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.