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and GOMRI 


The mission of Sea Grant is to 

enhance the practical use and 

conservation of coastal, marine 

and Great Lakes resources in 

order to create a sustainable 

economy and environment. 

There are 33 university–based 

Sea Grant programs throughout 

the coastal U.S. These programs 

are primarily supported by 

the National Oceanic and 

Atmospheric Administration 

and the states in which the 

programs are located. 

In the immediate aftermath 

of the Deepwater Horizon 

spill, BP committed $500 

million over a 10–year period 

to create the Gulf of Mexico 

Research Initiative, or GoMRI. 

It is an independent research 

program that studies the effect 

of hydrocarbon releases on 

the environment and public 

health, as well as develops 

improved spill mitigation, oil 

detection, characterization 

and remediation technologies. 

GoMRI is led by an independent 

and academic 20–member 

research board.

The Sea Grant oil spill science 

outreach team identifies the 

best available science from 

projects funded by GoMRI and 

others, and only shares peer-

reviewed research results.

  Texas • Louisiana • Florida 




Larissa J. Graham, Christine Hale, Emily Maung-Douglass, Stephen Sempier,  

LaDon Swann, and Monica Wilson

Nearly two million gallons of dispersants were used at the water’s 

surface and a mile below the surface to combat oil during the 

Deepwater Horizon oil spill. Many Gulf Coast residents have questions 

about why dispersants were used, how they were used, and what 

impacts dispersants could have on people and the environment.  

On April 20, 2010, an explosion on 

the Deepwater Horizon oil rig killed 

11 people. The rig was located 42 

miles southeast of Venice, Louisiana. 

The ruptured wellhead had released 

an estimated 4.9 million barrels of 

Louisiana sweet crude oil before 

responders capped it on July 19, 2010. 

Some of this oil was collected at the site 

of the wellhead. Scientists estimate that 

the remainder of the oil, or about 4.1 

million barrels (172 million gallons), were 

released into Gulf of Mexico waters.


Emergency responders attempted to 

clean up the oil and prevent it from 

reaching the shoreline by skimming 

and burning the oil at the surface. They 

also applied nearly two million gallons 

of dispersants to break up the oil. 

This spill was the largest application of 

dispersants in U.S. history.


The Deepwater Horizon site (NOAA photo)


This map shows the number of days that an area was oiled during the Deepwater Horizon oil spill. Responders found oil in areas 

that are shown in dark gray more days than those shown in light gray. In response to the oiling, responders applied dispersants to surface 

water. The orange lines represent where dispersant was sprayed using airplanes. Responders also sprayed dispersant from boats close to the 

wellhead and at subsurface depths (not shown on this map). (Environmental Response Management Application) 





Responders used two types of dispersants, Corexit 

9500A and Corexit 9527A, during the Deepwater 

Horizon oil spill. Both are on the federal guideline 

list of dispersants that can be used during a spill. 

Responders first applied Corexit 9527A until they 

exhausted the supply. Then they switched to Corexit 

9500A and continued using it throughout the cleanup 



 There are many different Corexit formulas 

and product types. When we use the word “Corexit” in 

this publication, we are referring to Corexit 9527A and 


During the spill, responders sprayed approximately one 

million gallons of dispersants over close to 300 square 

miles of oiled surface waters (Figure 1).



spraying dispersant from airplanes could not apply 

them within 2.3 miles of any vessel, within 3.45 miles 

of the shoreline or visible marine life, or in areas with 

water depths less than 33 feet deep.



sprayed dispersants from boats in areas close to the 

wellsite where crews were working. According to the 

U.S. Coast Guard, this method reduced the amount of 

dispersant in the air, increasing the safety of the working 

environment. The same U.S. Coast Guard report states 

that emergency responders applied 98 percent of the 

dispersants more than 11 miles offshore.


For the first time in U.S. history, responders used 

dispersants at very deep depths. They applied 

approximately 771,000 gallons of Corexit 9500A nearly 

one mile beneath the sea surface at the source of the 

leak. They did this using a remotely operating vehicle to 

inject dispersant at the site of the wellhead.




A typical commercial dispersant contains solvents and 

surfactants. Solvents help keep the chemicals mixed 

and help them dissolve into the oil.  Surfactants allow oil 

and water to mix easily. 


