From Beachapedia

What are Dispersants and How Do They Work?

A boat sprays dispersant on the water

Dispersants, like Corexit 9500/9527 (used in Gulf of Mexico oil spills), are a mixture of solvents, surfactants and other chemicals that are designed to make oil more soluble in water. Dispersants consist normally of one or more surfactants. Surfactants are long molecules that are hydrophilic (water-seeking) on one end and oleophilic (oil-seeking) on the other. One end grabs an oil molecule, the other, a water molecule. By reaching across the oil-water boundary, the surfactant lowers the tension that keeps the two substances separate.

The primary natural force behind the breakdown of a large oil slick is turbulence. In a sink full of water, swishing that oily measuring cup vigorously gets more oil off than gently gliding it around. But if you add a bit of soap (dispersant), the measuring cup cleanup is even easier. Oil dispersant is that dish soap, lowering the tension between oil and water and allowing small droplets of oil to break away from the larger clumps on the sea surface by transferring it into the water column. Dispersants cause the oil slick to break up and form water-soluble micelles that are rapidly diluted. The oil then spreads throughout a larger volume of water rather than clinging to the thin surface layer of a water body.

The use of dispersants following an oil spill has been coined an ‘environmental tradeoff’ by industry and the government. By breaking down a large oil slick into small droplets, dispersants do increase the surface area of the oil available to oil-consuming bacteria. Theoretically this should allow the bacteria to degrade the oil faster, however, we don’t really know what effects, ill or otherwise, the dispersant chemicals may be having on the bacterial communities themselves. There is concern that the dispersants could bind to the micelles in a way that would hinder the microbes from degrading oil.

Dispersant use can help dissipate surface slicks of oil, slowing down their movement to coastal areas, beaches and wetlands, but it in turn, relocates the oil to the water column in the deeper waters where the oil spills occur. Good news, at least initially, for beach-goers and the animals that make their homes in coastal wetlands, but it presents a whole new challenge to deep water organisms, especially small, free floating plankton that live and eat in the water column. The smaller size of dispersed oil droplets may even be facilitating uptake of oil by these smaller sea creatures as well as into the tissue and organs of larger species. This seems to be a point of much controversy currently.

The other tradeoff is that using dispersants doesn’t allow us to try to capture the spilled oil by skimming or sucking it off the surface. So while the sea surface is cleaner, the oil is still out in the water, out of sight and, as some fear, out of mind.

The U.S. Bureau of Safety and Environmental Enforcement (BSEE) operates a National Oil Spill Response Research and Renewable Energy Test Facility known as Ohmsett (the Oil and Hazardous Material Simulated Environmental Test Tank) in Leonardo, N.J. The facility features a large saltwater test tank that allows for full-scale testing of oil spill response equipment and technologies. This tank has a large wave generator to simulate the type of conditions seen in the open ocean. Read an article which describes a test which simulated the use of dispersants on an oil spill and discusses the environmental trade-offs. To help answer some of these trade-off questions, NOAA, in between spills, continues to study dispersants and their potential effects on marine resources.

Recent Dispersant Use

Since the Deepwater Horizon drilling platform exploded in the Gulf of Mexico on April 20, 2010, BP has applied almost two million gallons of dispersants, both on the surface and beneath Gulf waters. Government officials acknowledge that the quantity and manner in which dispersants have been applied in the Gulf are unprecedented. The application of dispersant at the source of the discharge, 5,000 feet under the surface of the water, was also unprecedented.

Although included in the Oil Pollution Act of 1990 as a tool for minimizing the impact of oil spills, chemical dispersants are controversial (NRC, 2005) because of the toxicity of dispersed mixtures and their potential negative impacts on ocean life. Another point of controversy is that once oil is dispersed in deep water, it cannot be recovered. Several studies have indicated that oil, when combined with dispersants in the water column, is more toxic to marine species than either oil or dispersant alone. Interestingly, that conclusion was not supported by recent tests conducted by EPA, however, not everyone agrees that the EPA accurately represented the conclusions that can be drawn from their initial assessments.

