In 2010, Corexit EC9500A and Corexit EC9527A are being used in large quantities in the Deepwater Horizon oil spill. TheEnvironmental Protection Agency (EPA) had pre-approved both forms of Corexit for uses in emergencies such as the Gulf oil spill.Corexit 9580 was used during the 1989 Exxon Valdez oil spill disaster in Alaska. On May 19, 2010 the EPA gave BP 24 hours to choose less toxic alternatives to Corexit, selected from the list of EPA-approved dispersants on the National Contingency Plan Product Schedule, and begin applying them within 72 hours of EPA approval of their choices; or, if BP could not find an alternative, to provide a report on the alternative dispersants investigated and reasons for their rejection. BP took the latter option, citing safety and availability concerns with alternatives. Sea Brat 4, the only effective alternative that is available in quantities large enough for the spill and is less toxic, was rejected by BP because of the risk that components would break down into nonylphenol, which persists in the environment and is toxic to marine life. BP had used Corexit EC9500A and Corexit EC9527A by late May, applying 800,000 US gallons (3,000,000 l) total, but more accurate estimates run as high as 1,000,000 US gallons (3,800,000 l) underwater. By late April 2010, Nalco, the maker of Corexit, says that it has been deploying only Corexit 9500.
The proprietary composition is not public, but the manufacturer's own safety data sheet on Corexit EC9527A says the main components are 2-butoxyethanol and a proprietary organic sulfonate
The relative toxicity of Corexit and other dispersants are difficult to determine due to a scarcity of scientific data. The manufacturer's safety data sheet states "No toxicity studies have been conducted on this product," and later concludes "The potential human hazard is: Low." According to the manufacturer's website, workers applying Corexit should wear breathing protection and work in a ventilated area. Compared with 12 other dispersants listed by the EPA, Corexit 9500 and 9527 are either similarly toxic or 10 to 20 times more toxic. In another preliminary EPA study of eight different dispersants, Corexit 9500 was found to be less toxic to some marine life than other dispersants and to break down within weeks, rather than settling to the bottom of the ocean or collecting in the water. None of the eight products tested are "without toxicity", according to an EPA administrator, and the ecological effect of mixing the dispersants with oil is unknown, as is the toxicity of the breakdown products of the dispersant. Corexit 9527, considered by the EPA to be an acute health hazard, is stated by its manufacturer to be potentially harmful to red blood cells, the kidneys and the liver, and may irritate eyes and skin. The chemical 2-butoxyethanol, found in Corexit 9527, was identified as having caused lasting health problems in workers involved in the cleanup of the Exxon Valdez oil spill. According to the Alaska Community Action on Toxics, the use of Corexit during the Exxon Valdez oil spill caused people "respiratory, nervous system, liver, kidney and blood disorders". Like 9527, 9500 can cause hemolysis (rupture of blood cells) and may also cause internal bleeding.
According to the EPA, Corexit is more toxic than dispersants made by several competitors and less effective in handling southern Louisiana crude. On May 20, 2010, the EPA ordered BP to look for less toxic alternatives to Corexit, and later ordered BP to stop spraying dispersants, but BP responded that it thought that Corexit was the best alternative and continued to spray it. Reportedly Corexit may be toxic to marine life and helps keep spilled oil submerged. There is concern that the quantities used in the Gulf will create 'unprecedented underwater damage to organisms.' Nalco spokesman Charlie Pajor said that oil mixed with Corexit is "more toxic to marine life, but less toxic to life along the shore and animals at the surface" because the dispersant allows the oil to stay submerged below the surface of the water. Corexit 9500 causes oil to form into small droplets in the water; fish may be harmed when they eat these droplets. According to its Material safety data sheet, Corexit may also bioaccumulate, remaining in the flesh and building up over time. Thus predators who eat smaller fish with the toxin in their systems may end up with much higher levels in their flesh.[