By allowing oil and water to mix, the oil slick breaks into 

many smaller oil droplets. The smaller droplets mix into 

the water column and then are eventually carried away 

by currents, attach to particles in the water column and 

settle to the bottom, or evaporate.


 Nature’s “oil-eating” 

microbes are found in our oceans and have adapted over 

time to use most of the chemicals in oil as food. In the 

process, they naturally remove oil from the environment. 

Smaller oil droplets are more available to these “oil-

eating” microbes than large oil slicks (Figure 2). 



Responders consider trade-offs when determining the 

most effective cleanup method(s) that should be used 

during an oil spill.


 Applying dispersants was just one of 

many strategies that emergency responders used at the 

Deepwater Horizon site during the spill. Other response 

activities included burning the oil at the water surface 

and mechanically recovering it using skimmers or other 


Emergency responders may use dispersants when 

other cleanup methods may not be as practical. For 

example, the spill is very large, the water may be too 

rough for other methods, or the spill may be too far 

away from land to use mechanical recovery equipment.



Under these conditions, responders may choose to 

apply dispersants to break up an oil slick. They may also 

choose to use dispersants because they can be deployed 

rapidly by aircraft, cutting down on the response time.



Dispersants and other response methods can prevent an 

oil slick from reaching the shoreline. This can reduce the 


Dispersants, which are one strategy used to respond to an oil spill, contain molecules that have one end that is attracted to water 

and one end that is attracted to oil. When responders apply dispersants to an oil slick, these molecules attach to the oil, allowing the oil slick 

to be broken up into smaller oil droplets. These smaller droplets then mix into the water column and where they are “eaten” and further 

degraded (broken down) by microbes and other organisms.  (Florida Sea Grant/Anna Hinkeldey)


oiling of coastal wildlife and sensitive habitats, such as 

coastal wetlands, mangroves, and beaches. Once these 

habitats are oiled, they are very difficult to clean without 

causing further damage to the area.


However, dispersants are not always effective. When 

oil is dispersed, it is removed from the water’s surface 

and harder to clean up mechanically.


 Dispersants may 

not work well on some types of oil and spraying them 

on patchy oil slicks can be inefficient.


 They also do not 

work well under very windy conditions because they can 

drift in the wind during application or waves can wash 

them off the slick. When this happens, the dispersant 

cannot penetrate, mix with, and break down the oil.



Applying dispersants beneath the water’s surface, at 

the site of the wellhead, was a new technique used 

during the Deepwater Horizon oil spill. One reason that 

this method was approved was related to the health 

of response workers and the ease of application. Deep 

sea application would minimize human contact with 

dispersants and could also occur at night and during foul 

weather. This technique may have been more effective 

at dispersing the oil because Corexit was injected 

right into the wellhead. This also allowed for a quicker 

response time because dispersants could be applied at 

night and in foul weather.





National Contingency Plan (NCP)


the framework to authorize dispersant use. This plan 

outlines which dispersants emergency responders 

can apply during a spill and how dispersant use will be 



Emergency responders use 

Special Monitoring of 

Applied Response Technologies (SMART)


to monitor surface dispersant use. SMART guidelines 

do not look at the effects or impacts of dispersed oil. 

Instead, they provide information about how to apply 

dispersants and how well they are working.


Responders use the SMART monitoring program to 

determine if dispersants are effectively breaking up oil 

and, if so, how quickly.


 The U.S. Coast Guard, National 

Oceanic and Atmospheric Administration (NOAA), U.S. 

Environmental Protection Agency (EPA), U.S. Centers for 

Disease Control and Prevention, and the U.S. Bureau of 

Safety and Environmental Enforcement were all part of 

developing this program.


 During the Deepwater Horizon 

oil spill, trained observers flew over the oil slick to see if 

the dispersant was working to break up the oil. Sampling 

teams collected water samples to measure the amount 

of dispersed oil in the water column.


 Agencies used 

this information to modify how they were applying the 



In 2010, the U.S. EPA collected air samples off the coast 

of Louisiana for four days after the aerial application 

of dispersants. They tested the air for two chemicals, 

2-butoxyethanol and propylene glycol. Scientists took 

samples 30 minutes after dispersant application and did 

Controlled burning (left) and skimming (right) are two response techniques used to remove oil from the water surface 

after an oil spill. (NOAA photos)


not find either chemical.


