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By Deborah Blum
The standard toxicity test for chemical compounds is called the LD50. LD stands for Lethal Dose and 50 indicates 50 percent. In other words, LD50 means the lowest dose at which a material kills half of the test subjects.
The results are usually given in milligrams of compound per kilograms of body weight. Many of these tests are conducted on laboratory rats. To give you a few rat results: the LD50 of table sugar (sucrose) is 29,700 mg/kg. For table salt (sodium chloride, NaCl) it's 3,000 mg/kg. Really poisonous substances, though, measure in the single digits: Sodium cyanide (NaCN), for instance, possesses an LD50 score of 6.4 mg/kg.
Basically, the lower the number, the deadlier the compound. Poisons in water and air are usually measured in lethal concentration rather than dose - in other words an LC50. Which got me wondering about the oil pouring into the Gulf of Mexico from BP's shattered oil rig. Not to mention the chemical dispersants being used in attempt to break down the spreading oil. What kind of lethal concentration might be building up in those waters?
The U.S. Environmental Protection Agency data on dispersants provide the LC50 in parts per million. Of course, these tests aren't done on rats but sea creatures, in this case Menidia, a small silvery fish that likes to hover near the water's edge and Mysidopsis, a tiny brine shrimp.
As has been earlier reported, Corexit, the compound chosen by BP, has some of the lowest LC50 numbers on the list, meaning that it's among the most poisonous. Also, it's among the least effective on Louisiana crude (the type flowing from the Deepwater break). Why the EPA went along with this choice remains a mystery to me - or maybe I just think the answer would depress me - but under public pressure the agency has now ordered BP to find an immediate alternative.
Nearly 700,000 gallons of Corexit have already been poured into gulf waters. But that pales, obviously, beside the amount of Louisana crude, now estimated at a minimum of 6 million gallons. So, I wondered, what is the LC50 of Louisiana crude on small salt water dwellers?
Of course, I realize, that comparing lethal concentrations is not straightforward. The results differ by species and by time as well as by amount of poison, The EPA numbers for Corexit 9500 (the formula used most heavily by BP) show that at 2.62 ppm, the dispersant kills half the silver fish in 96 hours/ four days. At a slightly higher concentration - 3.4 ppm - the compound kills half the little shrimp in two days.
As for crude oils, a very decent analysis by the American Petroleum Institute shows that all are toxic, but their effects vary with thickness and with the different chemistry seen in say, oil from the Gulf of Mexico and oil from Kuwait. The best estimate I've seen for South Louisiana Crude - after hours of exasperated research - comes from thesis work done at Louisiana State University several years ago. For instance, the study found that Louisiana crude had an LC50 of 4250 ppm for the warm-water loving killifish.
This suggests that crude oil is less acutely poisonous than chemical dispersants. But here's the really interesting finding in that terrific little study. Adding a dispersant - specifically Corexit 9500 - made the oil more poisonous. A lot more poisonous.
The "dispersed" oil had an LC50 of 317.7 ppm, making it more than 11 times more lethal in its effects. The study found a similar worsening for white shrimp, although not quite as dramatic. "Dispersed oils were more toxic than crude oils," noted the report.
Oh, definitely. Still, you might argue that this is only a master's thesis conclusion. But as it turns out there are plenty of other studies raising very similar warnings and they go back quite a ways. A report in the journal Environmental Toxicology a decade ago concluded that "LC50 values indicate that dispersed oil combinations were significantly more toxic to these organisms than .. crude oil." Another study, this time of snails and amphipods reached exactly the same conclusion.
To be fair, a study of the Australian octopus found no increased toxicity. But don't you wonder what we're doing out there in the fragile environment of the Gulf, whether we're reducing the spill damage or just turning the whole area into one ever-more poisonous bowl of toxic soup?
And don't you wish our officials gave any indication that they knew more about it than we do? I love doing this kind of research but in this case I'd much rather have our country's so-called regulators waving the LC50 red flag ahead of me.
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