Fate of Dispersants in the Environment

Depending on a number of factors, including water and air temperature, wind speed and wave action, dipersant chemicals will eventually biodegrade. According to a recent Environmental Protection Agency study, the key active ingredient in the dispersants, dioctyl sodium sulfosuccinate (DOSS), degrades very rapidly under conditions similar to those found at the Gulf surface during the Deepwater Horizon disaster. Meanwhile, in the much colder temperatures found in the deep sea, the breakdown is quite slow. The results fit well with previous observations in the Gulf. Months after the spill ended, teams still detected DOSS at very low concentrations in the deep sea. Use of dispersants in cold water conditions is therefore a concern for both deep sea oil spills and for spills in Arctic waters. A paper published in Environmental Science and Technology in June 2014 indicated that DOSS can remain on Gulf of Mexico beaches for nearly 4 years:

"DOSS was found to persist in variable quantities in deep-sea coral communities (6-9000 ng/g) 6 months after the spill, and on Gulf of Mexico beaches (1-260 ng/g) 26-45 months after the spill. These results indicate that the applied dispersant, which was thought to undergo rapid degradation in the water column, remains associated with oil in the environment and can persist for ~4 years."

Tradeoffs and Controversy

The scientific literature is inconclusive on the impact of dispersants to the marine environment. One long-term study did show that dispersants reduced the “persistence of oil in sub-tidal and intertidal sediments compared to untreated oil.” Studies have shown that early treatment with COREXIT® EC9500A, helps decrease the formation of "mousse" from the spilled oil. But in toxicity studies, it has been shown that COREXIT 9500A combined with fuel oil #2 is more lethal than either fuel oil #2 or the dispersant alone.

At a Senate hearing on June 15, 2010, EPA Administrator, Lisa Jackson stated, “In the use of dispersants we are faced with environmental tradeoffs.” Those tradeoffs were apparent when the Wall Street Journal reported on June 1:

"A federally convened group of scientists is set to recommend that BP PLC and the government continue spraying chemicals into the Gulf of Mexico to help prevent leaking oil from washing ashore, even though the scientists have serious concerns about the potential long-term damage to sea life."

Other scientists believe that the use of dispersants does not represent a science-based, quantifiable “tradeoff” but rather amounts to a large-scale experiment on the Gulf of Mexico ecosystem that runs contrary to a precautionary approach, an experiment where the costs may ultimately outweigh the benefits.

On July 10, 2010 the journal Nature reported concerns expressed by scientists about the implications of the use of dispersants (Nature News, July 10, 2010). David Valentine, a geo-microbiologist at the University of California, Santa Barbara, described BP’s use of dispersants as “an experiment that’s never been performed before – to dump that much of an industrial chemical into the ocean.”

Susan Shaw, a marine toxicologist and director of the Marine Environmental Research Institute, responded to the EPA’s announcement on June 30 that its initial round of toxicity testing on eight dispersants, including Corexit 9500 found no "biologically significant" endocrine-disrupting effects on the small estuarine fish and mysid shrimp tested. "We already know that dispersants are less toxic than oil if you compare the two," said Shaw. "But because Corexit contains a petroleum solvent, we're actually putting petroleum solvent on top of a petroleum spill. So it's increasing the hydrocarbons in the water column." Furthermore, said Shaw, the dispersant can increase the toxicity of the oil for those marine organisms that encounter it. "It's like a delivery system. The [dispersed] oil enters the body more readily and it goes into the organs faster."

Dispersion is thought to speed up oil degradation because tiny droplets can be more readily metabolized by oil-eating microbes. Samantha Joye, a biogeochemist at the University of Georgia in Athens disagrees: "It assumes that the dispersant doesn't impact the microbial community, and we have no idea if that's true or not. There's just as good a chance that this dispersant is killing off a critical portion of the microbial community as it is that it's stimulating the breakdown of oil."

Dispersants applied by BP have resulted in widely disseminated undersea plumes of oil, confirmed by NOAA on June 8, 2010. Samples were collected by scientists from University of South Florida on the MV Weatherbird II and tested by NOAA's lab. Subsequently, the plumes have migrated outward from the discharge source and there is concern that over time, oil may travel with prevailing currents (the Gulf “loop current” and the Gulf Stream) to the Florida Keys, Cuba, and the eastern seaboard of the US. A further revelation, as reported by NPR on September 10, was research by Samantha Joye, a professor in the Department of Marine Sciences at the University of Georgia, of a substantial layer of oily sediment stretching for dozens of miles in all directions from the site of the blown-out oil well. The large quantities of dispersed oil in these plumes can enter the marine food chain and bioaccumulate, or build-up, in animal tissue, potentially impacting marine ecosystems over a broad geographical area and for many years to come.