 The U.S. EPA also collected 

onshore air quality samples along shoreline sites in 

Louisiana, Mississippi, Alabama, and Florida. They 

tested these samples for two dispersant chemicals that 

were most likely to be in the air in measurable amounts 

after spraying, 2-butoxyethanol and dipropylene 

glycol monobutyl ether. They found low levels of both 

chemicals in the air, but all were below levels that 

are likely to cause negative health effects. All of the 

data from these monitoring efforts are available on 

U.S. EPA’s website a




According to the U.S. Coast Guard’s 


Coordinator Report

, the U.S. EPA and the U.S. Coast 

Guard were satisfied with how dispersants were used 

during the Deepwater Horizon oil spill. These agencies 

stated that dispersants had been an important tool 

for reducing the oil spill’s impact and had prevented 

more damage to marshes, wetlands, beaches, and 

coast’s economy. They also stated that the information 

collected during the spill supported continued 

application of dispersants.





The dispersants that emergency responders use today 

are less harmful to the environment than the more toxic 

products used prior to 1970.


 However, they still contain 

chemicals that can have a negative impact on aquatic 

life (Tables 1 & 2). 

The U.S. EPA was concerned that dispersants could  

hurt aquatic wildlife and so, in May 2010, they 

directed BP to scale back on the use of dispersants. 

To understand how dispersants could affect aquatic 

wildlife, the U.S. EPA conducted independent toxicity 

studies in the laboratory.



The U.S. EPA tested eight approved dispersants, 

including Corexit 9500A, on two aquatic species: Gulf 

mysid, a small shrimp, and the inland silverside, a fish 

that resides in coastal waters in the Gulf of Mexico.



These laboratory tests looked at the acute toxicity 

of dispersants. Determining the acute toxicity of a 

substance can help scientists determine the lethal 

concentration. However, knowing the acute toxicity 


How a substance can affect you will depend on how harmful it is and how long and how often you are exposed 

to it. Every substance is poisonous at some level. Take water, for example. Although we need water to survive, 

drinking too much water can also kill us because it dilutes the concentrations of other nutrients that our body 

needs to function. Researchers determine how much of a substance is “toxic” by exposing animals to different 

doses of a chemical for varying amounts of time. Scientists have conducted most of the tests on small fish or 

shrimp to determine how harmful chemicals found in dispersants are to animals. It is important to note that the 

toxicity of a chemical  will vary among different species, differing lengths of exposure time, and the other factors 

that might be affecting that species at the time. 


The potential health effects of the two dispersants that emergency responders used during the Deepwater Horizon oil spill are 

shown above. This information was compiled from the Material Safety Data Sheets for each chemical. 

Dispersant used during the 

Deepwater Horizon oil spill

Corexit 9500A


Corexit 9527A

Potential health effects to humans based on animal tests

Repeated or prolonged exposure may cause eye and skin irritation, dry the skin leading 

to discomfort and dermatitis, or irritate the respiratory tract.

May be harmful if swallowed, effect the liver and kidney, and/or damage or irritate the 

gastro-intestinal tract. Harmful if absorbed through skin or by inhalation. Repeated 

or prolonged exposure may irritate the respiratory tract or existing skin condition. 

Excessive exposure may cause central nervous system effects, nausea, vomiting, 

anesthetic, or narcotic effects. 


A member of U.S. Coast 

Guard National Strike Force 

on an overflight mission, 

observing an airplane 

spraying dispersant. 

(NOAA photo)

does not help scientists understand the effects that the 

substance can have on the reproduction, growth, or 

development of an animal. Corexit 9527A was not tested 

during these studies because the dispersant was no 

longer being used in response efforts.

The laboratory experiments showed that none of the 

dispersants, when tested alone, were more toxic to the 

shrimp or silversides than oil.


Corexit 9500A and another 

dispersant (JD-2000) were less toxic to silversides than 

the other dispersants that were tested.


The U.S. EPA was also concerned about how oil mixed 

with dispersant could affect aquatic life. They conducted 

additional studies and found that Corexit 9500A, when 

mixed with oil, had similar toxicity as the seven other 

dispersants that were tested.


