If thick layers of surface oil reach salt marshes, the oil coats and kills the plants and is very difficult to remove.  Birds and other wildlife in coastal habitats can become coated as well.  So, if surface oil approaches these critical wildlife habitats, dispersants may be used to minimize environmental damage and speed up the natural breakdown of the product.

When oil is floating on the surface, only the bottom layer of oil is accessible to naturally occurring oil eating microbes.  These organisms feed on oil and help break it down.  By using a dispersant, the layer of oil on the surface is broken into smaller particles and mixed within the water column.  This results in much more surface area of the oil being available for the microbes to feed on, speeding up the natural breakdown of the product.  Used in this manner, dispersants can help protect near-shore habitats and animals and may be the best option available if containment and collection has failed. 

However, the decision to use dispersants is a trade-off.  While the dispersants may help protect valuable near-shore habitats and wildlife, using them may expose previously unaffected portions of the environment to toxins.  Both oil and dispersants can be considered toxic to a variety of marine species.    Certain studies suggest dispersed oil could be more toxic than either compound separately.  Once dispersed, oil could impact organisms located throughout the water column and may travel in underwater currents to locations previously unaffected by the spill.  Also, the microbes that break down the oil require oxygen and can use up much of the oxygen in the water, suffocating other species. 

As BP has used large amounts of dispersants, these issues have raised public concern and many questions about how toxic these materials may be.  Unfortunately, the answer isn’t as simple as we would like.  To understand the issue better, let’s review what “toxic” actually means in science.

When something is said to be toxic, it means that a certain amount, or concentration, of the substance has been shown to negatively impact the health of certain organisms.  To test this, scientists expose specific organisms to the compound in increasing amounts.  For aquatic life, scientists often test algae, small microscopic organisms, and small fish.  After mixing the chemical into the water, they can look for changes in growth, behavior, reproduction, and of course death in the organisms.  Using these experiments, they estimate how toxic a substance may be to a variety of other organisms, including humans.   

A substance that is highly toxic to a microscopic organism may require such a high concentration to harm a human that we would not consider it toxic to us at all.  Humans use many “toxic” products regularly, however we don’t use enough of them at any one time or in a way that would cause negative health impacts to us or our environment.  Many of our household cleaning products fall into this category.  Also, animals that live in water have a longer and different exposure to dissolved compounds compared to humans getting splashed by the water or going for an occasional swim.

For example, Dawn dishwashing detergent can be considered a simple dispersant.  When used on your dishes, or even to clean oiled wildlife, it is effective and safe for you and the animals that come in contact with it.  However, put Dawn into your fish tank and at a relatively low concentration it becomes toxic to the fish in your aquarium.  Dispersants and oil can coat the gills of fish, not allowing them to absorb enough oxygen from the water to survive. 

BP contracted workers who are loading up airplanes or boats with dispersants are required to wear personal protective equipment as touching and breathing the dispersant at that stage is considered toxic to humans.  However, when sprayed and mixed with water, scientists believe the concentration becomes low enough to not cause significant human health impacts.  Also, by the time any dispersant applied at the leak site reaches the Florida Panhandle, the concentration should be even lower.  However, the possible human health impacts of swimming in water with dispersed oil is still being examined.

The biggest concerns about using dispersants with the Deepwater Horizon event are the massive amounts being used and the unknown toxicity of dispersed oil to many species in the Gulf.  Some organisms may be more sensitive to the dispersants or dispersed oil than the toxicity tested species.  Fish eggs, larvae, and young in general have been shown to be more sensitive than adults to toxins.  Also, scientists simply do not know how fast or even where the dispersed oil is spreading.  This means they don’t know what concentration any particular organism is encountering at any particular time.  The obvious concern is that these compounds could be killing large numbers of organisms without us even being aware of it.

Protecting salt marshes and preventing birds and other animals from being coated with oil by using dispersants continues to be a tool used to fight this spill.  Unfortunately, the risks to the rest of the environment in using extreme amounts of dispersants on a spill this size are largely unknown.  Research is being conducted on these issues, but it is unknown if results will come soon enough to help decide if and when to use dispersants.