Corexit

Corexit is a proprietary brand name for a series of chemical dispersants developed and manufactured by Nalco Environmental Solutions (now part of Ecolab Inc.) used to mitigate oil spills in marine and coastal environments. The chemical is designed to break down oil slicks into smaller droplets, enhancing the natural biodegradation process by increasing the oil’s surface area and dispersing it into the water column.
Corexit gained global attention following its extensive use during the 2010 Deepwater Horizon oil spill in the Gulf of Mexico, one of the largest environmental disasters in history. While credited with helping reduce the visible surface oil, the chemical’s environmental and health impacts have been the subject of significant scientific and regulatory debate.

Composition and Chemical Properties

Corexit is not a single substance but a family of dispersant formulations, each containing a mixture of surfactants, solvents, and stabilisers. These components work collectively to reduce the interfacial tension between oil and water, promoting the breakup of oil slicks into tiny droplets.
Major Corexit formulations include:

• Corexit 9527A: Developed in the 1970s; contains 2-butoxyethanol, a glycol ether solvent known for its dispersing ability but also its toxicity.
• Corexit 9500A: Introduced as a less toxic, more effective alternative and widely used during the Deepwater Horizon spill.

Key ingredients (by class):

• Anionic and non-ionic surfactants – e.g., dioctyl sodium sulfosuccinate (DOSS), which lowers surface tension.
• Solvents – such as propylene glycol and petroleum distillates, which help dissolve and distribute surfactants.
• Stabilisers and additives – to improve performance under varying temperature and salinity conditions.

The mixture’s overall function is to allow oil to mix with seawater, forming emulsions that bacteria can more easily degrade.

Mechanism of Action

When applied to an oil spill—either sprayed from aircraft, ships, or underwater at wellheads—Corexit disperses the oil into microscopic droplets.
The process involves three main stages:

1. Contact: Surfactant molecules attach to the oil–water interface.
2. Dispersion: The surfactants lower interfacial tension, causing the slick to fragment into small droplets.
3. Dilution and Biodegradation: The droplets disperse through the water column, where natural microbial activity breaks them down more rapidly.

This mechanism reduces the amount of surface oil, helping prevent slicks from reaching shorelines and affecting marine birds or coastal habitats. However, it also transfers a significant portion of the oil into the subsurface environment, raising questions about long-term ecological impacts.

Historical Use

Corexit has been employed in several major oil spill responses since the 1960s, including:

• The 1978 Amoco Cadiz spill (France).
• The 1989 Exxon Valdez spill (Alaska).
• The 2010 Deepwater Horizon disaster (Gulf of Mexico).

Its widespread use has made it one of the most studied and controversial dispersant products in environmental management history.

Corexit and the Deepwater Horizon Spill

During the Deepwater Horizon oil spill (April–July 2010), approximately 7 million litres (1.84 million gallons) of Corexit (mostly Corexit 9500A and 9527A) were applied—both on the ocean surface and directly at the Macondo wellhead nearly 1,500 metres below sea level.
This was the first time dispersants had been injected subsea at such depth and scale. The method aimed to break up oil before it could surface, reducing visible slicks and limiting coastal contamination.
Results:

• The treatment significantly reduced surface oil slicks and protected coastal wetlands from direct contamination.
• However, it led to the formation of deepwater plumes of dispersed oil, whose effects on marine ecosystems and human health remain contentious.

The U.S. Environmental Protection Agency (EPA) and independent researchers later conducted studies assessing the ecotoxicity, biodegradability, and long-term environmental consequences of Corexit use.

Environmental Impact

The environmental effects of Corexit depend on multiple factors, including dosage, water temperature, salinity, and the type of oil spilled.
Positive aspects:

• Enhanced microbial degradation of oil droplets.
• Reduction of oil fouling on shorelines and wildlife.
• Easier containment of dispersed oil in the open sea.

Adverse effects:

• Toxicity to marine life: Laboratory and field studies have shown that Corexit formulations can be toxic to fish larvae, crustaceans, plankton, and corals.
• Bioaccumulation: Some components (such as DOSS) persist in marine sediments and may enter food webs.
• Synergistic toxicity: When mixed with crude oil, Corexit can increase the oil’s toxicity, especially to early life stages of marine organisms.
• Deepwater contamination: The dispersal of oil into subsurface layers may have caused long-term damage to benthic ecosystems.

In the Gulf of Mexico, research indicated damage to coral reefs, declines in plankton populations, and physiological stress in fish species exposed to dispersed oil mixtures.

Human Health Concerns

Exposure to Corexit—especially through aerosolised spray during large-scale dispersant operations—has been linked to respiratory, dermatological, and neurological symptoms among cleanup workers and residents.
Reported health effects include:

• Skin and eye irritation.
• Headaches, nausea, and dizziness.
• Coughing and shortness of breath.
• Fatigue and chemical sensitivity.

Subsequent medical monitoring under the NIH Gulf Long-Term Follow-Up Study (GuLF STUDY) found associations between dispersant exposure and chronic respiratory issues and mental health stressors among affected populations.
Although regulatory agencies classified Corexit’s acute toxicity as “moderate,” concerns remain regarding chronic low-dose exposure and cumulative ecological impacts.

Regulation and Oversight

In the United States, the use of chemical dispersants like Corexit is regulated under the Oil Pollution Act of 1990 and the EPA’s National Contingency Plan (NCP).
Key regulatory aspects include:

• Dispersants must be listed on the EPA’s NCP Product Schedule before deployment.
• Environmental and toxicity tests determine their suitability for use under specific conditions.
• The Coast Guard and EPA jointly oversee deployment decisions in collaboration with local authorities.

Following the Deepwater Horizon incident, the EPA introduced stricter requirements for:

• Toxicity testing and disclosure of dispersant ingredients.
• Limitations on subsea application.
• Development of less harmful, more biodegradable alternatives.

Alternatives and Research

Post-2010 research has focused on developing next-generation dispersants with lower toxicity and higher biodegradability. Alternatives include:

• Biosurfactants: Naturally derived surfactants from microbial or plant sources (e.g., rhamnolipids, sophorolipids).
• Mechanical and bioremediation techniques: Skimming, in-situ burning, and use of oil-degrading bacteria.
• Solid-phase sorbents: Materials that absorb or adsorb oil without introducing chemical dispersants.

These innovations aim to balance spill mitigation efficiency with minimal ecological disruption.

Global Perspectives and Controversy

While dispersants like Corexit are part of standard oil-spill response strategies worldwide, their use remains controversial. Supporters argue that dispersants are necessary emergency tools to minimise surface impacts and accelerate biodegradation, while critics highlight the trade-off between visible mitigation and hidden ecological damage.
Countries such as Canada, Australia, and members of the European Union have established strict guidelines for dispersant approval and environmental impact evaluation. Some regions have restricted Corexit use altogether, citing unresolved concerns about marine toxicity.