 Oil alone and oil mixed 

with each of the dispersants that were tested had similar 

toxicity to shrimp, except when oil was mixed with one of 

the dispersant tested (Nokomis 3-AA). The combination 

of oil and this type of dispersant was more toxic to the 

shrimp than oil alone.


The extent of damage that dispersants, dispersed oil, and 

oil can have on aquatic life is still under debate. Some 

peer-reviewed research studies have found dispersants 

to be less toxic than oil alone, and other studies have 

shown that dispersants or oil plus dispersant are more 

toxic than oil alone.



The On-Scene Coordinator Report

This report documents the response efforts during the 

Deepwater Horizon oil spill. It provides a timeline of the 

spill and the efforts to address the potential impacts to 

public health related to the spill. This report is available at













sodium sulfosuccinate 



(Found in Corexit 9527A 

only, not Corexit 9500A)

Propylene glycol

Distillates, petroleum

hydrotreated light

Dipropylene glycol 

monobutyl ether

Sorbitan, mono-(9Z)-9-


Sorbitan, mono- 




Sorbitan, tri-(9Z)-9-

octadecenoate, poly(oxy-

1,2-ethanediyl) derivatives

Other substances 

that contain this 



Certain cosmetics, 

gelatin, chocolate 

powder, beverages, 


Certain cleaners, 

soaps, cosmetics, 

lacquers, paints 


preservatives in 

food, medicines 


Cleaners, paints, 



degreasers, paints

Skin cream, air 


Baby bath, mouth 

wash, face lotion 

Insect spray,  

food additive 

Potential health effects to humans 

based on animal tests


Causes eye, skin, and respiratory tract irritation. May be 

harmful if absorbed through the skin or inhaled. Harmful if 

swallowed. May cause gastrointestinal irritation with nausea, 

vomiting, and diarrhea. 

Causes eye and skin irritation. May be toxic to blood, kidneys, 

liver, and central nervous system. Repeated or prolonged 

exposure to the substance can produce target organs 

damage, including injury to red blood cells, kidney or the liver. 

May cause adverse reproductive effects, birth defects, or 

cancer. Severe over-exposure can result in death. 

Causes mild eye, skin, respiratory, and gastrointestinal 

irritation. May affect the behavior or central nervous system, 

brain, metabolism, blood, respiration, cardiovascular system, 

endocrine system, urinary system, and liver. Prolonged or 

repeated inhalation may affect behavior, central nervous 

system, and spleen. May be toxic to central nervous system. 

May mutate genetic material or cause adverse reproductive 

effects and birth defects. 

Causes skin irritation. Repeated skin contact has resulted in 

irritation and skin cancer in animals. May cause drowsiness or 

dizziness. May be fatal if swallowed or enters airways. 

The chemical, physical, and toxicological properties of this 

chemical have not been thoroughly investigated. 

Not classified as a hazardous substance or mixture.

Not classified as a hazardous substance or mixture.

Not classified as a hazardous substance or mixture.

Chemicals in  

Corexit 9500A


Corexit 9527A


The chemicals that are found in Corexit 9500A and 9527A, the two dispersants that were used during the Deepwater Horizon 

oil spill.


 The Chemical Abstracts Service number (CAS) or unique identifier for each chemical substance, toxicity level based on tests 

on animal subjects, and the potential health effects are listed for each chemical. This information was provided by the 

Food and Drug 

Administration (a), U.S. Department of Health and Human Services (b), Agency for Toxic Substances and Diseases Registration 

(c), and Material Safety Data Sheets (d). 


More information about these ongoing studies can be found on GoMRI’s website at 



. Other publications focusing on dispersants are on the Sea Grant Oil Spill Science Outreach website at 




Many questions remain about the impacts of dispersants 

and dispersed oil on the ecosystem, environment, and 

human health.  The Gulf of Mexico Research Initiative 

(GoMRI) and other research programs are looking 

at alternative dispersant use and technology, how 

dispersants react with oil under various conditions, the 

environmental impacts of dispersants, and the impacts 

that dispersants have on human health. 

This work was made possible in part by a grant from The Gulf of Mexico 

Research Initiative, and in part by the Sea Grant programs of Texas, 

Louisiana, Florida and Mississippi-Alabama. The statements, findings, 

conclusions and recommendations do not necessarily reflect the views of 

these organizations.



