Gas-powered bus. Refuelling of a bus which uses natural gas (methane) as a fuel in order to reduce airborne pollution. Methane is a simple hydrocarbon which burns much more efficiently than petrol or diesel. Vehicles which use methane can reduce emissions of carbon monoxide and unburnt hydrocarbons by up to 85-90%. They also reduce by up to 32% the emissions of carbon dioxide, a greenhouse gas. Emissions of sulphur and nitrogen oxides, a major cause of acid rain, are also reduced. Photographed in Bristol, Britain.