In April 2015, five years after the Deepwater Horizon disaster, an article What Have We Learned About Using Dispersants During the Next Big Oil Spill? was published on NOAA's Response and Restoration Blog. The article concludes as follows:

"The biggest lesson learned is one we already knew. Once oil is spilled there are no good outcomes and every response technology involves trade-offs. Dispersants don’t remove oil from the environment, but they do help reduce the concentration of the oil by spreading it out in the water (which ocean currents and other processes do naturally), while also increasing degradation rates of oil. They reduce the amount of floating oil, which reduces the risk for some organisms and environments, but increases the risk for others. We also know that some marine species are even more sensitive to oil than we previously thought, especially for some developmental stages of offshore fish including tuna and mahi mahi.

But we also know, from the Exxon Valdez and other spills, that oil on the shore can persist for decades and create a chronic source of oil exposure for birds, mammals, fish, and shellfish that live near shore. We don’t want oil in the water column, and we don’t want oil in our bays and shorelines. Basically, we don’t want oil spills at all. That sounds like something everyone can agree with. But until we stop using, storing and transporting oil we have the risk of spills. The decision to use dispersants or not use dispersants will never be clear cut. Nor will it be done without a lot of discussion of the trade-offs. The many real and heart-felt concerns about potential consequences aren’t dismissed lightly by the responders who have to make tough choices during a spill."

Human Health Impacts

Most of what is known about the toxicity of dispersants and dispersed oil is based on acute (short-term) toxicity tests. The scientific literature suggests that acute toxicity tests with death as the primary endpoint may not adequately assess the chronic (long-term) impacts of chemically-dispersed oil. Long-term studies are needed to adequately determine delayed effects due to metabolism of chemically-dispersed oil, bioaccumulation, or photo-enhanced toxicity. Here's a comprehensive report on the health hazards of crude oil and the known ingredients of Corexit and an article summarizing many of the human health impacts from exposure to oil and dispersants from the Gulf oil spill disaster. Oil spill cleanup crews who responded to the April 2010 Deepwater Horizon oil spill display "significantly altered" blood profiles, liver enzymes and somatic symptoms compared to an unexposed control group in new research published in the American Journal of Medicine, which suggests that oil spill cleanup workers are at risk of developing liver or blood related disorders. Read more on this.

The properties that facilitate the movement of dispersants through oil also make it easier for them to move through cell walls, skin barriers, and membranes that protect vital organs, underlying layers of skin, the surfaces of eyes, mouths, and other structures. The exact makeup of the dispersants is kept secret under competitive trade laws, however the manufacturer is required to reveal any toxic substances contained in the formula. According to datasheets submitted to EPA by the manufacturer Nalco, Corexit 9527 contains 2-butoxyethanol. It has been documented that people exposed to higher than recommended levels of 2-butoxyethanol for several hours reported irritation of the nose and eyes, headaches, a metallic taste in their mouths, and vomiting.

According to a news report on June 3, 2010, over a dozen oil cleanup workers had reported health problems such as dizziness, headaches, chest pain, and nausea and were being treated in local medical centers. As of September 20, 2010, the Louisiana Department of Health and Hospitals had received 411 reports of health complaints believed to be related to exposure to pollutants from the oil spill, including cases of heat stress. Three hundred twenty-five (325) reports came from workers and 86 from the general population. Most frequently reported symptoms included headache, dizziness, nausea, vomiting, weakness/fatigue and upper respiratory irritation. Perhaps even more disturbing is the fact that 2-1/2 years after the Deepwater Horizon disaster many residents of the Gulf states were still experiencing a wide variety of such health effects.

Chemicals in crude oil and dispersants can cause a wide range of health effects in people and wildlife. The chemicals can impair normal growth and development through a variety of mechanisms, including endocrine disruption and direct fetal damage. Some of the classes of chemicals, such as polynuclear aromatic hydrocarbons (PAHs), cause mutations that may lead to cancer and multi-generational birth defects (Burns and Harbut, 2010).

