A U.S. Air Force Reserve plane sprays dispersants over the Deepwater Horizon oil spill in the Gulf of Mexico.

Corexit[1] (often styled COREXIT)[2] is a product line of solvents primarily used as a dispersant for breaking up oil slicks. It is produced by Nalco Holding Company which is associated with BP and Exxon.[3] Corexit was the most-used dispersant in the Deepwater Horizon oil spill in the Gulf of Mexico, with COREXIT 9527 having been replaced by COREXIT 9500 after the former was deemed too toxic.[4] Oil that would normally rise to the surface of the water is broken up by the dispersant into small globules that can then remain suspended in the water.[5]



In 2010, Corexit EC9500A and Corexit EC9527A were used in large quantities in the Deepwater Horizon oil spill.[6][7] The Environmental Protection Agency (EPA) had pre-approved both forms of Corexit for uses in emergencies such as the Gulf oil spill.[8] 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,[9] 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.[10] BP took the latter option, citing safety and availability concerns with alternatives.[11] 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.[12]

BP had used Corexit EC9500A and Corexit EC9527A by late May, applying 800,000 US gallons (3,000,000 l) total,[13] but more accurate estimates run as high as 1,000,000 US gallons (3,800,000 l) underwater.[14] By late April 2010, Nalco, the maker of Corexit, says that it has been deploying only Corexit 9500.[15]


Corexit 9527

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 with a small concentration of propylene glycol.[16][17]

Corexit 9500

In response to public pressure, the EPA and Nalco released the list of the six ingredients in Corexit 9500, revealing constituents including sorbitan, butanedioic acid, and petroleum distillates.[4] Corexit EC9500A is made mainly of hydrotreated light petroleum distillates, propylene glycol and a proprietary organic sulfonate.[18] Environmentalists also pressured Nalco to reveal to the public what concentrations of each chemical are in the product; Nalco considers that information to be a trade secret, but has shared it with the EPA.[19] Propylene glycol is a chemical commonly used as a solvent or moisturizer in pharmaceuticals and cosmetics, and is of relatively low toxicity. An organic sulfonate (or organic sulfonic acid salt) is a synthetic chemical detergent, that acts as a surfactant to emulsify oil and allow its dispersion into water. The identity of the sulfonate used in both forms of Corexit was disclosed to the EPA in June 2010, as dioctyl sodium sulfosuccinate.[20]


The relative toxicity of Corexit and other dispersants are difficult to determine due to a scarcity of scientific data.[4] 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."[21] According to the manufacturer's website, workers applying Corexit should wear breathing protection and work in a ventilated area.[22] Compared with 12 other dispersants listed by the EPA, Corexit 9500 and 9527 are either similarly toxic or 10 to 20 times more toxic.[8] 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.[23] 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.[23]

The first analysis of the 57 chemicals found in Corexit formulas 9500 and 9527 was conducted by Earthjustice and Toxipedia Consulting Services in the summer of 2011. Results showed the dispersant could contain cancer-causing agents, hazardous toxins and endocrine-disrupting chemicals.[24]

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.[15][25] 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.[26] 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".[17] Like 9527, 9500 can cause hemolysis (rupture of blood cells) and may also cause internal bleeding.[5]

According to the EPA, Corexit is more toxic than dispersants made by several competitors and less effective in handling southern Louisiana crude.[27] On May 19, 2010, the EPA ordered BP to change to a different dispersant than Corexit, or to produce a report within 24 hours on alternatives considered and reasons for their rejection.[28] BP took the latter option, sending its report the next day.[29] On May 26, the EPA told BP to reduce the use of dispersants; surface use was prohibited unless a request for exemption in specific circumstances was granted, while subsurface use was capped at 15,000 gallons per day.[30]

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.'[31] 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.[32] Corexit 9500 causes oil to form into small droplets in the water; fish may be harmed when they eat these droplets.[5] According to its Material safety data sheet, Corexit may also bioaccumulate, remaining in the flesh and building up over time.[33] Thus predators who eat smaller fish with the toxin in their systems may end up with much higher levels in their flesh.[5] The influence of Corexit on microbiological communities is a topic of ongoing research[34].


The oil film will be dispersed in small droplets which intermix with the seawater. The oil is then not only distributed in two dimensions (on the surface) but is dispersed in three (in the water).

In handling Louisiana crude Corexit EC9500A (formerly called Corexit 9500) was 54.7% effective, while Corexit EC9527A was 63.4% effective.[35][36] The EPA lists 12 other types of dispersants as being more effective in dealing with oil in a way that is safe for wildlife.[8] One of those tested was Dispersit, which was 100% effective in dispersing Gulf oil but is more toxic to inland silverside and shrimp than Corexit.[37]

Evidence from researchers at Woods Hole Oceanographic Institute shows Corexit did not degrade as promised. Studies conducted in January 2011 indicated that the 800,000 gallons of Corexit applied at BP's Macondo well-head "did nothing to break up the oil and simply drifted into the ecosystem".[38]


UK authorities have an approved list of products which must pass both "sea/beach" and "rocky shore" laboratory toxicity tests, following a review of approval procedures over a decade ago.[39] Corexit did not pass the rocky shore test when submitted for renewal of its inclusion on the list, and was dropped. Although it has been omitted from the approved list since 1998, existing stocks which pre-date the removal may be permitted for use away from rocky shorelines, subject to prior approval.

