THE alternative fuel industry has hit back at research which suggests gas-powered vehicles could be more of a health hazard than diesel. After comparing diesel and natural gas vehicles, Harvard University Centre for Risk Analysis in America suggests vehicles powered by gas could produce high levels of ultra-fine particulates.

Experts say the levels could be higher than diesel emissions, and the results for liquefied petroleum gas would be similar. The findings could present fleet managers with headaches after many have chosen to operate 'cleaner' and cheaper-to-run LPG or compressed natural gas (CNG) vehicles.

David Ropeik, director of risk communications at the Centre for Risk Analysis, said: 'We did not study LPG, but we can say that since the molecule is similar to compressed natural gas, its environmental characteristics are likely to be similar.'

But the Government says its own research shows natural gas produces lower particle levels than diesel, and companies involved in the production and use of gas-powered vehicles have leapt to the defence of alternative fuels.

A DETR spokesman said: 'We will be looking at why this research suggests natural gas produces more ultra-fine particles than diesel. We're concerned because there are studies which suggest particulates cause health problems, and we don't really understand why.'

He added the department was undertaking a programme of research into different types of particulates, and a programme to measure the emissions of different types of vehicles and fuels.

Tom Fiddell, director-general of the LPG Association, said: 'There was no test of LPG in the Harvard study and the two fuels are quite different - natural gas is a compressed gas and LPG is a liquid. The report was off track on the smaller particles because LPG is good on PM10s and should be good on smaller particles.'

Gordon Wise, of Autogas 2000, added: 'Whatever fuel you run you are always going to have particulates of some sort, and I think both LPG and natural gas are going to be the best of the bunch. You're never going to get a perfectly clean fuel - even to produce electricity there is a pollution problem.'

Ron Mariner, assistant fleet manager of Hampshire police which took on 15 dual-fuel Ford Fiestas in 1998, said: 'We must wait for the Government's own report before making a decision, but it is worrying for fleet operators who have spent money on LPG and for oil companies investing large amounts of money in installing facilities for LPG around the country.'