Switching vehicles from diesel to liquefied petroleum gas might pose health risks, research carried out at America's Harvard University suggests. The university's Centre for Risk Analysis said a study had found that the combustion of natural gas might generate more 'ultra-fine' particles than diesel. Ultra-fine particles are 25 times smaller than the fine particles generated by diesel and could reach deeper into the lungs. The report said it was likely that LPG, which was almost identical to natural gas, would produce similar ultra-fine particles. 'Several studies indicate that ultra-fine particles may have an even more dramatic impact on health than those in the fine category,' the report said. However, Dr Frank Lee Sen-chun, associate professor in chemistry at Baptist University, was cautious about the findings, saying there was no immediate cause for concern. 'More research has to be done to confirm the result scientifically. In this interim period, we still need to do something to fight the pollution problem brought by diesel fuel,' Dr Lee said. Diesel combustion generates particulates, nitrogen oxides and sulphur dioxides, the main air pollutants in the SAR. By contrast, gas combustion produces fewer particulates but more carbon dioxide, contributing to the global greenhouse effect. The Government has begun issuing grants to more than 18,000 taxis to help them switch from diesel to LPG. Trials are being staged for LPG-powered mini-buses. It is hoped the total emission of respirable particulates by vehicles can be reduced by 60 per cent by 2003 and 80 per cent by 2005. A spokeswoman for the Environmental Protection Department said the findings had not been confirmed. 'There is no health standard for particulates of this size and no literature to support the health impact,' she said, adding LPG was still a cleaner fuel than diesel.