road vehicle use in California releases as much greenhouse gas as 1.5 million car trips from San Francisco to Los Angeles, according to a report from the Center for Biological Diversity and Clean Air Initiative.

Yet California continues to exempt the most-polluting off-road vehicles from state air-quality laws, the study said. The loophole should be closed, said the Center for Biological Diversity.

Pollution from off-road vehicles in California has doubled in the past 15 years, the study reported. Some off-road vehicles emit as much pollution in a single hour as more than 30 cars. Off-road vehicles include dirt bikes, all-terrain vehicles and snowmobiles.