Dispersants clear the visible oil, but what happens to the marine life? (Image: Petty Officer 2nd Class Andrew Kendrick/US Coast Guard)

As arguments rage over how to clean up the Deepwater Horizon oil spill, an examination of toxicity tests reveals flaws in the data used to determine the safety of dispersants.

The US Environmental Protection Agency and BP have locked horns over the toxicity of the dispersants being used to break up the oil spewing from the Deepwater Horizon well. Now, New Scientist has learned that huge variability in the safety test results submitted by different manufacturers makes it very difficult to judge which of the available dispersant chemicals poses the least threat to marine life.

“It screams to me that I can’t make a judgement on any of these data,” says Carys Mitchelmore, a toxicologist at the University of Maryland’s Chesapeake Biology Laboratory in Solomons, and co-author of a 2005 National Research Council report on the use of oil dispersants.


The EPA commissioned a new round of tests in late May, but it remains unclear when these will be completed.

As of 14 June, more than 3.34 million litres of dispersant had been sprayed onto oil on the sea surface. At least a further 1.52 million litres had been pumped into the oil gushing from the stricken well some 1500 metres below sea level. The use of such large volumes at depth is unprecedented, and marine biologists are concerned about possible toxicity to organisms, including shrimp and fish larvae.

BP vs EPA

So far BP has used a dispersant called Corexit EC9500A made by Nalco Energy Services of Sugar Land, Texas. But on 20 May, the EPA ordered BP to find a less toxic alternative.

The company quickly responded, stating that only five dispersants met the EPA’s requirements. Only one, called Sea Brat #4, made by Alabaster of Pasadena, Texas, was stockpiled by BP. This contained a chemical that would degrade into nonylphenol; this is a hormone disrupter likely to harm the reproductive systems of marine organisms. “BP continues to believe that Corexit EC9500A remains the best alternative,” the company concluded.

Since then, there has been an uneasy stand-off, with the EPA telling BP on 26 May to stop surface spraying, and limit its subsea use of dispersant to a maximum of some 57,000 litres on any given day.

Toxicity tests

However, a close examination of the EPA’s website reveals that the agency’s assumptions about dispersant toxicity are based on unreliable data.

When it demanded that BP find less toxic dispersants, the EPA referred to a table summarising experiments in which a fish called Menidia beryllina and Mysidopsis bahia shrimp were exposed to a mixture of dispersant and diesel fuel oil in a ratio of 1:10.

These are standard experiments that must be submitted for inclusion on the EPA’s National Contingency Plan Product Schedule, which lists products authorised for use on an oil spill. They determine the concentrations in parts per million (ppm) required to kill half of the animals in a given time.

But they are only part of a series of tests that are required by the EPA: the dispersant must also be tested alone, the fuel oil must be tested alone, and finally the lab must run controls testing a “reference toxicant” known as dodecyl sodium sulphate (DSS).

Looking across all the listed dispersants, test results for the fuel oil alone are highly variable, with the concentrations required to kill half of the fish varying from 5.95 to 201.8 ppm. This could indicate that some of the tested oil samples had lost their most toxic volatile components.

This is a major problem. The tests reveal varying toxicity for the different dispersant-oil mixes. But given the significant differences between results for samples of oil alone, it becomes very difficult to compare the results for the mixes. “It’s absurd,” says Joannie Docter, president of GlobeMark Resources in Atlanta, Georgia, which makes a dispersant called JD-2000.

Incomplete data

The DSS control experiments pose even bigger problems, says Mitchelmore. The concentrations of DSS required to kill half of the fish, for example, vary from 1.14 ppm for the tests submitted for Sea Brat #4, to 159.6 ppm for the controls submitted for Nokomis 3-F4, made by Mar-Len Supply of Hayward, California.

Presumably, this huge variation reflects inconsistencies in the testing procedures used by the labs hired by manufacturers to run the tests. If so, it throws the EPA’s conclusions about the products’ relative toxicities into serious doubt.

Docter has complained to the EPA about its reliance on “incomplete and misleading” data; Nalco has similarly drawn the agency’s attention to the huge variability in the results supplied by different dispersant manufacturers. “They acknowledged that there are some shortcomings,” says David Horsup, vice-president for research and development with Nalco Energy Services.

Mitchelmore argues that the EPA should also run tests on the growth and reproduction of shrimp and fish, to judge the dispersants’ longer-term effects. Those tests are not yet being run.

The EPA says it has commissioned a single laboratory to retest all the authorised dispersants, running the standard tests but using Louisiana crude rather than fuel oil. Establishing uniform conditions and ensuring the animals are all of the same age takes time, the agency told New Scientist. “These tests take more than a few days to run.”