The chief executive of BP says crews have been able to reduce the amount of oil reaching the Gulf’s surface from a massive underwater leak by using chemicals at the gusher’s source — but environmental advocates are raising questions about the plan’s safety.

“We are adamantly opposed to dispersants being used at the well-head as we believe it adds more toxins and less value to the clean up process,” says Mobile Baykeeper Casi Callaway. “Certain dispersants may be useful at the shore/grassbed line, but we can’t endorse this action until we know what specific dispersants are to be used!”

Speaking this morning on NBC’s “Today” show, BP CEO Tony Hayward said BP is injecting dispersant chemicals into the oil as it pours out of the underwater well. He said it was a new approach that appeared to be having an impact on the amount of oil reaching the surface slick.

It’s still unknown exactly what compounds are being deployed. Many of the dispersants used in the oil industry are proprietary chemicals whose exact composition is unknown.

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Last week ProPublica reported that the dispersants contain potentially harmful toxins:

“There is a chemical toxicity to the dispersant compound that in many ways is worse than oil,” said Richard Charter, a foremost expert on marine biology and oil spills who is a senior policy advisor for Marine Programs for Defenders of Wildlife and is chairman of the Gulf of the Farallones National Marine Sanctuary Advisory Council. “It’s a trade off — you’re damned if you do damned if you don’t — of trying to minimize the damage coming to shore, but in so doing you may be more seriously damaging the ecosystem offshore.”

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(This story originally appeared at Facing South.)