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Posted by on Jun 3, 2010 in Toxicology

What the Hell Is Corexit?

What the Hell Is Corexit?


Corexit, the chemical dispersant being used by BP to break up its massive and growing oil spill, is not the cause of physical symptoms among cleanup workers, says the product’s manufacturer, Nalco.

Several news sources, including the NYT, are reporting today that the Naperville, Illinois-based company is defending the safety of Corexit, “when used as directed,” although Nalco advises that BP’s direct application of Corexit to the spewing oil well is “unprecedented.” The Naperville Sun Times says that 993,000 gallons of Corexit have been sprayed or dumped in the Gulf of Mexico as of yesterday. In May, the EPA asked BP to back off on its use of Corexit in the Gulf spill.

So what’s in Corexit? It’s hard to know exactly, because part of the formula is proprietary. According to the material safety datasheet for Corexit 9500, the “clear, hazy, amber” liquid contains

  • 10%-30% hydrotreated light petroleum distillates (a mineral spirit-type solvent, as far as I can tell);
  • 1%-5% propylene glycol (a widely used solvent and chemical cousin of ethylene glycol); and
  • 10%-30% “organic sulfonic acid salt,” which is proprietary (the EPA evidently has the full formula, according to the NYT).

For humans, Corexit appears to be merely a short-term irritant; it is not defined as hazardous or toxic by EPA standards. Safety precautions (eg, gloves, splash goggles) are intended to keep the product away from the skin and eyes. Filter masks are recommended when air concentrations are expected to reach a certain threshold.

Today’s PubMed search for “Corexit” returns 59 articles, dating back to 1974. No article pertains to human safety, and 37 articles concern the product’s effect on sea life. A search for “Corexit 9500” returns 22 articles, dating back to 1996; 12 pertain to animal or plant effects.

The upshot: Products like Corexit 9500 are very effective oil dispersants, but they may increase (at least temporarily) the concentrations of toxic polycylic aromatic hydrocarbons (PAH) in oil-contaminated water, presumably through their dispersant effects. And there are evidently A TON of variables to consider when deciding to use dispersantslike, the concentration of the crude oil, the “weathered” condition of the oil, water salinity, oil-exposure conditions (eg, whether declining or continuous), and the myriad, myriad, myriad species at risk and their life cycles. 

Singer et al (1996). Comparison of acute aquatic effects of the oil dispersant Corexit 9500 with those of other Corexit series dispersants. Corexit 9500 was found to be similarly “toxic” to other Corexit products on early-life stages of the red abalone and kelp forest mysid. The authors, from the University of California, Santa Cruz, wrote that Corexit 9500 is a “reformulation of a long-time industry ‘standard,’ Corexit 9527, to allow use on higher viscosity oils and emulsions.”

George-Ares and Clark (2000). Aquatic toxicity of two Corexit dispersants. Two Exxon employees described the in-vitro “low to moderate toxicity” of Corexit 9500 and Corexit 9527 on “most aquatic species.” They also described the variables affecting toxicity (such as species, life stage, duration of exposure, and temperature) and addressed environmental factors that inform the use of dispersants.

Pollino and Holloway (2002). The toxicity of testing of crude oil and related compounds using early life stages of the crimson-spotted rainbowfish (Melantotaenia fluviatilis). Australian academicians determined that Corexit 9500 and Corexit 9527 were less acutely toxic than naphthalene and crude oil-water-dispersant mixtures on the larvae of freshwater rainbowish.

Ramachandran et al (2004). Oil dispersant increases PAH uptake by fish exposed to crude oil. Canadian researchers concluded that the use of dispersants, like Corexit 9500, actually increases the exposure of fish to toxic crude-oil hydrocarbons.

