The Environmental Protection Agency has directed oil giant BP to use a less toxic form of chemical dispersants to break up the oil spill in the Gulf of Mexico.

White House press secretary Robert Gibbs said the Obama administration wants BP to use the least toxic dispersants available.

"Our feeling and EPA's feeling is, given the extent to which we have to continue to use them, to use the least toxic of them makes the most sense," Gibbs said Thursday.

Gibbs said he was not aware of any test results or other data that caused EPA to seek a different dispersant. Under the order, which BP received Wednesday night, the company has 24 hours to identify at least one approved dispersant product that is effective, available in large quantities and meets specified toxicity limits.

The notice says BP must begin using only the approved alternative within 72 hours of submitting its list of alternatives to the EPA and getting EPA approval.

The order comes just days after EPA Administrator Lisa Jackson publicly described use of dispersants as a trade-off between health risks and limiting further environmental damage.

Louisiana Gov. Bobby Jindal, members of Congress and environmental groups have raised questions about use of the dispersants, which shoot chemicals thousands of feet beneath the sea. The chemicals break apart the oil and keep it from reaching the surface.

One of the chief agents being used, called Corexit 9500, is identified as a "moderate" human health hazard that can cause eye, skin or respiratory irritation with prolonged exposure, according to safety data documents.

A spokeswoman for the EPA declined immediate comment.

A spokesman for BP said Thursday the company is only using established dispersant products that are preapproved for use in the Gulf of Mexico.

One of the criteria for selecting Corexit was the manufacturer's ability to supply the large volumes needed to deal with the response to the massive spill, said BP spokesman Mark Salt

Corexit "is very effective in causing oil to form into small isolated droplets that remain suspended until they are either eaten by naturally occurring microbes, evaporate or are picked up or dissolved," Salt said.

Jackson used less glowing language, calling the dispersant "the lesser of two environmental outcomes no one wants to have to deal with. But we also need to be able to answer questions about what's out there and what's available for use," she said Tuesday after a Senate environment committee hearing.

In her testimony, Jackson said the long-term effects of the dispersants on aquatic life are still unknown, and said the EPA would work to ensure that "the dispersants that are used are as non-toxic as possible."

The agency has been working with manufacturers, BP and with others to get less toxic dispersants to the response site as quickly as possible, Jackson said.

Rep. Edward Markey, D-Mass., chairman of a House global warming committee, praised Jackson for acting swiftly to address concerns that the dispersant BP chose to use is more toxic than other available chemicals.

"The effect of long-term use of dispersants on the marine ecosystem has not been extensively studied, and we need to act with the utmost of caution," Markey said.

Earlier this week, Markey sent a letter to Jackson raising questions about the potential toxicity of Corexit, and whether the chemical agent could be contributing to reports of large undersea "plumes" of oil suspended thousands of feet below the water's surface.