Dispersing agents, also called dispersants, are chemicals that contain surfactants and/or solvent compounds that act to break petroleum oil into small droplets. In an oil spill, these droplets disperse into the water column where they are subjected to natural processes, such as waves and currents, that help to break them down further. This helps to clear oil from the water's surface, making it less likely that the oil slick will reach the shoreline.
Heavy crude oils do not disperse as well as light to medium weight oils. Dispersants should not be used on gasoline or diesel spills for example. Dispersants are most effective when applied immediately following a spill, before the lightest materials in the oil have evaporated, however, dispersant manufacturers have claimed that the "window-of-opportunity" to apply dispersants effectively is widening.
Environmental factors, including water salinity and temperature, and conditions at sea also influence the effectiveness of dispersants. Studies have shown that most dispersants work best at salinities close to that of normal seawater. EPA policy does not allow the use of dispersants in freshwater unless authorized by an On-Scene Coordinator to protect human health. The effectiveness of dispersants also depends on water temperature. While dispersants can work in colder water, they work best in warm water.
Some countries rely almost exclusively on dispersants to combat oil spills because frequently rough or choppy conditions at sea make mechanical containment and cleanup difficult. However, dispersants have not been used extensively in the United States because of possible long term environmental effects, difficulties with timely and effective application, disagreement among scientists and research data about their environmental effects, effectiveness, and toxicity concerns.
New technologies that improve the application of dispersants are being designed. The effectiveness of dispersants is being tested in laboratories and in actual spill situations, and the information collected may be used to help design more effective dispersants. Dispersants used today are less toxic than those used in the past, but long term cumulative effects of dispersant use are still unknown.