At yesterday’s hearing on the BP Disaster, Peter DeFazio and Jerrold Nadler hammered BP America President Lamar McKay on the relative toxicity and efficacy of the dispersant Corexit as compared to some other dispersants. They pointed out that Corexit is one of the most toxic of the approved dispersants and is not as effective as others. Here’s a chart of the relative toxicity and efficacy from the EPA (click to enlarge).

In addition, on Monday, Edward Markey wrote EPA Administration Lisa Jackson asking why BP was using Corexit rather than a less toxic dispersant. Among other questions Markey asked were:

It is my understanding that the main dispersants applied so far are from a product line called Corexit, some of which had their approval rescinded in Britain more than a decade ago, because laboratory tests found them harmful to sea life that inhabits rocky shores.

a. How did EPA ensure that this dispersant’s toxicity to aquatic life was evaluated?

b. Was its toxicity to mollusks and other sea life that inhabit the Gulf of Mexico evaluated, and if so, what were the results? If not, why not?

c. If EPA relied on toxicity studies for coastal morphologies different from that of the Gulf Coast, what was done to evaluate the applicability of those studies for the use of the dispersants in the Gulf of Mexico environment?

d. Was the toxicity to other subsurface aquatic life evaluated? If so, please provide details, and if not, why not?

Late yesterday, the EPA informed BP it’s going to have to switch to another, less toxic, dispersant within three days.

The Environmental Protection Agency informed BP officials late Wednesday that the company has 24 hours to choose a less toxic form of chemical dispersants to break up its oil spill in the Gulf of Mexico, according to government sources familiar with the decision, and must apply the new form of dispersants within 72 hours of submitting the list of alternatives.

The move is significant, because it suggests federal officials are now concerned that the unprecedented use of chemical dispersants could pose a significant threat to the Gulf of Mexico’s marine life. BP has been using two forms of dispersants, Corexit 9500A and Corexit 9527A, and so far has applied 600,000 gallons on the surface and 55,000 underwater.

I guess all these hearings aren’t entirely a waste of time.

(Updated with efficacy table.)

Update: Here’s EPA’s order to BP to use a less toxic dispersant. And here’s some data from the dispersant monitoring.

Update: According to Nadler’s office, the maker of Dispersit got an order from BP for 60,000 gallons today.