Aerial spraying of chemical dispersants in the Gulf
After the April 20th explosion of the Deepwater Horizon rig in the Gulf of Mexico, BP began using chemical dispersants to combat the plume of oil spilling into the ocean. The use of dispersants is generally considered to protect sensitive shorelines at the expense of deeper waters. The oil binds to the dispersants and rather than drifting ashore as a devastating slick, the oil is broken up into smaller particles which then sink to the ocean floor and are ultimately broken down by microbes into carbon dioxide and water. The dispersants are themselves inherently toxic and their effects on marine life when bound to oil are not entirely certain.
The EPA has pressed BP to reduce its usage of the dispersant Corexit 9500, and to determine if any less toxic products were available. The EPA also pursued its own toxicity testing on the compounds and released its results this week. They report that of eight dispersants tested, Corexit 9500 is no more toxic than any other product, and that Corexit in combination with oil is no more toxic than oil alone. The testing was performed on two organisms: the mysid shrimp, Americamysis bahia, an aquatic invertebrate, and the inland silverside, Menidia beryllina, a small estuarine fish.
The test was two-fold:
1) acute toxicity testing involves exposing the animals to variable concentrations of oil alone and oil-dispersant mixtures and determining how much of each it takes to kill half of the animals. That amount was similar for oil and oil plus dispersant. Dispersants alone are somewhat less toxic than oil.
2) in vitro testing of potential endocrine disruption involves exposing human-derived cells (in a laboratory) to the oil and to the oil-dispersant mix. The cells were then examined to see if there were any changes in the way the cells respond to estrogen or testosterone. No such changes were observed, and so both the oil and dispersant were deemed not to be endocrine disrupting in mammalian cells.
As with any study, this one has limitations. While somewhat reassuring, the question of the long-term effects of these compounds is not answered by these tests. The effect on other organisms like shellfish, phytoplankton, bacteria, and so on, is not known. And, as the EPA itself points out, the test they performed on human-derived cells does not test all the ways a chemical can cause endocrine disruption. The capacity for endocrine disruption was also not tested on non-mammalian cells, so it remains unclear what impact they might have on fish, invertebrates, or other ocean dwelling organisms.
As the oil spill gradually fades from the media spotlight, the creatures in the Gulf will continue their exposure to these compounds and thus their uncontrolled experiment on their effects. We will continue to follow this story and share both future results released by the EPA, as well as from any other credible source.