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Oil comes in many forms that differ from each other in their behaviour. The difference is defined  under 3 main headings , Viscosity, Volatility, and Toxicity.

Viscosity refers to an oil resistance to flow.

Volatility refers to how quickly the oil evaporates (into the air).

Toxicity refers the poisonous elements of oil and how it can adversely affect the environment.

Oil is classified into groups that outline their behaviour and persistence.Knowing the behaviour of the oil helps determine its effects on the environment and how to appreciate a clean -up.

Group 1

Non-persistent light oils (petrol, aviation fuel)

Highly volatile (evaporate within 1-2 days). Does not leave a residue behind after evaporation

Localised, severe impacts to water column and intertidal resources.

Clean up can be dangerous due to high flammability and toxic air hazard.

Group 2

Persistent light oils (eg Diesel)

Moderately volatile, will leave residue (up to one third of spill amount after a few days).

Will “oil” intertidal resources with long - lasting contamination potential .

Clean-up can be very effective.

Group 3

Medium oils ( most crude)

About one-third will evaporate within 24 hours.

Oil contamination of intertidal areas can be severe and long lasting.

Oil impact to Wildlife can be severe.

Cleanup most effective if conducted promptly.

Group 4

Heavy oils (Heavy crude oil)

Little or no evaporation or dissolution.

Heavy contamination of intertidal areas likely. 

Severe impact on wildlife.

Long-term contamination of sediments possible.

Weathers very slowly .

Shoreline cleanup difficult.

Group 5

Sinking oils (slurry oil and residual oils)

If spilled on shoreline will behave similarly to Group 4.

Will sink quickly in water. 

No evaporation or dissolution when submerged.

Severe impact to animals living in sediment.

Can be removed by dredging.