As this year’s holiday season begins, few shoppers eagerly packing their carts with gifts stop to think about where the items originated — and how they got to their local stores. But the truth about the world’s largest container ships, which bring millions of products from China to the West, isn’t exactly warm and fuzzy.

Massive container ships like the legendary Emma Maersk, which is a quarter of a mile long, consume astonishing amounts of fuel as they pull thousands of tons of merchandise across the seas. In the process, they spew black smoke into the air and leave a brown haze across the surface of the water.

Unlike power stations or cars, which are themselves responsible for a hefty amount of pollution, mega ships are allowed to burn the cheapest, dirtiest fuel available — and according to award-winning science writer Fred Pearce, just 16 of those ships produce more pollution than all the cars in the world.

The chemicals found in this thick, high-sulphur fuel have been linked to thousands of human deaths, and cause untold damage to oceanic ecosystems. Ship emissions expert James Corbett of the University of Delaware calculates a worldwide death toll of about 64,000 a year.

“Thanks to decisions taken in London by the body that polices world shipping, this pollution could kill as many as a million more people in the coming decade — even though a simple change in the rules could stop it,” writes Pearce in The Daily Mail.

Though ships have long gotten around strict pollution laws that govern emissions on land, the International Maritime Organisation, the U.N. body that governs the world’s shipping, has finally set a requirement for cleaner fuel but has given the shipping industry 12 years to make the switch.

“However you look at it, the super-ships are rogues on the high seas, operating under pollution standards long since banished on land; warming the planet and killing its inhabitants. Santa’s sleigh, they are not,” says Pearce.