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From NPR News, this is ALL THINGS CONSIDERED. I'm Melissa Block.

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And I'm Michele Norris.

The chemical dispersants that BP poured into the Gulf of Mexico to dilute the oil from its broken well seem to have disappeared. That's what government scientists told a Senate panel today. But many warned that out of sight should not lead to out of mind. As NPR's Christopher Joyce reports, the long-term effects of these chemicals remain a mystery.

CHRISTOPHER JOYCE: BP and the federal cleanup teams in the Gulf had a choice in the early days of the spill: let the oil rise to the surface, form a slick and travel wherever, or use dispersants to break it up. They chose dispersants - almost two million gallons of them. Republican Senator John Barrasso of Wyoming said the circumstances were unforgiving.

Senator JOHN BARRASSO (Republican, Wyoming): They leave responders with a catch-22: either you are blamed for dumping chemicals in the Gulf or you allow the oil to devastate the Gulf.

JOYCE: Barrasso spoke at a hearing before a panel of the Senate Environment and Public Works Committee, which is weighing new legislation to regulate the use of chemical dispersants. Those dispersants - BP used two kinds of a chemical called Corexit - are toxic. Government scientists say cleanup workers were warned and took safety precautions. As for wildlife exposed to it? Well, nobody really knows.

Dr. PAUL ANASTAS (Assistant Administrator, Office of Research and Development, U.S. Environmental Protection Agency): But what we do know right now is this: We aren't seeing dispersants in our monitoring results.

JOYCE: That's Paul Anastas, a scientist with the Environmental Protection Agency. EPA has looked in deep water, as well as along the Gulf coastline - no dispersants showed up.

But scientists who study marine life point out that doesn't mean the chemicals haven't affected life in the Gulf. Among those scientists is toxicologist Ronald Kendall at Texas Tech University, who implored the senators not to think that the worst is over.

Dr. RONALD KENDALL (Chairman, Department of Environmental Toxicology, Texas Tech University): This is uncharted territory. We need science now.

JOYCE: Kendall said what's happening in the Gulf right now is a huge uncontrolled experiment.

Dr. KENDALL: Did we really understand the environmental toxicology of such a massive use in the deep water of a substance such as Corexit? And I say we did not.

JOYCE: Tests by EPA done since the oil spill show the Corexit dispersant is slightly less toxic than the oil itself. And when it mixes with the oil, the combination is no more toxic than oil by itself. And by now, the Corexit is highly diluted in a vast body of water.

But toxic is a hard thing to pin down. Tests done before and after the spill looked at short-term effects of dispersants on only a few species of fish and shrimp. Many of those tests were done by the manufacturer, not by government regulators. EPA's Anastas acknowledged that science's work is just beginning.

Dr. ANASTAS: I am not suggesting that we have perfect knowledge. I am not suggesting that we don't need more information and more monitoring. I am actually saying straight out that it is important to keep on asking these hard questions.

JOYCE: A lot of marine scientists are asking those questions and wondering where the money will come from to answer them, especially once the drama in the Gulf goes the way of the oil slicks.

Dana Wetzel is a chemist who spent years studying dispersants at the Mote Marine Laboratory in Florida.

Dr. DANA WETZEL (Program Manager, Mote Marine Laboratory): We cannot just sweep this under the rug and say, okay, we're collecting all the oil, and it's going away. We really don't have all that much to worry about anymore. I contend that that's absolutely so wrong.

JOYCE: A lot of oil was not collected or burned off and could remain under the Gulf in great clouds of oil and dispersant. Wetzel says the potential long-term effects of this chemical cocktail on everything from coral reefs to whale sharks are unknown.

Dr. WETZEL: Will they reproduce as they normally would? Will they have the same number of offspring? It doesn't end there. Will those offspring reproduce? You know, is their immune system compromised?

JOYCE: The government and BP are hiring scientists to figure these things out, but because their findings may figure in lawsuits against BP, not all the results may be made public.

Christopher Joyce, NPR News.

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