MORE than a week after an explosion on the Deepwater Horizon drilling rig in the Gulf of Mexico, 11 of the rig's workers are still unaccounted for. The US Coast Guard reckons that 5,000 barrels of oil are leaking out every day, though this is really an informed guess. The great depth of the leaks from the rig makes staunching them a huge technical challenge. Despite the development of some fancy new techniques to deal with oil spills there is a sense of inevitability about the oil's arrival on shore.

The spill is embarrassing for those Republicans who have campaigned for more off-shore drilling. It is embarrassing too for Barack Obama, who accepted more drilling off the coast to ease the passage of legislation on climate change. On April 30th the White House announced that new off-shore drilling would be suspended until the cause of the accident could be verified.

Every type of disaster, whether tsunami (Asian), volcano (Krakatoa), or hurricane (Katrina), has its own metric. When it comes to oil spills the Exxon Valdez rules the waves. The oil spill (from a tanker) happened in Prince William Sound in Alaska in 1989. Although the amount of oil spewing into the Gulf today sounds scary, it is quite small compared with Exxon Valdez, which itself was a relatively small spill. Five thousand barrels a day is the equivalent of 210,000 gallons of oil. The Exxon Valdez spilt 11m gallons, so it will take some time for the spill in the Gulf of Mexico to equal it.

However one of the first lessons from the Exxon Valedez is that it is not the quantity of oil released that is important, but where it ends up. What made that spill synonymous with environmental devastation was that the oil that reached the coast then slurped over 1,200 miles of pristine shoreline. The scale of the current disaster in the Gulf of Mexico will be directly related to where the oil comes ashore. Louisiana's coastline features some important and vulnerable habitats and nature reserves. There is also a thriving fishing industry, providing shrimp and oysters.

The concentration of the oil is important too. Marine lubricants, oils used to lubricate marine engine parts and hydraulic equipment, leak from the back of all the world's ships. Every year four-and-a-half times the quantity spilled by the Exxon Valdez is poured into the ocean and nobody bats an eyelid. This does not mean that it is harmless (environmental groups are campaigning to reduce it), but the point is that if oil is dispersed into the ocean where it biodegrades naturally, its effects are less damaging. Indeed, much more oil seeps from the sea bed around the world naturally than from spills and shipping, says Simon Boxall from the University of Southampton.

But when a large amount of oil is released in a single place it causes direct hazards for marine wildlife. The best way of minimising damage is to protect sensitive areas with floating dams, or booms. However experts reckon that the size of the spill will exhaust the world's supply of booms very quickly. The region's best hope is that the wind blows most of the slick offshore, in which case little needs to be done. Intervention with dispersants and surfactants can create more damage than it prevents.

Crude oil is a natural product which disperses and biodegrades in the ocean fairly rapidly, although some of it may remain over the longer term. A study released earlier this year found evidence of ongoing environmental damage in Prince William Sound 21 years after the Exxon Valdez. Older reports found that about 20,000 gallons of oil could be found there 14 years after the disaster. Prince William Sound is recovering. But it has taken an awfully long time to do so.