BP PLC continues to stockpile and deploy oil-dispersing chemicals manufactured by a company with which it shares close ties, even though other U.S. EPA-approved alternatives have been shown to be far less toxic and, in some cases, nearly twice as effective.

After the Deepwater Horizon rig exploded and a deepwater well began gushing crude in the Gulf of Mexico three weeks ago, BP quickly marshaled a third of the world's available supply of dispersants, chemicals that break surface oil slicks into microscopic droplets that can sink into the sea.

But the benefits of keeping some oil out of beaches and wetlands carry uncertain costs. Scientists warn that the dispersed oil, as well as the dispersants themselves, might cause long-term harm to marine life

So far, BP has told federal agencies that it has applied more than 400,000 gallons of a dispersant sold under the trade name Corexit and manufactured by Nalco Co., whose current leadership includes executives from BP and Exxon. And another 805,000 gallons of Corexit are on order, the company said, with the possibility that hundreds of thousands of more gallons may be needed if the well continues spewing oil for weeks or months.

But according to EPA data, Corexit ranks far above dispersants made by competitors in toxicity and far below them in effectiveness in handling southern Louisiana crude.

Of 18 dispersants whose use EPA has approved, 12 were found to be more effective on southern Louisiana crude than Corexit, EPA data show. Two of the 12 were found to be 100 percent effective on Gulf of Mexico crude, while the two Corexit products rated 56 percent and 63 percent effective, respectively. The toxicity of the 12 was shown to be either comparable to the Corexit line or, in some cases, 10 or 20 times less, according to EPA.

EPA has not taken a stance on whether one dispersant should be used over another, leaving that up to BP. All the company is required to do is to choose an EPA-approved chemical, EPA Administrator Lisa Jackson told reporters yesterday during a conference call aimed at addressing questions about dispersants being used in efforts to contain the Gulf spill.

"Our regular responsibilities say, if it's on the list and they want to use it, then they are preauthorized to do so," Jackson said.

One explanation for BP's reliance on Nalco's Corexit, which its competitors say dominates the niche market for dispersants because of its industry ties, was its availability in large quantities at the time of the Gulf spill.

"Obviously, logistics and stockpiles and the ability for the responsible party to pull the materials together," Jackson said. "I'm sure that has a lot to do with the ones that they choose."

Nonetheless, experts question BP's sustained commitment to Corexit, given apparently superior alternatives.

"Why wouldn't you go for the lesser toxic formulation?" said Carys Mitchelmore, an assistant professor of environmental chemistry and toxicology at the University of Maryland's Center for Environmental Science. Mitchelmore testified on Capitol Hill this week about dispersants and co-authored a 2005 National Academy of Sciences report on the chemicals.

BP spokesman Jon Pack defended the use of Corexit, which he said was decided in consultation with EPA. He called Corexit "pretty effective" and said the product had been "rigorously tested."

"I'm not sure about the others," Pack said. "This has been used by a number of major companies as an effective, low-toxicity dispersant."

BP is not considering or testing other dispersants because the company's attention is focused on plugging the leak and otherwise containing the spill, Pack said.

"That has to be our primary focus right now," he said.

Nalco spokesman Charlie Pajor said the decision on what to use was out of his company's hands. He also declined to comment on EPA comparison tests, saying only that lab conditions cannot necessarily replicate those in the field. "The decision about what's used is made by others -- not by us," he said.

Nalco's connections

Critics say Nalco, which formed a joint venture company with Exxon Chemical in 1994, boasts oil-industry insiders on its board of directors and among its executives, including an 11-year board member at BP and a top Exxon executive who spent 43 years with the oil giant.

"It's a chemical that the oil industry makes to sell to itself, basically," said Richard Charter, a senior policy adviser for Defenders of Wildlife.

The older of the two Corexit products that BP has used in the Gulf spill, Corexit 9527, was also sprayed in 1989 on the 11-million-gallon slick created by the Exxon Valdez grounding in Alaska's Prince William Sound.

Cleanup workers suffered health problems afterward, including blood in their urine and assorted kidney and liver disorders. Some health problems were blamed on the chemical 2-butoxyethanol, an ingredient discontinued in the latest version of Corexit, Corexit 9500, whose production Nalco officials say has been ramped up in response to the Gulf of Mexico disaster.

Among Corexit's competitors, a product called Dispersit far outpaced Corexit 9500, EPA test results show, rating nearly twice as effective and between half and a third as toxic, based on two tests performed on fish and shrimp.

Bruce Gebhardt, president of the company that manufactures Dispersit, U.S. Polychemical Corp., said BP asked for samples of his company's product two weeks ago. Later, he said, BP officials told him that EPA had wanted to ensure they had "crossed all their T's and dotted all their I's" before moving forward.

Gebhardt says he could make 60,000 gallons a day of Dispersit to meet the needs of spill-containment efforts. Dispersit was formulated to outperform Corexit and got EPA approval 10 years ago, he said, but the dispersant has failed to grab market share from its larger rival.

"When we came out with a safer product, we thought people would jump on board," he said. "That's not the case. We were never able to move anyone of any size off the Corexit product."

He added, "We're just up against a giant."