For years, it was thought by American government scientists who measure air pollution that diesel locomotive engines were relatively clean and emitted far less health-threatening emissions than those of diesel trucks or other vehicles; however, the scientists discovered that because they used faulty estimates of the amount of fuel consumed by diesel locomotives, they grossly understated the amount of pollution generated annually. After revising their calculations, they concluded that the annual emissions of nitrogen oxide, a major ingredient in smog and acid rain, and soot would be by 2030 nearly twice what they originally assumed. In Europe, where most major railways have been electrified, there is less concern.