Alternative dispersants which are approved by the EPA are listed on the National Contingency Plan Product Schedule[9] and rated for their toxicity and effectiveness.[40]


  1. ^
  2. ^
  3. ^ By PAUL QUINLAN of Greenwire (2010-05-13). "Less Toxic Dispersants Lose Out in BP Oil Spill Cleanup". Retrieved 2010-06-10. 
  4. ^ a b c David Biello (18 June 2010). "Is Using Dispersants on the BP Gulf Oil Spill Fighting Pollution with Pollution?". Scientific American. Retrieved 19 June 2010. 
  5. ^ a b c d Gaelin Rosenwaks (June 5, 2010). "Oil spill's environmental costs". Toronto Sun. Retrieved June 25, 2010. 
  6. ^ Juliet Eilperin. "Post Carbon: EPA demands less-toxic dispersant". The Washington Post. Retrieved June 26, 2010. 
  7. ^ New York Times, "less toxic dispersants lose out in BP oil spill cleanup", May 13, 2010
  8. ^ a b c Mark Guarino (May 15, 2010). "In Gulf oil spill, how helpful – or damaging – are dispersants?". Christian Science Monitor. Retrieved 26 June 2010. 
  9. ^ a b "National Contingency Plan Product Schedule". Environmental Protection Agency. 2010-05-13. Retrieved 2010-05-21. 
  10. ^ "Dispersant Monitoring and Assessment Directive - Addendum 2". 
  11. ^ "Dispersant Monitoring and Assessment Directive – Addendum". Environmental Protection Agency. 2010-05-20. 
  12. ^ Jamie Anderson (May 23, 2010). "BP to persist with Corexit 9500 dispersant". The Money Times. Retrieved June 26, 2010. 
  13. ^ Paul Quinlan (2010-05-24). "Secret Formulas, Data Shortages Fuel Arguments Over Dispersants Used for Gulf Spill". New York Times. Retrieved 2010-05-24. 
  14. ^ Juliet Eilperin (2010-05-20). "Post Carbon: EPA demands less-toxic dispersant". Washington Post. Retrieved 2010-05-20. 
  15. ^ a b Shelley DuBois (June 15, 2010). "Company profile of NALCO, maker of Corexit for BP oil spill". Fortune. Retrieved June 25, 2010. 
  16. ^ "Safety Data Sheet Product Corexit EC9527A". Retrieved 2010-05-16. 
  17. ^ a b "Chemicals Meant To Break Up BP Oil Spill Present New Environmental Concerns". ProPublica. Retrieved 2010-05-07. 
  18. ^ "Safety Data Sheet Product Corexit EC9500A". Nalco. p. 1. Retrieved 2010-05-16. 
  19. ^ Anne Mulkern (June 25, 2010). "Maker of Controversial Dispersant Used in Gulf Oil Spill Hires Top Lobbyists". New York Times. Retrieved June 25, 2010. 
  20. ^ Schor, Elana (2010-06-09). "Ingredients of Controversial Dispersants Used on Gulf Spill Are Secrets No More". The New York Times. 
  21. ^ "Safety Data Sheet Product Corexit EC9500A". Nalco. pp. 5–6. Retrieved June 11, 2010. 
  22. ^ Sanjay Gupta (June 10, 2010). "Anderson Cooper 360: Blog Archive - How will the oil spill affect my health?". Retrieved June 25, 2010. 
  23. ^ a b CNN Wire Staff (June 30, 2010). "Dispersants appear to break up in Gulf, EPA says". CNN. Retrieved July 1, 2010. 
  24. ^
  25. ^ "Material Safety Data Sheet: Corexit EC9527A". NALCO. May 11, 2010. Retrieved May 30, 2010. "may cause injury to red blood cells (hemolysis), kidney or the liver" 
  26. ^ Elana Schor (June 9, 2010). "Ingredients of Controversial Dispersants Used on Gulf Spill Are Secrets No More". New York Times. Retrieved June 25, 2010. 
  27. ^ "Less toxic dispersants lose out in bp oil spill cleanup", The New York Times, May 13, 2010
  28. ^ "Dispersant Monitoring and Assessment Directive – Addendum 2"
  29. ^ "BP response to EPA Monitoring and Assessment Directive - Addendum 2"
  30. ^ "EPA Dispersant Monitoring and Assessment Directive ‐ Addendum 3"
  31. ^ Dugan, Emily (May 30, 2010). "Oil spill creates huge undersea 'dead zones'". The Independent. Retrieved May 30, 2010. 
  32. ^ "Nalco dispersant makes oil more toxic to marine life, group says". Daily Herald. June 15, 2010. Retrieved June 25, 2010. 
  33. ^ Bill Riales (June 18, 2010). "BP Dispersant Getting Independent Lab Test". WKRG News 5. Retrieved June 25, 2010. 
  34. ^ Fulmer, P. A.; Hamdan, L. J.: Effects of COREXIT EC9500A on bacterial communities influenced by the Deepwater Horizon oil spill, AGU Fall Meeting Abstracts, December 2010
  35. ^ Environmental Protection Agency, NCP Product Schedule, Accessed May 16, 2010
  36. ^ Environmental Protection Agency, NCP Product Schedule, Accessed May 16, 2010
  37. ^ Michael J. Hemmer, Mace G. Barron, and Richard M. Greene (June 30, 2010). "Comparative Toxicity of Eight Oil Dispersant Products on Two Gulf of Mexico Aquatic Test Species". Retrieved August 30, 2011. 
  38. ^
  39. ^ Oil spill treatment products approved for use in the United Kingdom. Marine Management Organisation. May 18, 2010. Retrieved June 13, 2010. 
  40. ^ "National Contingency Plan Product Schedule Toxicity and Effectiveness Summaries". Environmental Protection Agency. May 13, 2010. Retrieved May 21, 2010. 

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