The leaked hydrogen from fuel cells could cause as much as a 10% decrease in the ozone layer if it accumulates in the atmosphere, new research shows. According to conventional wisdom, cars that run on hydrogen fuel cells, are environmentally friendly because they only emit water vapour. However, a study by the California Institute of Technology, reports that the leakage that would inevitably occur could cause big problems in the upper atmosphere. Researchers estimate that, if hydrogen were to replace fossil fuels entirely, four to eight times more hydrogen would be released than at present, and would result in a doubling or tripling of inputs to the atmosphere from all sources natural or man-made.