Studies of the effects of the Exxon Valdez oil spill have shown that the environmental damage caused by oil spills is greater than was previously thought. It is now thought that the impact on marine life can be serious at less than one part per billion of pollution.
Heavier components of crude oil, such as polynuclear aromatic hydrocarbons (PAHs) appear to cause the most damage; while they are relatively unreactive they persist in the water much longer than volatile components such as toluene.
Experiments have shown that less than half a part per billion of PAHs is sufficient to burst the cell membranes of marine life. Researchers have theorized that PAHs affect the structure of the DNA of fish.