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What is an Oil Spill?



An oil spill is the release of a liquid petroleum hydrocarbon into the environment due to human activity and is a form of pollution. Oil spills stem from accidents involving tankers, barges, pipelines, refineries, and storage facilities, often while the oil is being transported to its users.

For instance, the April 2010 Gulf of Mexico oil spill involved crude oil that was released from the explosion of the off-shore drilling rig.

During an oil spill cleanup, workers may encounter many types of crude oil, including fresh and weathered, which contain carcinogenic volatile aromatic compounds like benzene, toluene and naphthalene.

The Four Types of Oil

The first thing we need to discuss is the various types of oil that might require cleanup. Spill responders group oil into four basic types, which you can see here, along with a general summary of how each type can affect shorelines.

jet fuel
Jet fuel is an example of a very light oil.

Type 1: Very Light Oils (Jet Fuels, Gasoline)

  • highly volatile (should evaporate within 1-2 days)
  • high concentrations of toxic (soluble) compounds
  • localized, severe impacts to water column and intertidal resources
  • no cleanup possible

Type 2: Light Oils (Diesel, No. 2 Fuel Oil, Light Crudes)

  • moderately volatile and will leave residue (up to one-third of spill amount) after a few days
  • moderate concentrations of toxic (soluble) compounds
  • will "oil" intertidal resources with long-term contamination potential
  • cleanup can be very effective

Type 3: Medium Oils (Most Crude Oils)

  • about one-third will evaporate within 24 hours
  • oil contamination of intertidal areas can be severe and long-term
  • oil impacts to waterfowl and fur-bearing mammals can be severe
  • cleanup most effective if conducted quickly

Type 4: Heavy Oils (Heavy Crude Oils, No. 6 Fuel Oil, Bunker C)

  • little or no evaporation or dissolution
  • heavy contamination of intertidal areas likely
  • severe impacts to waterfowl and fur-bearing mammals (coating and ingestion)
  • long-term contamination of sediments possible
  • weathers very slowly
  • shoreline cleanup difficult under all conditions
Crude oil is a mixture of hydrocarbons.

What is Crude Oil?

  • Crude oil is a mixture of hydrocarbons and consists of light, medium and heavy chemicals.
  • The hydrocarbons in crude oil are mostly alkanes, cycloalkanes and various aromatic hydrocarbons while the other organic compounds contain nitrogen, oxygen and sulfur, and trace amounts of metals such as iron, nickel, copper and vanadium. The exact molecular composition varies widely.
  • The light parts, such as benzene, xylene, toluene and ethyl benzene generally evaporate into the air in the first 24 hours of a spill (usually before reaching the shore).
  • The medium and heavy parts (consistency much like motor oil) is what cleanup operations on the land and near shore areas focus on.
  • Complex mixture of carcinogenic substances.

Health Hazards and Exposure

Health hazards generally associated with crude oils: Inhalation of the toxic volatile hydrocarbon components, such as benzene, and dermatitis from repeated or prolonged skin contact can cause dermatitis or skin cancer.

Occupational Exposure Limits (OEL)

  • Cleanup workers typically work more than 8 hours/day for 7-14 days in a row.
  • Workers should be informed that OELs based on standard times are not appropriate for monitoring.
  • OELs don't include skin contact, absorption and ingestion, which are common in cleanups.
  • Check with your site supervisor for additional guidance!

Be cautious during cleanup operations. If you are unsure, ask your supervisor before proceeding!

Weathered Crude Oil

Weathered crude or "mousse" is crude petroleum that has lost an appreciable quantity of its more volatile components and has mixed with sea water and organic matter. This is caused by evaporation and other natural causes during the spill landing on the shore and during oily waste handling, storage and treatment or disposal.

The various types of oil differ in how they weather (chemically or physically change when exposed to the elements). Most crude oil blends will emulsify quickly when spilled, creating a stable mousse that presents a more persistent cleanup and removal challenge.

