The chemicals BP is now relying on to break up the steady flow of leaking oil from deep below the Gulf of Mexico could create a new set of environmental problems. 
 
 Even if the materials, called dispersants, are effective, BP has already bought up more than a third of the world’s supply. If the leak from 5,000 feet beneath the surface continues for weeks, or months, that stockpile could run out. 
 
 On Thursday BP began using the chemical compounds to dissolve the crude oil, both on the surface and deep below, deploying an estimated 100,000 gallons. Dispersing the oil is considered one of the best ways to protect birds and keep the slick from making landfall. But the dispersants contain harmful toxins of their own and can concentrate leftover oil toxins in the water, where they can kill fish and migrate great distances. 
 
 The exact makeup of the dispersants is kept secret under competitive trade laws, but a worker safety sheet for one product, called Corexit, says it includes 2-butoxyethanol, a compound associated with headaches, vomiting and reproductive problems at high doses. 
 
 “There is a chemical toxicity to the dispersant compound that in many ways is worse than oil,” said Richard Charter, a foremost expert on marine biology and oil spills who is a senior policy advisor for Marine Programs for Defenders of Wildlife and is chairman of the Gulf of the Farallones National Marine Sanctuary Advisory Council. “It’s a trade-off – you’re damned if you do, damned if you don’t -- of trying to minimize the damage coming to shore, but in so doing you may be more seriously damaging the ecosystem offshore.” 
 
 BP did not respond to requests for comment for this article. 
 
 Dispersants are mixtures of solvents, surfactants and other additives that break up the surface tension of an oil slick and make oil more soluble in water, according to a paper published by the National Academy of Sciences. They are spread over or in the water in very low concentration – a single gallon may cover several acres. 
 
 Once they are dispersed, the tiny droplets of oil are more likely to sink or remain suspended in deep water rather than floating to the surface and collecting in a continuous slick. Dispersed oil can spread quickly in three directions instead of two and is more easily dissipated by waves and turbulence that break it up further and help many of its most toxic hydrocarbons evaporate. 
 
 But the dispersed oil can also collect on the seabed, where it becomes food for microscopic organisms at the bottom of the food chain and eventually winds up in shellfish and other organisms. The evaporation process can also concentrate the toxic compounds left behind, particularly oil-derived compounds called polycyclic aromatic hydrocarbons, or PAHs. 
 
 According to a 2005 National Academy of Sciences report, the dispersants and the oil they leave behind can kill fish eggs. A study of oil dispersal in Coos Bay, Ore. found that PAH accumulated in mussels, the Academy’s paper noted. Another study examining fish health after the Exxon Valdez spill in Alaska in 1989 found that PAHs affected the developing hearts of Pacific herring and pink salmon embryos. The research suggests the dispersal of the oil that’s leaking in the Gulf could affect the seafood industry there. 
 
 “One of the most difficult decisions that oil spill responders and natural resource managers face during a spill is evaluating the trade-offs associated with dispersant use,” said the Academy report, titled Oil Spill Dispersants, Efficacy and Effects. “There is insufficient understanding of the fate of dispersed oil in aquatic ecosystems.” 
 
 A version of Corexit was widely used after the 1989 Exxon Valdez spill and, according to a literature review performed by the group the Alaska Community Action on Toxics, was later linked with health impacts in people including respiratory, nervous system, liver, kidney and blood disorders. But the Academy report makes clear that the dispersants used today are less toxic than those used a decade ago. 
 
 “There is a certain amount of toxicity,” said Robin Rorick, director of marine and security operations at the American Petroleum Institute. “We view dispersant use as a tool in a toolbox. It’s a function of conducting a net environmental benefit analysis and determining the best bang for your buck.” 
 
 Charter, the marine expert, cautioned the dispersants should be carefully considered for the right reasons. 
 
 “Right now there is a headlong rush to get this oil out of sight out of mind,” Charter said. “You can throw every resource we have at this spill. You can call out the Marine Corps and the National Guard. This is so big that it is unlikely that any amount of response is going to make much of a dent in the impacts. It’s going to be mostly watching it happen.” 
 
 Ryan Knutson contributed to this report ||||| Scientists studying video of the gushing oil well have tentatively calculated that it could be flowing at a rate of 25,000 to 80,000 barrels of oil a day. The latter figure would be 3.4 million gallons a day. But the government, working from satellite images of the ocean surface, has calculated a flow rate of only 5,000 barrels a day. 
 
