Crude Comments

The latest news from the Gulf of Mexico offers both relief (the "top kill" approach to ending the oil spill may be working) and dismay (the amount of oil pouring into the water is now thought to be closer to 20,000 barrels a day rather than the 5,000 barrels that BP has insisted on for weeks.)

In other words - at worst case - the U.S. Geological Survey estimates that the spill amount may be closer to 39 million gallons of oil so far, rather than the 11 million previously suspected. Now, I've spent the last week or so focusing on the chemical dispersants used to break down the oil, partly because they're a lot more poisonous than crude oil and partly because as they spread through the oil, they make it more poisonous too.

But given the ratio - about a million gallons of dispersant to that 39 million or so figure - it makes sense to me to spend a moment also obsessing about the chemistry of crude oil. Oil spills wreak most havoc, as we know, by simply enveloping living things - from grass to birds to insects to fish - rolling out a dense, greasy blanket that suffocates, immobilizes, and kills.

But - and this is not news - oil is also just plain poisonous without any help from dispersants at all. This is why you don't find people generally lapping up gasoline down at the corner station or setting up petroleum bars where people can grab a quick shot of Sweet Louisiana Crude. So what's the chemical recipe that makes them so hazardous?

As the American Petroleum Institute points out, all crude oils are slightly chemically different, according to where they are extracted. Emphasis on the word slightly. The average crude oil, according to API, is about 84 percent carbon and 14 percent hydrogen - in other words, no surprise again - a hydrocarbon fuel. If the remaining two percent contains sulfur - which mixes with hydrogen to form the toxic, famously smelly hydrogen sulfide gas - the crude is considered "sour". A sulfur-free crude, like that from Louisiana, then is tagged as sweet. Other trace elements typically include nitrogen, oxygen, and a sprinkling of minerals, ranging from arsenic to vanadium.

And, yes, arsenic is poisonous but here the amount is usually not large enough to be alarming. The toxicity issues really arise from the way that those atoms of carbon and hydrogen bond together, combining into an assortment of materials long linked to health problems. Petroleum experts actually have an acronym for four famously troublesome compounds in crude oil: BTEX. This stands for benzene, toluene, ethylbenzene and xylenes.

Benzene is probably the best known of these compounds, because it's been flagged as a human carcinogen for a couple decades. I've always rather admired the elegant structure of a benzene molecule, which is a beautifully arranged ring of six carbon atoms and six hydrogen atoms:


But while the benzene ring has an elegant structure, the compound itself is considered so dangerous that the U.S. Environmental Protection Agency has set the maximum permissible level in drinking water at .005 mg/l. The problem with benzene is that it directly damages bone marrow, making it suspect in environmentally induced leukemias. We didn't learn this from studying oil spills, of course, but from occupational exposures to benzene in factories that process petroleum products, from tobacco smoke, which also contains benzene, and from studies at gas stations.

The second compound flagged by BTEX is toluene, which is sometimes refered to as methylbenzene, because it consists of a benzene ring with some additional carbon and hydrogen atoms attached to it (known as a methyl group).

It looks like this:


Is toluene also poisonous? Well, if I tell you that it is used as an industrial solvent in the making of everything from paint thinners to dynamite does that hint at an answer? Like all the BTEX compounds, toluene is a "volatile aromatic hydrocarbon" which basically means that it evaporates easily when heated and mixes poorly with water, tending to float on top. Because these are not readily water soluble, the body has a difficult time washing them away once they enter cells and toluene appears particularly toxic to nervous system cells.

As for the other two, ethylbenzene and xylene are also built around benzene rings and like toluene have additional clusters of carbon and hydrogen attached to the basic ring. And, yes, they are also poisonous. Oh, and remember, that these compounds are volatile, meaning that they evaporate easily? This is great for breaking down the oil slick. But it also means that as the sun heats an oil slick, the rising vapors are rich with BTEX compounds, which can be both inhaled and, as they settle on skin, absorbed there.

These are not the only toxic compounds in crude oil but they make enough of a point for me. Far out in the Gulf, we can be somewhat reassured that dilution works wonderfully in the favor of aquatic creatures living there. By some estimates, even at worse case, the mixing bowl of the Gulf contains 5 billion drops of water for every one drop of spilled oil.

But once the oil washes ashore, we have an environment awash in benzene, toluene, ethylbenzene and xylene. And that's why biologists worry when oil-soaked birds groom their feathers and swallow the oil. And that's why health officials worry about clean-up workers and Gulf coast residents breathing tainted air and touching oil-soaked materials.

And that's why there's still no one lining up at the BTEX bar for just a quick drink of even the sweetest crude on earth.


