Almost every Christmas gift you gave or received two months ago was transported vast distances across the ocean, spending weeks inside a shipping container. What powers these epic journeys across the globe? Unfortunately, it’s not reindeers. It’s the black, sludgy dregs of the refining process known as heavy fuel oil. Each tonne, when burned, releases several thousand times the amount of sulphur and tiny lung-damaging particles that petrol or diesel cars do, while also contributing to dangerous climate change.