Fuller et al (2004). Comparative toxicity of oil, dispersant, and oil plus dispersant to several marine species. Scientists at Texas A&M observed that crude oil with dispersant was equally or less toxic that crude oil alone on 2 fish and 1 shrimp species. “Unweathered” crude oil (dominated by “soluble hydrocarbon fractions”) was more toxic than weathered oil (which was dominated by “colloidal oil fractions”). In declining exposure conditions, weathered and unweathered oil with dispersant were equally toxic to a standardly tested fish species, Menidia beryllina. Both media were dominated by the less toxic “colloidal oil fractions.” The consistent finding in this variable-results study: declining-exposure conditions were less toxic than continuous-exposure conditions.

Couillard et al (2005). Effect of dispersant on the composition of the water-accommodated fraction of crude oil and its toxicity to larval marine fish. Researchers from the Canadian Department of Fisheries and Oceans concluded that Corexit 9500, when added to seawater-accommodated fractions of light crude oil, multiplied the concentrations of PAH and was associated with higher mortality rates in larval mummichog.

Liu et al (2006). Field investigation on the toxicity of Alaska North Slope crude oil (ANSC) and dispersed ANSC crude to Gulf killifish, Eastern oyster and white shrimp. Investigators at Louisiana State University found that Corexit 9500 was an effective oil dispersant and facilitated the rapid reduction of hydrocarbon concentrations. At testing conditions, most of the tested juvenile organisms (>83%) survived “well” after 24 hours of exposure. A crude oil concentration higher than 30 ppm was required for “any significant toxic effect.”

Ramachandran et al (2006). Influence of salinity and fish species on PAH uptake from dispersed crude oil. Water salinity reduced PAH exposure (by reducing PAH solubility) and the efficiency of dispersants (but only at the highest tested salinity). The Canadian authors concluded that the risk of PAH exposure from dispersed oil will be greatest where salinity is lowestthat is, in coastal waters.

Anderson et al (2009). Preliminary investigation of the effects of dispersed Prudhoe Bay Crude Oil on developing topsmelt embryos, Atherinops affinis. Again, Corexit 9500 increased the hydrocarbon concentrations in water-accommodated oil fractions and this effect appeared to adversely affect the survival of topsmelt embryos, according to researchers of the University of California, Davis.

Jung et al (2009). Biochemical changes in rockfish, Sebastes schlegeli, exposed to dispersed crude oil. Korean investigators confirmed that oil dispersants, like Corexit 9500, increase the exposure of fish to oil hydrocarbons.

Lin et al (2009). Characterization of the metabolic actions of crude versus dispersed oil in salmon smolts via NMR-based metabolomics. Taiwanese scientists concluded that “dispersant treatment significantly decreased the lethal potency of crude oil to salmon smolts,” and described several variable metabolic effects that may be useful for monitoring sublethal actions of dispersed oil on fish.

Duarte et al (2010). Acute effects of chemically dispersed crude oil on gill ion regulation, plasma ion levels and haematological parameters in tambaqui (Colossoma macropomum). Investigators in the Amazon reported that chemically dispersed crude oil impairs gill function (ie, ion regulation) in tambaqui to a greater extent than untreated crude oil or Corexit 9500 alone.

Video still of burning Deepwater Horizon rig from YouTube.

06/07/10 addendum: BP’s use of 1 million or so gallons of dispersant may also confound the cleanup effort in the Gulf. It’s certainly to BP’s advantage to obscure the scope of the spill, and Admiral Thad Allen of the Coast Guard says that dispersants “have succeeded at fragmenting one giant spill into ‘hundreds of thousands’ of mini spills,” reports today’s Politics Daily. BP’s use of dispersant directly on the wellhead is also likely to prevent crude oil from rising to the surface, where it is easier to spot and clean up.  

bmartin (1082 Posts)

A native East Tennessean, Barbara Martin is a formerly practicing, board-certified neurologist who received her BS (psychology, summa cum laude) and MD from Duke University before completing her postgraduate training (internship, residency, fellowship) at the Hospital of the University of Pennsylvania in Philadelphia. She has worked in academia, private practice, medical publishing, drug market research, and continuing medical education (CME). For the last 3 years, she has worked in a freelance capacity as a medical writer, analyst, and consultant. Follow Dr. Barbara Martin on and Twitter.