Even in high winds, usually over 70% of a Fuel Oil No. 6 spill will persist as floating or beached oil for a week or longer. On the other hand, over 90% of the diesel in a small spill in the marine environment is either evaporated or naturally dispersed into the water column in time frames of a couple of hours to a couple of days.

Weathered Crude Oil Terms

Weathering is a series of chemical and physical changes that cause spilled oil to break down and become heavier than water. Winds, waves, and currents may result in natural dispersion, breaking a slick into droplets which are then distributed throughout the water. These droplets may also result in the creation of a secondary slick or thin film on the surface of the water.

Adsorption (sedimentation) is the process by which one substance is attracted to and adheres to the surface of another substance without actually penetrating its internal structure.

Dispersion Chart
Click to Enlarge

Biodegradation is the degradation of substances resulting from their use as food energy sources by certain micro-organisms including bacteria, fungi, and yeasts.

Dispersion is the distribution of spilled oil into the upper layers of the water column by natural wave action or application of chemical dispersants.

Dissolution is the act or process of dissolving one substance in another.

Emulsification is the process whereby one liquid is dispersed into another liquid in the form of small droplets.

Evaporation occurs when the lighter substances within the oil mixture become vapors and leave the surface of the water. This process leaves behind the heavier components of the oil, which may undergo further weathering or may sink to the ocean floor.

Oxidation occurs when oil contacts the water and oxygen combines with the oil to produce water-soluble compounds. This process affects oil slicks mostly around their edges. Photo Oxidation is a sunlight-promoted chemical reaction of oxygen in the air and oil.

Health Risks of Weathered Crude Oil

There is a potential dermatitis hazard when weathered crude oil contacts the skin.

Health risks associated with working while exposed to weathered crude oil include the following:

  • Potential dermatitis hazard from skin contact.
  • Inhaling oil droplets/oily particles put into the air during cleanup operations can be irritating to eyes, nose, throat and lungs.
  • Evaporation that occurs during the first 24 to 48 hours after the spill greatly reduces inhalation hazards from the toxic volatile components, such as benzene.

NOTE: Even if air sampling shows no detectable levels or very low levels of volatile organic compounds (VOCs), there still may be health effects present.


Tarballs are dark-colored pieces of oil and the remnants of an oil spill.

Tarballs, the little, dark-colored pieces of oil that can sometimes stick to your feet when you go to the beach, are actually remnants of oil spills. When crude oil (or a heavier refined product) floats on the ocean surface, its physical characteristics change.

During the first few hours of a spill, the oil spreads into a thin slick. Winds and waves tear the slick into smaller patches that are scattered over a much wider area. Various physical, chemical, and biological processes change the appearance of the oil. These processes are generally called "weathering." Weathering processes eventually create a tarball that is hard and crusty on the outside and soft and gooey on the inside, not unlike a toasted marshmallow.

Tarballs are very persistent in the marine environment and can travel hundreds of miles. There is no magic trick to making tarballs disappear. Once tarballs hit the beaches, they may be picked up by hand or by beach-cleaning machinery. If the impact is severe, the top layer of sand containing the tarballs may be removed and replaced with clean sand.



Before beginning this quiz, we highly recommend you review the module material. This quiz is designed to allow you to self-check your comprehension of the module content, but only focuses on key concepts and ideas.

Read each question carefully. Select the best answer, even if more than one answer seems possible. When done, click on the "Get Quiz Answers" button. If you do not answer all the questions, you will receive an error message.

Good luck!

1. An oil spill is considered a form of pollution.

2. During an oil spill cleanup, workers may encounter many types _____.

3. Jet fuel is a type of _____.

4. How much medium oil type should evaporate in 24 hours?

5. This occurs when oil contacts the water and oxygen combines with the oil to produce water-soluble compounds.

Have a great day!

Important! You will receive an "error" message unless all questions are answered.