The U.S. Environmental Protection Agency released results of its second round of toxicity testing on dispersants—chemicals used to break up the oil that spewed for nearly 90 days into the Gulf of Mexico from BP's Macondo well. This new round focused on the specific oil in question—Louisiana sweet crude—alone and in conjunction with eight dispersants, including COREXIT, the chemical employed in bulk by BP in the Gulf.

"The eight have similar toxicities to one another when mixed with crude oil," EPA Assistant Administrator and chemist Paul Anastas said during an August 2 press briefing to announce the results. "The dispersant and oil mixtures were no more toxic to test species than the oil alone," except for Nokomis 3-AA, which BP has not reported using during the Gulf disaster.

Specifically, the oil alone is moderately toxic to juvenile silverside fish (Menidia beryllina) and mysid shrimp (Americamysis bahia), both natives of the Gulf of Mexico, according to tests conducted at a single laboratory under EPA contract. Similarly, the oil mixed with seven of the eight dispersants is also moderately toxic.

The EPA found that only COREXIT 9500A (the primary dispersant used in the Gulf disaster), Nokomis 3-4F and ZI-400 made the dispersed oil less toxic than oil alone to the shrimp, though all the dispersants registered as "moderately toxic." The least toxic dispersant, per the EPA tests, is ZI-400.The levels at which the eight tested dispersants killed half the tested animals ranged widely, from as little as 0.39 milligrams-per-liter for the shrimp to 13.1 mg/L for fish. Oil alone killed half the shrimp at 2.7 mg/L and roughly 7 percent of fish at the highest concentration tested: 2.9 mg/L. In general, the EPA data matched that provided by industry. "While there are differences in the data we are presenting today with the data submitted to [government by industry], there are not significant differences," Anastas said.

One mystery: COREXIT 9500A showed a "non-petroleum hydrocarbon chemical peak," meaning it was breaking down into unspecified hydrocarbon compounds on its own, though this was not included in the final results. It remains unclear what this result might mean for both toxicity testing and in terms of the dispersant's long-term effect. The EPA also did not test COREXIT 9527, which BP reported using in relatively small quantities at the beginning of the spill.

Anastas called the decision to employ roughly 7 million liters of COREXIT a "wise choice," especially given that oil "degrades 50 percent faster when dispersant is used." He added: "It was interesting to see that the dispersant-oil mixture was about the same toxicity as the oil alone."

According to Anastas, roughly 100 times more oil is present in the Gulf than dispersant. And dispersant use ceased on July 19 when the well was capped.

Of course, the new data does not determine where the dispersed oil has ended up; the U.S. National Oceanic and Atmospheric Administration has detected large plumes of it throughout the water column and some scientists have found dispersed oil in crab larvae, among other animals. Nor do the tests mimic the actual conditions faced by the dispersant-oil mixture in the cold, dark, deep waters of the Gulf of Mexico. And it does not dispel fears that the dispersed oil may have long-term toxic effects as it breaks down, both in the water and in Gulf sealife. "In toxicology, it's quite often not the original compound that's the toxic entity," says toxicologist Carys Mitchelmore of the University of Maryland, who co-authored a 2005 National Research Council report on dispersants.

"A very important longer-term research question is to characterize the various metabolites and see if there are any metabolites of concern in the breakdown products," Anastas said in response to a question from Scientific American.

Image: U.S. Air Force Master Sgt Paul Tatar, a C-130 Hercules aircraft aerial spray aircraft maintainer, refills a C-130 with oil-dispersing chemicals on the runway of the Stennis International Airport, Miss, on May 9, 2010. U.S. Air Force photo by Tech. Sgt. Prentice Colter.