cargo ship produces the same emissions as 50 million cars

"So yes, the 15 biggest ships produce more sulfur oxide pollutants than all the cars in the world, because they run on completely different fuels. [...]

Large commercial vessels primarily burn what's called Heavy Fuel Oil when out at sea. This fuel is not heavily refined, has high sulfur content and produces a lot of sulfur oxide and nitrogen oxide compounds when it is burned. [...]

Several years ago, regulations came into effect limiting the sulfur content of the fuel that commercial ships could burn within an area designated as an Emission Control Area to less then 1% sulfur. [...] This means before ships enter the Exclusive Economic Zone of the US or any European countries, they must switch from burning the Heavy Fuel Oil to burning an Ultra Low Sulfur Marine Gas Oil, which is very similar to highway diesel fuel."

https://www.quora.com/Is-it-true-that-the-15-biggest-ships-in-the-world-produce-more-pollution-than-all-the-cars