WASHINGTON - BP Plc is making oil leaking into the Gulf of Mexico more toxic to marine life by mixing it with Nalco Holding Co.'s dispersants, an environmental group said.

A plane sprays dispersant over the oil leaked from the Deepwater Horizon wellhead in the Gulf of Mexico. More than 1.2 billion gallons of dispersants have been sprayed into the Gulf, according to the Deepwater Horizon Unified Command linking companies and government agencies responding to the spill.(Reuters) Oil and Nalco's Corexit dispersants combined have higher toxicity than either substance alone, Richard Denison, a senior scientist at the Environmental Defense Fund, said today by telephone. Denison yesterday posted on his website an analysis based on technical product bulletins that Nalco provided to the U.S. Environmental Protection Agency.

"Dispersed oil is more toxic than undispersed oil," Denison wrote in his posting. "The dispersant maker's own test data demonstrate that the combination of oil plus dispersant is quite a bit more toxic than the dispersant alone and -- even more significant -- the combination is more acutely toxic than the oil by itself."

More than 1.2 billion gallons of dispersants have been sprayed into the Gulf, according to the Deepwater Horizon Unified Command linking companies and government agencies responding to the spill. BP has applied Corexit from planes and from the seafloor to help keep the worst oil spill in U.S. history from damaging wetland habitats.

According to Denison, 2.61 parts per million of a mixture containing 1 part Corexit EC9500A and 10 parts oil killed half the silverside fish in a 96-hour study cited in a Nalco bulletin. Oil alone requires a higher concentration -- 10.72 parts per million -- to achieve the same toxicity, and Corexit requires 25.2 ppm, Denison said, citing the bulletin.

Corexit reduces total environmental damage, said Charlie Pajor, a spokesman for Naperville-based Nalco who didn't dispute Denison's findings.

"It's more toxic to marine life, but less toxic to life along the shore and animals at the surface because the oil is not at the surface," Pajor said. "It's generally less environmentally harmful than allowing the oil to migrate to the surface."

Nalco rose 44 cents, or 2 percent, to $22.50 at 12:41 p.m. in New York Stock Exchange composite trading. The shares declined 14 percent this year through yesterday.

Based on the total used so far, the dispersant-oil mixture is capable of killing half the fish across 4 square miles down to the ocean floor, Denison said. If diluted 10-fold, the oil-dispersant mix would kill 5 percent of fish across 40 square miles, he said.

Nalco's dispersants are approved by the U.S. Environmental Protection Agency, said Tristan Vanhegan, a BP spokesman. He declined to comment specifically on the Environmental Defense report because he said he hadn't seen it.

"We continue to use Corexit while looking for alternative products," Vanhegan said. "The EPA has asked us to look for less toxic products."

Nalco Chief Executive Officer J. Erik Fyrwald met with members of Congress on May 26 to assure them that Corexit is safe after the EPA demanded BP limit its use.