His comments follow a study by Harvard University which said natural gas could produce a higher number of ultra-fine particles than diesel. The study said ultra-fine particles penetrate deeper into the lungs and can do more damage than larger particles.
Natural gas reduces emissions of fine particulates (smaller than 2.5 microns), but could generate more ultra-fine particles (smaller than 0.1 micron) than diesel.
Murray said: 'Not only are the effects of ultra-fine particles not totally understood, but other factors may be at play and it would be unwise to jump to any conclusions. Research is being undertaken in the US and in the UK by the DETR. Early indications suggest that, in terms of particle mass and numbers of ultra-fine particles, the results are better for natural gas than diesel.'