Ford and Toyota are to begin marketing some models as partial zero-emission vehicles (PZEVs) in next month, but will not pass on the cost of equipping them to purchasers.

There is some controversy regarding the true cost of making vehicles compatible with new emissions controls, with the state's clean air officials putting it at $200 and carmakers at $500.

Whatever the true amount, producers say it is enough for PZEVs to be prohibitively expensive, which is why most have no plans to roll them out across the US, in addition to California, where they are required.

General Motors spokesman Kevin Cullen told USA Today that his company was 'fairly certain our customers would be unwilling to pay it.

'If the sticker price reflects the difference in costs, we don't think there'll be a whole lot of PZEVs sold in the open market', he explained.

Ford and Toyota are bucking this trend to demonstrate their commitment to environmental issues, and to spread the general cost of California PZEV hardware.

The companies may just be hurrying the inevitable though, as analysts predict a national convergence with Californian regulations over the next few years.

Former EPA worker Tom Austin, now of Sacramento clean-air research company Sierra Research, said 'it's only a matter of time before essentially all gasoline-fuelled passenger cars and light trucks are PZEVs.

'Once the technology has been demonstrated, once it becomes clear it's something that can be manufactured and will perform in customer hands - even if the costs don't come down specifically - it'll be difficult for the evolution towards the near-zero-emission vehicles to stop.'