EPA data show dispersants plus oil are more toxic than either alone

Richard Denison, Ph.D., is a Senior Scientist.

In an earlier post, I noted in haste some apparent discrepancies between EPA and BP acute toxicity data on the Corexit® dispersants.  Little did I realize that the data mixup was actually telling me something much more significant:  that the dispersant maker’s own test data demonstrate that the combination of oil plus dispersant is quite a bit more toxic than the dispersant alone and – even more significant – the combination is more acutely toxic than the oil by itself.

Let me repeat that:  The data indicate that dispersed oil is more toxic than undispersed oil.  EPA has posted the dispersant manufacturer Nalco’s “Technical Product Bulletins” for each of the dispersants that have been used in the Gulf:  Corexit® EC9527A and Corexit® EC9500A.

Section VII of each of the bulletins shows the toxicity data for a) dispersant alone, b) the reference oil used in the test, No. 2 fuel oil, and c) a mixture of dispersant and test oil at a 1:10 ratio.  Here are the data (remember, the lower the value, the more toxic the substance):

    LC50* values in parts per million (ppm)
    Menidia (inland silverside fish)
96-hour test
(mysid shrimp)
48-hour test
Corexit® EC9500A Dispersant alone 25.20 14.57
No. 2 fuel oil alone 10.72 16.12
Dispersant : oil
(1 : 10)
2.61 3.40
Corexit® EC9527A Dispersant alone 32.23 24.14
No. 2 fuel oil alone 10.72 16.12
Dispersant : oil
(1 : 10)
4.49 6.60
*LC50 = concentration at which 50% of the test organisms are killed within the duration of the test.

To put these numbers in perspective, consider this calculation (thanks to Sayer Ji at Gulf Oil Spill Truth for starting me down this path of calculations):

Just 2.61 parts per million of a 1:10 mixture of Corexit® EC9500A and oil kills 50% of silverside fish within 96 hours, at least under the test conditions.

  • Assuming uniform mixing, that means that 1 gallon of that mixture added to 383,142 gallons of water would be sufficiently toxic to do the same.
  • So far BP has added at least 1.2 million gallons of pure Corexit® dispersants to the Gulf.  Assuming it’s mixed with 10 times as much oil, that’s enough dispersant to render equally toxic nearly 7,000,000 Olympic swimming pools worth of water.  (There are about 660,000 gallons in an Olympic-sized swimming pool, the dimensions of which are about 164 x 82 x 6 feet.)

Assuming a water depth of 5000 feet at the spill site, that much water would occupy a block that is 10,600 x 10,600 x 5,000 feet, or 4 square miles on the surface of the water extending all the way down to the ocean floor.  That’s the volume of water that would be sufficiently toxic to kill half of the fish residing in it within 4 days.  Diluting that another 10-fold (into the water underneath 40 square miles of the Gulf) would still kill 5% of the fish, and another 10-fold (400 square miles) would still kill 0.5% of the fish.

Now obviously these volumes are a tiny fraction of the volume of the Gulf, or even of the region of the Gulf where the dispersant have been sprayed and injected.

But that still adds up to a lot of extra toxicity to release into the Gulf, especially when you consider it could well be increasing, rather than decreasing the acute lethal toxicity of the oil.

And what about less lethal effects, e.g., killing only 1 of every 1000 fish … or killing more sensitive organisms … or effects from exposures longer than 96 hours … or sublethal effects, which can be expected to occur at much lower concentrations?

We know nothing about those possible effects because the testing for them hasn’t been done – unless you count this huge, uncontrolled experiment we’ve now embarked on.

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  1. justmeint
    Posted June 15, 2010 at 4:08 am | Permalink

    This entire toxic scenario is frightening….
    Did You Know?
    BP engineers alerted federal regulators at the Minerals Management Service that they were having difficulty controlling the Macondo well (Deepwater Horizon) six weeks before the disaster, according to e- mails released by the Energy and Commerce Committee.

    “I don’t think this would have happened on Exxon’s watch,” Tom Bower, author of “The Squeeze: Oil, Money and Greed in the 21st Century,” said in a June 11 Bloomberg Television interview. “They’d be much more careful and much more conscious of the need to supervise subcontractors.”

