The Environmental Protection Agency released  data today from its first round of toxicity testing on dispersants that could be used on the oil spill in the Gulf of Mexico, but the agency's chief scientist said that not enough was yet known to make a recommendation about which one is best to use.
Since the spill began, almost 1.6 million gallons of dispersant have been sprayed onto the water and released at the site of the leaking well. Dispersants break up the oil into smaller droplets, which break down faster and are less likely to coat birds or damage wetlands. But the dispersants themselves can be toxic to other marine life, such as shrimp and fish.
BP has been using a dispersant called Corexit 9500, which appeared to be particularly toxic. On 20 May, EPA asked BP to find a less toxic alternative. When BP said it couldn't find a safer substitute, EPA began to test eight of the 14 dispersants that it had previously approved for oil spills. These tests aren't entirely new: Companies must submit toxicity data when they request approval for dispersants, but variation in methods made it difficult to compare the results. In addition, the existing tests looked at a mixture of dispersants and fuel oil, not the crude oil now gushing into the gulf.
EPA's National Health and Environmental Effects Research Laboratory supervised acute toxicity on two species native to the gulf: a small fish, Menidia beryllina, and mysid shrimp, Americamysis bahia. Corexit 9500 and seven other dispersants are all about equally toxic, with some slightly more toxic to fish and others more toxic to shrimp. Ranging from the category of "practically non-toxic" to "slightly toxic," all the dispersants are less toxic than oil, said Paul Anastas, EPA's chief scientist, at a teleconference about the results.
However, experts say that the combination of dispersant and oil can be more toxic than either alone. EPA is testing these mixtures, and Anastas says he expects results within the next few weeks. Only after considering these further data will EPA consider whether to order BP to change its use of dispersants. (The average daily amount used has dropped by 68% since 23 May, when EPA told BP to lower the volumes.)
Another important question is the risk of chronic effects, such as reproductive harm, from exposure to dispersants over the weeks to months they take to degrade, says Carys Mitchelmore, environmental toxicologist at the University of Maryland's Center for Environmental Science in Solomons. It's not clear whether EPA plans to do this. Perhaps the biggest problem with dispersants, Mitchelmore points out, is that they allow droplets of oil to reach organisms below the surface that otherwise wouldn't have been exposed.