Here’s what happened. Over 2 million gallons of oil dispersant were used to dissipate surface oil slicks from Deepwater Horizon. Unfortunately, the dispersant didn’t magically make the oil disappear. Instead, it broke the oil into tiny droplets that remain suspended underwater. The size of the droplets, along with the chemical properties of the dispersant, may make the combination more toxic than oil alone. Scientists are concerned that the blend can more easily penetrate cell walls, skin, and membranes. Those fears are being fanned by the fact that gulf coast crab larvae---which are a food source for other marine life---now test positive for oil dispersant.
Using dispersant to contain the spill was a known, calculated risk. It prevented some of the oil from reaching shore, and theoretically makes it easier for oil-eating bacteria to help with clean up. However, dispersant has never before been used in such large quantities. Its toxicity and consequences at this concentration are somewhat unknown.