We think of oil as being a single substance, but there actually are many different kinds of oil. Oil types differ from each other in their viscosity, volatility, and toxicity. Viscosity refers to an oil's resistance to flow. Volatility refers to how quickly the oil evaporates into the air. Toxicity refers to how toxic, or poisonous, the oil is to either people or other organisms.

When spilled, the various types of oil can affect the environment differently. They also differ in how hard they are to clean up.

Spill responders (and the Code of Federal Regulations) group oil into five basic groups, which you can see below, along with a general summary of how each group can affect shorelines. For oil spill planners and responders needing more technical information about the characteristics of different oils, please refer to the Oil Fact Sheets page.

Group 1: Non-Persistent Light Oils (Gasoline, Condensate)

• Highly volatile (should evaporate within 1-2 days).
• Do not leave a residue behind after evaporation.
• High concentrations of toxic (soluble) compounds.
• Localized, severe impacts to water column and intertidal resources.
• Cleanup can be dangerous due to high flammability and toxic air hazard.

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

• Moderately volatile; 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.

Group 3: Medium Oils (Most Crude Oils, IFO 180)

• 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.

Group 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.

Group 5: Sinking Oils (Slurry Oils, Residual Oils)

• Will sink in water.
• If spilled on shoreline, oil will behave similarly to a Group 4 oil.
• If spilled on water, oil usually sinks quickly enough that no shoreline contamination occurs.
• No evaporation or dissolution when submerged.
• Severe impacts to animals living in bottom sediments, such as mussels.
• Long-term contamination of sediments possible.
• Can be removed from the bottom of a water body by dredging.

A Case of Mistaken Identity: Can You Spot the False Oil Positives? Read a blog post that features photos of different kinds of oil, as well as some things commonly confused with spilled oil.

How Toxic Is Oil? Learn about some ways that oil can cause harm to living organisms.

The Toxicity of Oil: What's the Big Deal? Explore why oil is toxic and how different recipes for oil can have various toxic effects on living things.

Fingerprinting Oil: Learn about the process known as "fingerprinting,” which scientists use to distinguish between oils.

What are Natural Seeps? Learn how oil or natural gas naturally enters the ocean through fractures and sediments, at areas known as "seeps.”