Potential human health effects include burning skin, difficulty breathing, headaches, heart palpitations, dizziness, confusion, and nausea — which have already been reported by some workers — as well as chemical pneumonia and internal bleeding (Burns and Harbut, 2010, US EPA 2010). These relatively minor, acute, health effects are easier to recognize than the more serious, long-term effects that don't have obvious signs and symptoms, such as lung, liver and kidney damage, infertility, immune system suppression, disruption of hormone levels, blood disorders, mutations, and cancer.

A study published in April 2015 by investigators at the University of Alabama at Birmingham suggests that Corexit EC9500A, the oil-dispersal agent widely used in the Gulf of Mexico following the Deepwater Horizon oil spill, contributes to damage to epithelium cells within the lungs of humans and gills of marine creatures. The study also identifies an enzyme that is expressed in epithelial cells across species that has protective properties against Corexit-induced damage. Article on this.

Toxic Impacts on Marine Life

Oil spill impacts to marine and other wildlife can occur by 1) physical contact (oiling), 2) toxicity, and 3) loss of food web niches. Some of the effects of this spill are visible – 6104 dead oiled birds, 593 sea turtles, 98 dolphins and other mammals (Consolidated Fish and Wildlife Collection Report, September 29, 2010). Many scientists suspect that the worst of the impacts on the Gulf are yet to come and will not be apparent without deliberate tracking and scientific assessment.

Since the 1970s, it has been known that application of dispersants to oil spills increases toxicity by increasing oil and hydrocarbon exposure to water column species. A review of the literature by Dye et al (1980) reported that "virtually every author who has investigated the toxicity of oil-dispersant mixtures reports dramatic increases in mortality compared to oil or dispersant alone, indicating the existence of supra-additive synergy." Today, many scientists are concerned about the likelihood of severe, acute impacts on a wide range of Gulf species that are now being exposed to Corexit and oil in the water column. For vulnerable species such as seagrass, corals, plankton, shrimp, crabs, and small fish, acute effects can be lethal, particularly during the spring spawning season. Studies have shown that coral larvae are extremely sensitive to the combined effects, with 0% fertilization rates in the presence of dispersant and dispersed oil, compared with 98% fertilization in the presence of oil alone.

As plumes of dispersed oil form in the water column, globules of oil and dispersant can envelop and kill floating plankton, fish eggs and larvae – and everything else at sensitive life stages. Laboratory experiments have indicated that dispersants increased toxic hydrocarbon levels in fish by a factor of up to 100 and may kill fish eggs. Planktivorous species like herring and whale sharks indiscriminately feed on these globules and may break the oil down to more toxic by-products. Already, vast numbers of bottom-feeders and filter-feeders have been decimated in heavily oiled areas such as Louisiana’s Barataria Bay (Shaw, CNN 2010). Depletion of these critical niches in the food web can set the stage for “trophic cascades,” causing the collapse of higher organisms (Peterson et al. 2003).

Air-breathing animals like dolphins and sperm whales are exposed to volatile petroleum fumes every time they surface for air. Taking oil into the blowhole can cause chemical pneumonia and liver and kidney damage. Skin contact with Corexit and oil can cause ulcers and burns to membranes of the eyes and mouth. Corexit 9527, which was used in the Gulf until supplies ran out in May 2010, contains the toxic solvent, 2-butoxyethanol, that can rupture red blood cells, causing animals to undergo internal bleeding, or hemolysis (Burns and Harbut, 2010, Nalco 2010). Fishermen in the Gulf have reported that dolphins spouting oil from the blowhole have approached their boats (Shaw, TEDXOilSpill, 2010).

The animals with primary exposure to the dispersed oil droplets are not the only affected group. At the top of the food web, large fish (amberjacks, tuna, grouper) and marine mammals are exposed to oil and dispersant through feeding on contaminated fish. As these chemicals are ingested, they can bioaccumulate in different species, exposing any larger animals further up the food chain to higher concentrations of oil. Bioaccumulation has been found to further increase the harmful effects of Polycyclic Aromatic Hydrocarbons (PAHs) found in oil.

