The Environmental Protection Agency told oil giant BP on Wednesday night that it has 24 hours to choose less toxic dispersants to apply to the Gulf oil spill, according to The Washington Post. The EPA's decision, expected to be announced later today, is a change of course for the agency, which had previously placed BP's dispersant on its approved list, and had also approved BP's underwater application of the dispersant as recently as last week.

On Tuesday, EPA administrator Lisa Jackson told a Senate panel that the agency is "working with manufacturers, with BP and with others to get less toxic dispersants to the response site as quickly as possible," indicating some some level of concern about the toxicity of the particular dispersants BP was using. According to the Post, the agency has asked BP to switch to a new dispersant within 72 hours of choosing alternatives.

As we've pointed out, the two dispersants BP chose to apply to the oil spill are from a line of dispersants called Corexit, and both were banned from use on oil spills in the U.K. more than a decade ago. The Corexit products had failed a U.K. toxicity test. The exact makeup of the dispersants is kept secret because according to the EPA, "the manufacturer has chosen to keep this information proprietary," and the agency is "obligated by law to protect this information."

Given that the Corexit dispersants were on the EPA's list of pre-approved products, they were fair game for BP to choose, even though they seem to be more toxic and less effective on south Louisiana crude than other dispersants on the list.

A version of Corexit, as we first reported last month, was used after the Exxon Valdez spill in 1989, and was later linked with human health problems, including respiratory, nervous system, liver, kidney and blood disorders. One of the two Corexit dispersants BP is using in the Gulf also contains a compound that, at high doses, is associated with headaches, vomiting and reproductive problems.

With at least 655,000 gallons of the chemicals already in the Gulf, it is the largest application of dispersants in the history of U.S. oil spills, according to the EPA.

What's more, the EPA and the Coast Guard had allowed BP to spray the Corexit dispersants underwater, near the source of the spill–a method that has never been used and is not the recommendedapplication of the Corexit products, according to the EPA's website. Independent scientists recently discovered giant plumes of dispersed oil forming in the deep waters of the Gulf and told The New York Times they suspected the undersea application of dispersants were a possible cause. (A handy FAQ from the EPA says there's "no information currently available" connecting dispersants to the giant plumes.)

We've put in calls to the EPA for a response and will update the post when we get it.