British Petroleum and government disaster-relief agencies are using a toxic chemical to disperse oil in the Gulf of Mexico, even though a better alternative appears to be available.

As the Deepwater Horizon oil spill spreads, BP and the U.S. Coast Guard have conducted tests with Corexit 9500, a chemical designed to break oil slicks into globules that are more quickly consumed by bacteria or sink into the water column before hitting shore.

The decision has been a controversial one. A few scientists think dispersants are mostly useful as public relations strategy, as they make the oil slick invisible, even though oil particles continue to do damage. Others consider Corexit the lesser of two evils: It's known to be highly toxic, adding to the harm caused by oil, but at least it will concentrate damage at sea, sparing sensitive and highly productive coastal areas. Better to sacrifice the deep sea than the shorelines.

But even as these arguments continue, with 230,000 gallons of Corexit on tap and more commissioned by BP, a superior alternative could be left on the shelf.

Called Dispersit, it's manufactured by the U.S. Polychemical Corporation and has been approved for use by the Environmental Protection Agency. Both Corexit and Dispersit were tested by the EPA, and according to those results, Corexit was 54.7 percent effective at breaking down crude oil from the Gulf, and Dispersit was 100 percent effective.

Not only did Corexit do a worse job of dispersing oil, but it was three times as lethal to silverfish – used as a benchmark organism in toxicity testing – and more than twice as lethal to shrimp, another benchmark organism and an important part of Gulf fisheries.

As for why Corexit is being used instead of Dispersit, authorities haven't yet said. According to the Protect the Ocean blog, U.S. Polychemical executive Bruce Gebhardt said the government had used Corexit before, and was sticking with what it already knows. Corexit makes up most dispersant stockpiles in the United States for this reason, though dispersant manufacture can be easily ramped up.

In a 1999 letter, the U.S. Coast Guard told U.S. Polychemical that "product information from planning mode evaluations remain on file to facilitate rapid review in the context of a spill." In that same year, the EPA added Dispersit to the National Oil and Hazardous Substances Pollution Contingency Plan, which determines what will be considered for use in an oil spill.

Relief agencies were not immediately available for comment about Dispersit. In a Tuesday press conference, Charlie Henry, the scientific support coordinator for the National Oceanic and Atmospheric Administration, said the potential effects of Corexit's use in the Gulf are unknown. "Those analyses are going on, but right now there's no consensus," he said. "And we're just really getting started. You can imagine it's something we've never thought about."

Image: Coast Guard workers spray Corexit on oiled rocks in Berkeley, California, in 2007./United States Coast Guard.

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Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.