Sunlight can also affect the toxicity of dispersed oil through photosensitivity and photomodification. Photosensitization can occur when PAHs bioaccumulate in the tissues of aquatic organisms and form free-radicals when these organisms are exposed to sunlight. Photosensitization can lead to longer-term impacts which differ from the initial effects of PAH toxicity. Photomodification occurs when the chemically-dispersed oil is exposed to sunlight and is transformed into a more toxic chemical. Studies have demonstrated that chemically dispersed oil is significantly more toxic than oil alone when exposed to sunlight. Free-floating, translucent larvae and animals living on or in the sea bottom in shallow water will be the most susceptible to photo-enhanced toxicity.

A Mote Marine Laboratory study of Corexit 9500 showed that use of the dispersant in the BP oil spill disaster caused great harm to coral. The findings were published in the peer-reviewed journal PLOS ONE.


Citing lessons learned during the 2010 BP Deepwater Horizon oil spill, the Environmental Protection Agency in January 2015 proposed sweeping changes in regulations for the use of chemical dispersants and other substances in future spills. The proposed rule (also see this Fact Sheet) includes more stringent standards for toxicity. It also would mandate that the inclusion of the chemicals in regional spill response plans, and the way they are used, be reviewed every five years. The rule would ban the use of dispersants in freshwater.

"Our emergency officials need the best available science and safety information to make informed spill response decisions when evaluating the use of specific products on oil discharges," said Mathy Stanislaus, assistant administrator for EPA's Office of Solid Waste and Emergency Response, in a news release announcing the proposed rule.

"These requirements are anticipated to encourage the development of safer and more effective spill mitigating products, and would better target the use of these products to reduce the risks to human health and the environment," said a summary statement included with the proposed rule.


Massive amounts of dispersants were used during the Gulf oil spill in an attempt to disperse the oil, promote its degradation and prevent it from reaching shore where it could impact beaches and coastal wetlands. Although use of dispersants did help to limit coastal impact from the oil, there are many concerns regarding short and long-term effects to wildlife, human health and the overall Gulf ecosystems. It may take years before a complete evaluation of the benefits and liabilities associated with dispersant use are known, until then dispersant use will likely remain a extremely controversial topic.


A Closer Look at Dispersants

Biodegradability of Corexit 9500 and Dispersed South Louisiana Crude Oil at 5 and 25 °C (Environmental Science and Technology)

Comparative Toxicity of Louisiana Sweet Crude Oil (LSC) and Chemically Dispersed LSC to Two Gulf of Mexico Aquatic Test Species (EPA)

Consensus Statement

Dispersant (Wikipedia)

Dispersants: A Guided Tour (NOAA)

Dispersant makes oil from spills 52 times more toxic

Earthjustice report: The Chaos of Clean-Up

EPA Info on Dispersant Use/BP Spill

Fate of Dispersants Associated with the Deepwater Horizon Oil Spill (Environmental Science and Technology)

Gulf Oil Spill Health Hazards

How Do Oil Dispersants Work?

Marine & Environmental Research Institute

NBC Video Report on Dispersant Use in the Gulf

Oil dispersants an environmental ‘crapshoot’

Oil Dispersant Study Released by EPA, But Big Questions Remain

Oil Dispersants Used During Gulf Spill Degrade Slowly In Cold Water (Chemical and Engineering News)

Scientists Call for End to Use of Dispersants in Gulf

Scientists to Back Dispersant Use, Despite Concerns

Susan Shaw – TED presentation

Swimming Through the Spill ...

Toxicity Aside, Dispersants Could Undermine Natural Oil-Eaters

Toxicity of Deepwater Horizon Source Oil and the Chemical Dispersant, Corexit® 9500, to Coral Larvae

Unfinished Business: The Unspoken Link Between Dispersants and Sick Children in the Gulf of Mexico

What BP Doesn’t Want You to Know About the 2010 Gulf Spill (Newsweek)

What Have We Learned About Using Dispersants During the Next Big Oil Spill? (NOAA)

This article is part of a series on Clean Water which looks at various threats to the water quality of our oceans, and the negative impacts polluted waters can have on the environment and human health.

For information about laws, policies, programs and conditions impacting water quality in a specific state, please visit Surfrider's State of the Beach report to find the State Report for that state, and click on the "Water Quality" indicator link.