  Texas • Louisiana • Florida 



1.  National Commission on the BP Deepwater Horizon 

Oil Spill and Offshore Drilling. (2010). The Use of 

Surface and Subsea Dispersants During the BP 

Deepwater Horizon Oil Spill, Staff Working Paper No. 4. 

RetrievedNovember 23, 2015, from 




2.  Griffiths, S. K. (2012). Oil Release from Macondo Well 

MC252 Following the Deepwater Horizon Accident. 

Environmental Science and Technology, 46, 5616-5622. 

3.  Coastal Response Research Center, Research Planning 

Incorporated, and National Oceanic Atmospheric 

Administration. (2012). 

The Future of Dispersant Use 

in Oil Spill Response Initiative

. Retrieved October 10, 

2014, from



4.  US Coast Guard. (2011). 

On Scene Coordinator 

Report Deepwater Horizon Oil Spill: Submitted to 

the National Response Team, September 2011

Retrieved October 10, 2014, from 



5.  Lehr, B., Bristol, S., & Possolo, A. (2010). Oil 

Budget Calculator Deepwater Horizon. Technical 

documentation: A report to the National Incident 

Command. Retrieved August 13, 2015, from




6.  US Environmental Protection Agency. (2014). EPA’s 

Toxicity Testing of Dispersants. Retrieved October 

10, 2014, from



7.  National Research Council of the National Academies. 

(2005). Understanding Oil Spill Dispersants: Efficacy 

and Effects. Washington, D.C.: The National Academies 


8.  Coolbaugh, T., & McElroy, A. (2013). Dispersant Efficacy 

and Effectiveness. Retrieved from University of New 

Hampshire Coastal Response Research Center and 

Center for Spills in the Environment Web site: 




9.  US Environmental Protection Agency. (1994). National 

Oil and Hazardous Substances Pollution Contingency 

Plan. Retrieved October 10, 2014, from 



10.  US Coast Guard, National Oceanic Atmospheric 

Administration, US Environmental Protection Agency, 

Centers for Disease Control and Prevention, & Minerals 

Management Services. (2006) Special Monitoring of 

Applied Response Technologies. Retrieved October 10, 

2014, from


11.  US Environmental Protection Agency. (2014). Offshore 

air sampling for dispersant-related compounds. 

Retrieved October 13, 2014, from 



12.  US Environmental Protection Agency. (2014). Mobile 

Air Monitoring on the Gulf Coast: TAGA Buses. 

Retrieved October 13, 2014, from 




13. US Environmental Protection Agency. (2015). 

Questions and answers on dispersants. Retrieved 

November 20, 2015, from



Acute toxicity - Adverse (negative) effects after an organism 

is exposed to one or more doses of a substance over a short 

time period (often, less than 72 hours).  

Corexit 9527A and 9500A – dispersants approved for use in 

US waters and those that were used to minimize the presence 

of surface oil slicks during the Deepwater Horizon oil spill.

Dispersants – chemicals that are used during oil spill response 

efforts to break up oil slicks and can limit  floating oil from 

impacting sensitive ecosystems such as coastal habitats.

Microbes – very tiny organisms including bacteria, fungi, 

archaea, and protists.  Some microbes (bacteria and archaea) 

are the oldest form of life on earth.

National Contingency Plan (NCP) – a federal document that 

outlines response efforts for oil spills and other hazardous 

substance spills.

Special Monitoring of Applied Response Technologies 

(SMART) – a cooperative monitoring program that is used 

during oil spill response efforts to determine if dispersant 

application is effective.

Surfactants – compounds that work to break up oil. 

Dispersants contain surfactants that break the oil slick into 

smaller droplets that can more easily mix into the water 


Christine Hale 

Texas Sea Grant

Larissa Graham 

Mississippi-Alabama Sea Grant

Emily Maung-Douglass 

Louisiana Sea Grant

Stephen Sempier 

Mississippi-Alabama Sea Grant

LaDon Swann


Mississippi-Alabama Sea Grant

Monica Wilson 

UF/IFAS Florida Sea Grant 




Graham, L., Hale, C., Maung-Douglass, E., Sempier, S., Swann, 

L., and Wilson, M. (2016). Oil Spill Science: Chemical dispersants 

and their role in oil spill response. MASGP-15-015.

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