The U.S. Environmental Protection Agency released preliminary data Wednesday from its own toxicity testing for eight dispersants in a bid to corroborate potentially suspect industry-provided results. But questions remain about the safety of these chemicals that can be used to break up oil spills, including COREXIT 9500, which is being employed on a massive scale by BP in the Gulf of Mexico. The agency's results showed broadly similar impacts on silverside fish (Menidia beryllina) and mysid shrimp (Americamysis bahia) across a range of concentrations. And none of the dispersants showed significant capacity to disrupt the hormonal systems of animals, at least at the cellular level.

"In the tests we performed, all of the dispersants are roughly equal in toxicity and generally less toxic than oil," said EPA Assistant Administrator and chemist Paul Anastas in a press briefing on June 30. "The dispersant constituents are expected to biodegrade in weeks to months, rather than remaining in the ecosystem for years as oil might."

The eight dispersants all fell into the EPA categories of "slightly toxic" or "practically nontoxic." COREXIT 9500 proved slightly toxic to the mysid shrimp and practically nontoxic to the silverside fish. The eight dispersants were chosen because of their initial lower toxicities and widespread availability in large quantities. "Dispersants, like anything, are not without toxicity, nothing is," Anastas added. "The data is telling us that these are not posing the same types of hazard that we're seeing with the oil," including toxic constituents such as benzene, toluene, xylenes and various polycyclic aromatic hydrocarbons.

The EPA noted that its data closely matched industry provided data—except in two cases where the EPA found lower toxicity than industry. "Given the expected range of inter-laboratory variability, the results of the present study were consistent with test results reported in the [National Contingency Plan] Product Schedule," the EPA wrote in its preliminary report. Anastas added: "We recognize that there can be some inter-laboratory discrepancies or variability. We want to ensure through our own independent testing that we confirm those [industry] results."

The EPA stuck to its own guidelines, employing as a reference toxicant the surfactant sodium dodecyl sulfate, or SDS, but the use of this benchmark compound raised questions for at least one outside toxicologist. "It's well known in toxicology that it's just a horrible reference toxicant because it degrades quickly," says toxicologist Carys Mitchelmore of the University of Maryland. "[Toxicologists] haven't used that for 20 years," preferring copper sulfate or potassium chloride. 

In fact, it remains unclear whether anyone at EPA ever previously checked the industry-submitted data as required, for a variety of reasons. First, when Anastas was asked during the briefing by Scientific American if the agency had checked the industry data, he did not directly answer the question, nor did EPA respond to follow-up questions. Second, the industry-provided data appeared full of potential faults, including, in the case of one dispersant, use of the wrong reference toxicant—a test chemical which ensures consistency of results among various labs, in this case SDS. Third, there is no evidence that the agency had any requirements for acceptable toxicity levels in the industry data. "The magnitude of this event has raised important questions about how these previous, existing regulations [for dispersants in the National Contingency Plan Product Schedule] may need to be reexamined and revisited in ways that ask different questions and even better prepare us in the future," Anastas admitted.

That's a problem some think should have been resolved a long time ago. "It is ridiculous that we're in this situation. Thirty years ago with Ixtoc we used dispersants and dispersants have always been controversial," notes Mitchelmore, who co-authored a National Research Council report on dispersants in 2005. "In 2005, we were making the same recommendations that were made in 1989; that's ridiculous. There's not too much different in this technology since Ixtoc in 1979."

The EPA must now conduct specific toxicity tests for the Louisiana sweet crude oil that is actually spewing from the Macondo well, both alone and in conjunction with the various dispersants. "Once it's mixed with oil, that's where you get the most impact, that's where you see most of the toxicity," says toxicologist Sergio Alex Villalobos of Nalco, the maker of COREXIT 9500. Anastas suggests that testing will be completed before August, employing a suite of EPA labs as well as National Institutes of Health and contractor labs.

Through June 29, roughly 1.61 million gallons of dispersants—the bulk of it COREXIT 9500—had been applied to the spewing oil, both at the surface and beneath the sea. But the rate of application has slowed since the government ordered BP to slow its use on May 26. "In the month after EPA and the Coast Guard directed BP to ramp down dispersant use, the volume applied dropped nearly 70 percent from peak usage," Anastas said.

Ultimately, the decision to use dispersants was a tradeoff. "With a spill of this size and scope, dispersants are useful in breaking up the oil and preventing its spread, particularly to fragile wetlands," Anastas says. "I know many of you are interested to hear if this testing means EPA will order BP to switch dispersants. We are not making any such recommendation at this time. We have additional testing to do."

For example, more testing will be needed to determine if the breakdown of COREXIT 9500—either into other chemicals or when metabolized by animals—produces toxic products of its own. "In toxicology, it's quite often not the original compound that's the toxic entity," Mitchelmore notes.

Image: GULF OF MEXICO - Coast Guard Ens. Adam Mosley, a marine biology under graduate, logs sample data from oiled water in the Gulf of Mexico, May 27, 2010. The samples and field data Mosley collects help environmental scientists determine the effectiveness of dispersants used to break down oil. U.S. Coast Guard photo by Petty Officer 2nd Class Luke Pinneo.