 BP has resisted entreaties from scientists that they be allowed to use sophisticated instruments at the ocean floor that would give a far more accurate picture of how much oil is really gushing from the well. 
 
 “The answer is no to that,” a BP spokesman, Tom Mueller, said on Saturday. “We’re not going to take any extra efforts now to calculate flow there at this point. It’s not relevant to the response effort, and it might even detract from the response effort.” 
 
 The undersea plumes may go a long way toward explaining the discrepancy between the flow estimates, suggesting that much of the oil emerging from the well could be lingering far below the sea surface. 
 
 The scientists on the Pelican mission, which is backed by the National Oceanic and Atmospheric Administration, the federal agency that monitors the health of the oceans, are not certain why that would be. They say they suspect the heavy use of chemical dispersants, which BP has injected into the stream of oil emerging from the well, may have broken the oil up into droplets too small to rise rapidly. 
 
 BP said Saturday at a briefing in Robert, La., that it had resumed undersea application of dispersants, after winning Environmental Protection Agency approval the day before. 
 
 “It appears that the application of the subsea dispersant is actually working,” Doug Suttles, BP’s chief operating officer for exploration and production, said Saturday. “The oil in the immediate vicinity of the well and the ships and rigs working in the area is diminished from previous observations.” 
 
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 Many scientists had hoped the dispersants would cause oil droplets to spread so widely that they would be less of a problem in any one place. If it turns out that is not happening, the strategy could come under greater scrutiny. Dispersants have never been used in an oil leak of this size a mile under the ocean, and their effects at such depth are largely unknown. 
 
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 Much about the situation below the water is unclear, and the scientists stressed that their results were preliminary. After the April 20 explosion of the Deepwater Horizon, they altered a previously scheduled research mission to focus on the effects of the leak. 
 
 Interviewed on Saturday by satellite phone, one researcher aboard the Pelican, Vernon Asper of the University of Southern Mississippi, said the shallowest oil plume the group had detected was at about 2,300 feet, while the deepest was near the seafloor at about 4,200 feet. 
 
 “We’re trying to map them, but it’s a tedious process,” Dr. Asper said. “Right now it looks like the oil is moving southwest, not all that rapidly.” 
 
 He said they had taken water samples from areas that oil had not yet reached, and would compare those with later samples to judge the impact on the chemistry and biology of the ocean. 
 
 While they have detected the plumes and their effects with several types of instruments, the researchers are still not sure about their density, nor do they have a very good fix on the dimensions. 
 
 Given their size, the plumes cannot possibly be made of pure oil, but more likely consist of fine droplets of oil suspended in a far greater quantity of water, Dr. Joye said. She added that in places, at least, the plumes might be the consistency of a thin salad dressing. 
 
 Dr. Joye is serving as a coordinator of the mission from her laboratory in Athens, Ga. Researchers from the University of Mississippi and the University of Southern Mississippi are aboard the boat taking samples and running instruments. 
 
 Dr. Joye said the findings about declining oxygen levels were especially worrisome, since oxygen is so slow to move from the surface of the ocean to the bottom. She suspects that oil-eating bacteria are consuming the oxygen at a feverish clip as they work to break down the plumes. 
 
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 While the oxygen depletion so far is not enough to kill off sea life, the possibility looms that oxygen levels could fall so low as to create large dead zones, especially at the seafloor. “That’s the big worry,” said Ray Highsmith, head of the Mississippi center that sponsored the mission, known as the National Institute for Undersea Science and Technology. 
 
 The Pelican mission is due to end Sunday, but the scientists are seeking federal support to resume it soon. 
 
 “This is a new type of event, and it’s critically important that we really understand it, because of the incredible number of oil platforms not only in the Gulf of Mexico but all over the world now,” Dr. Highsmith said. “We need to know what these events are like, and what their outcomes can be, and what can be done to deal with the next one.”
– BP has begun using huge amounts of chemical dispersants to contain the oil slick that has tripled in only a day and is now the size of Puerto Rico. (AP has more on the growing size here.) And while those chemicals—being used above and below the surface—will help reduce the amount of oil that makes landfall, they create a whole new set of environmental concerns in the water, reports ProPublica. The makeup of the dispersants is secret under trade laws, but their toxins can kill fish and collect on the seabed to reenter the food chain. "There is a chemical toxicity to the dispersant compound that in many ways is worse than oil,” says an expert on marine biology. “It’s a trade off—you’re damned if you do damned if you don’t—of trying to minimize the damage coming to shore, but in so doing you may be more seriously damaging the ecosystem offshore."