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 where they are subjected to natural processes, such as waves and currents, which 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.
Environmental factors, including water salinity and temperature, and conditions at sea also influence the effectiveness of dispersants. The effectiveness of dispersants also depends on water temperature. While dispersants can work in colder water, they work best in warm water.
Subpart J of the National Oil and Hazardous Substances Pollution Contingency Plan (NCP) directs EPA to prepare a schedule of dispersants, other chemicals, and oil spill mitigating devices and substances that may be used to remove or control oil discharges.