    WELL excuse me your sainted Exxon……. and Chevron and ConocoPhillips.

    Let’s just take a look at a few of your past misdemeanours, and then we can consider again – if the moratorium on deepwater drilling should be lifted, and place it all firmly back into your nice clean hands!


  2. Sarah Vogel
    Posted June 15, 2010 at 10:04 am | Permalink

    Interesting post. Thank you for crunching the numbers. This data is even more unsettling considering today’s front page story in the NYTimes that suggests a shift by the oil industry to dispersants as the economically preferable clean up technology despite recommendations by the Coast Guard to increase mechanical response equipment. Makes me wonder what were the evaluative criteria by which dispersants were selected as method of ‘clean up’? And the point your blog raises, could they be making the situation worse.

  3. Jen Sass
    Posted June 15, 2010 at 11:37 am | Permalink

    Wow, Richard. Thanks for putting this together! Shocking.

  4. Posted June 15, 2010 at 12:30 pm | Permalink

    This is very important and useful information and it bodes ill for the ecosystem of the Gulf of Mexico. It also really raises questions about the potential human health effects – to clean-up workers and even to coastal residents – from the dispersed oil. Of course, the potential human health effects of the dispersant-oil mixture have not been assessed, so we remain ignorant of whether this same toxicity-magnification might also apply to people exposed to these chemicals.

  5. Posted June 16, 2010 at 8:58 am | Permalink

    Toxicity considerations alone may not be sufficient in this case – the purpose of the oil dispersant should also be to make the oil better available for eventual degradation. Some naturally occurring microbes have apparently been shown to be able to degrade oil, but not if it exists as a thick layer on the water surface. This argument, of course, does not speak against the need to carefully choose the best (and least toxic) dispersant available, but the fact that the dispersant may help to more quickly degrade the oil could maybe compensate for some “additional toxicity” of the mixture.

  6. Posted June 20, 2010 at 12:49 pm | Permalink

    We have a dispersant choice! JD 2000 Why hasn’t it been used for the spill…we could have done much better…..read and weap for our gulf….corporate greed over American safety…. It is all political…….
    Corexit/Nalco/Exxon…….all “good old boy business”

  7. Posted July 2, 2010 at 1:31 pm | Permalink

    Well there are some frightening reports of people getting sick at the work sites. But also potentially of Corexit mixed with acid rain. On Coastal Areas. Because there is inadequate testing of the effects of mixing these items and how they a} change as they break down as a mixture, and b} if that causes either or both to be more likely to enter into our Water Cycle, that scares people. So now, potentially in addition to killing ecosystems on the Coast and sickening workers and residents on the coast and in the marshlands, some of these chemicals could be dispersed or their constituents could be dispersed in rain on the Agricultural areas on the Coast. Just really strong acid rain would be bad enough and we know that is coming due to the burning of Crude at the rig. But no one is sure if the Corexit and other chemicals in the water are going to amplify the effects of that rain or make it a new, direct health threat to people, crops, wildlife and livestock.

  8. Posted July 2, 2010 at 1:38 pm | Permalink

    Even if that is not possible- that dispersant can get into the water cycle that way, the people there are so frightened they need to be reassured that its not possible. And I have to say that the EPA has screwed up being so slow and sometimes recalcitrant to test or to take charge. They will not be trusted, as they are seen as in cahoots with BP. So for the sake of sanity for the locals, this will have to be addressed by an Independent Third Party. That includes testing and of course making reports available. http://www.youtube.com/watch?v=Yvh7JfaLXqQ&fehttp://www.youtube.com/watch?v=q9ORQQb9Vkc&fe
    I have also read unverified reports from people reporting similar phenomenon in the Carolinas. It might just be acid rain, but even then, its most likely due to BP's activities, burning millions of gallons of crude in the Gulf. Gulf Moisture is a major component in Southern and SouthWestern weather. Its what helps drive our tornado seasons in Oklahoma in the spring and fall, but it is also where the majority of our rain comes from all season long.