As you may recall, the first round of the EPA’s dispersant testing didn’t tell us too much. At issue were the two million gallons of a dispersant called Corexit, which the agency had earlier been concerned was too toxic a choice. The agency tested the eight dispersants and found that most were roughly equal in toxicity taken alone, but did not test them in combination with oil, as is the case in the Gulf.

So today the agency announced the results of another round of testing—this time, the dispersant and oil together. Data released by the EPA showed that Corexit dispersant was “moderately toxic” to two species of shrimp and small fish, though the initial round of testing had been found to be “slightly toxic” to the shrimp and “practically nontoxic” to the fish. The new results show that the Corexit-oil mixture is on par with the “moderately toxic” Louisiana Sweet Crude oil. 

Compared with the other dispersants, Corexit was in the middle of the pack. A dispersant called ZI-400 scored the best in testing on both species. The EPA did not address specifically whether Corexit remains the best choice for future spills. (Another dispersant, Dispersit, however, which some early news reports touted as a far less toxic alternative to Corexit, was rated more toxic than Corexit for both species, and "highly toxic" to the fish.)

EPA scientist Paul Anastas said the results do not show any “enhancement of toxicity,” meaning that the mixture was not more toxic together than the oil alone, as some had anticipated. He said the decision to use dispersant “seems to be a wise decision” based on the data.

“The oil itself is the hazard that we’re concerned about, and as we said before, Enemy No. 1,” Anastas said.

That enemy, however, isn’t removed by the dispersant. It’s now dispersed throughout the water column—the effects of which we still don’t know, though Anastas said that dispersed oil biodegrades more easily.

He added that the new testing results are for short-term effects on these two sensitive species only, and do not indicate the long-term effects dispersed oil might have on the Gulf or the Gulf food chain.