This story was updated at 4:55 p.m. EDT.

U.S. EPA today released the results of new tests showing that eight federally approved dispersants are about as acutely toxic when mixed with oil as crude is alone and defended the value of the chemical products amid new congressional complaints about their use.

EPA's findings on the toxicity of oil, dispersant and a mixture of the two substances counter concerns raised by some scientists that dispersed oil, which is broken down into smaller droplets with potentially more exposed hydrocarbons, could pose a greater hazard to marine life than crude itself.

Paul Anastas, EPA assistant administrator for research and development, said the adverse effects of oil go beyond the toxicity being tested and described dispersants as an effective mitigation strategy for battling oil gushers like that in the Gulf of Mexico.

"The impacts of oil in terms of volume, toxicity and hazard certainly far outweigh what we've seen so far in the data about dispersants," Anastas told reporters.

The test results come at an opportune moment for oil-spill response officials who are facing new questions after a senior House Democrat chastised their decision to let BP PLC continue spraying dispersant on the surface of the Gulf nearly every day after a May directive that the chemical products' daily use be curtailed.

"BP carpet-bombed the ocean with these chemicals, and the Coast Guard allowed them to do it," Rep. Ed Markey (D-Mass.) said Saturday. Markey released documents showing discrepancies between the dispersant usage estimates BP provided before getting Coast Guard waivers for surface spraying and the totals that the company reported to members of Congress.

Asked about Markey's charges, Anastas said, "While it may be true that EPA may not have concurred on every decision or waiver of the amount of dispersants being used ... overall, we feel the use of dispersants was an important tool." That echoes an earlier EPA statement in which the agency noted that it "was not involved in day-to-day decisions about granting such waivers" to BP that allowed surface spraying to continue until a late June meeting of Coast Guard officials and EPA Administrator Lisa Jackson.

BP's chief operating officer, Doug Suttles, also defended the dispersant strategy during a public appearance yesterday. "What we did is what ourselves and the Unified Command, the Coast Guard and others felt was the appropriate amount," Suttles told reporters. "And we continue to modify that approach as we went."

Dispersant application has fallen to zero since a cap was fitted on BP's leaking Macondo wellhead 18 days ago, with the exception of a 200-gallon application on July 19, according to EPA.

EPA's new testing follows an earlier round of studies that examined the acute toxicity of the eight dispersants alone and concluded that Corexit 9500 -- BP's product of choice -- was no more acutely toxic than seven alternatives listed on the National Contingency Plan that governs oil spill response efforts (E&ENews PM, June 30). The tests did not examine chronic toxicity, which could help determine how dispersed oil affects the mortality or long-term health of marine species that may not die off as a short-term result of exposure.

Dispersants are used in a bid to break up crude into a form more easily biodegradable by oil-eating microbes, but scientists have warned that the rush of those bacteria to feed on smaller droplets could create oxygen-depleted zones where future life cannot be supported. Anastas said that monitoring of the Gulf is continuing and "to date, we have not seen dissolved oxygen fall below levels of concern."

The tests were conducted on juvenile specimens of two Gulf species, the mysid shrimp and the inland silverside. A mixture of Louisiana sweet crude and the dispersant Nokomis 3-AA proved more toxic than oil itself to the mysid, with other products falling in the moderate range of toxicity, including Corexit 9500.

The Nokomis and another alternative, Dispersit, both were found to be highly toxic to the silverside, with the ZI-400 dispersant ranking as slightly toxic. A mixture of Corexit and oil also tested as moderately toxic to the silverside. EPA's findings on the toxicity of oil itself to the silverside were inconclusive, however, making a relative comparison difficult.

Environmental Defense Fund senior scientist Richard Denison, commenting on the EPA results, said earlier data submitted by Corexit's manufacturer showed an oil-dispersant mix displaying greater acute toxicity than oil alone. Those earlier toxicity tests, Denison said, used No. 2 fuel oil -- a less toxic form than the Louisiana sweet crude that flowed into the Gulf.

"So the good news is that the dispersant doesn't appear to be increasing the acute aquatic toxicity of the oil released into the Gulf," Denison wrote on his blog. "The bad news is that the oil is pretty toxic, and the dispersant certainly doesn't help directly with that."

Click here to read the results of EPA's new dispersant toxicity tests.