Following last month's oil spill off the coast of Spain, European Union (EU) nations agree to ban single-hull tankers carrying heavy fuel in their waters. Heavy fuels are the heaviest grades of oil, including tar, bitumen and heavy crude oil.

Single-hulled ships have only one layer between the oil and the ocean. Newer ships are double-hulled, meaning that they have a second shell to help prevent a spill in case the first shell breaks.

The Prestige, which split in two on Nov. 19, was carrying 20 million gallons of heavy fuel. The Spanish government estimates that the ship has leaked 4.5 million gallons already.

The ban will come into effect next month. Under the agreement, the 15 EU nations will also enact a total ban on single-hulled vessels that are over 15 years old. This would take effect in 2010.

The EU will expect nations seeking entry into the union and Russia, which is a major oil exporter, to enact similar bans.

For more information about the Prestige oil spill click here.