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This whole situation exemplifies the admitted "cozy relationship" between government and big oil. Or, to put it more bluntly, government FOR big oil.
1. The day BP was finally forced to release a video clip, a Purdue prof calculated the leak to be 70,000 barrels a day, +/- 20%. BP and the Coast Guard both stuck with the "there is no way of knowing" strategy. Do they think we are all idiots?
2. The EPA directs BP to use less toxic dispersants. BP says no, they don't have access to a large enough quantity. I think I'll try telling the IRS I don't have enough money and see how that works.
3. I have read reports of police blockades at the entrance to some of Louisiana's barrier islands. Guess who ordered the blockade? You guessed it-BP. Pictures are bad PR.
-What happened to "We the People" and respect for science?

I agree completely about the cozy relationship. The stories about the so-called Minerals Management Services, or as Andy Revkin of the NYT now calls it, the Minerals Mismanagement Service have been appalling - right down to the fact that they let oil company employees fill out the safety inspection reports in pencil and then just traced over them in pen. Let's hope we learn something from it - because we'll be paying for it for a long time.

"But while the benzene ring has an elegant structure, the compound itself is so poisonous that the U.S. Environmental Protection Agency has set the maximum permissible level in drinking water at .005 mg/l."

Benzene is not that toxic. Its LD50 is more than 1000mg/kg and it's biodegradated quite rapidly.

Other hydrocarbons are as much problem - they are more chemically inert and more persistent, causing damage not only chemically, but physically by wetting oiled surfaces and forming a film.

By Alex Besogonov (not verified) on 27 May 2010 #permalink

You're absolutely right about benzene in terms of acute toxicity. But the reason the EPA sets such a low tolerance is because it's considered such a strong carcinogen. (Still dangerous/hazardous might be a better term in this case than poisonous, which implies acute toxicity, and I think i'll make a change there. Agreed, again, that the major harm caused by oil spills is the fact that they form a physical film - i called it a kind of greasy blanket in the post but film is probably a more precise image. But when we find clean up workers hospitalized, as was reported today, it's far more likely to be related to BTEX vapors - airborne toluene, for instance, is very likely to cause severe nausea and headaches, some of the symptoms reported.

It's not really accurate to equate use "as an industrial solvent in the making of everything from paint thinners to dynamite" with toxicity - after all, water (amongst many other things) is used in all of these processes!

And, of course, one of my chemistry professors liked to remind us that water is a "universal solvent." I think of that sometimes - revealing my total geek nature - when I'm soaking dishes and watching the water soften things up. But toluene is a different class of solvent - dissolving lacquers, for instance - and it was that kind of chemistry I was thinking of when I described it as a versatile industrial solvent.

Betsy, Don't you think this is a problem created under the previous administration? The current administration has just shut down the offshore leases that had been approved by this agency SINCE the rupture of this wellhead. That agency was proceeding according to Bush/Cheney mandates to keep enriching the rich, and da*n the rest of us. When the President spoke on it, he emphasized our need for alternate fuel sources, and I cannot help but think that nothing short of a nuclear disaster (foreign or domestic) is as world-threatening as this "accident." Are you willing to reduce your carbon footprint? Or do you absolutely NEED an SUV?

Of course this problem evolved during prior administrations. (plural) It takes much longer than one year for any agency to fall to such ruin with or without a catalyst! I also realize how we must all make sacrifices to reduce our carbon footprint. We have extracted nonrenewable resources for the sake of comfort and ease of mobility for too long. Eisenhower with his interstate system and suburban sprawl are some of the contributing factors. Change will not come easy. I am all for imposing a sizable tax on gasoline to support the development of alternative fuel sources. We have the technology! I live 35 miles from where I work, so maybe I will have to move closer. You don't see as much suburban sprawl in Europe and gas prices are substantially higher. Not many SUVs in Europe either! Americans are the "spoiled brats" of the Earth's family. I do fault our leadership for leading us down this reckless path with wanton abandon. Lip service is very ineffective.

One more thought. When Iraq set all those oil wells on fire, I remember thinking "Let them burn- then we'll HAVE to switch to alternative energies!" Necessity is the mother of invention. That was erroneous thinking. We need fossil fuels for plastics, drug research, and many other uses other than gasoline. We need to preserve these resources for future generations. Can we do it?

Please don't suggest burning the crude, products of combustion ARE acutely toxic! The carbon particles of smoke stay in your body and cause slow death later in life too.

great post

i want you always make new articles like this.I hope you will read my this comment.When Iraq set all those oil wells on fire, I remember thinking "Let them burn- then we'll HAVE to switch to alternative energies!" Necessity is the mother of invention. That was erroneous thinking. We need fossil fuels for plastics, drug research, and many other uses other than gasoline.
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