The dispersant used after the Gulf¹ of Mexico oil spill is no more toxic² than oil alone, the US Environmental Protection Agency (EPA) has said.

美国环保局称，处理墨西哥湾泄漏原油所使用的分散剂的毒性不会比石油大。

The EPA study follows concerns raised by congressional investigators³ that dispersant may have been more widely used than the government ordered.

Meanwhile a new US government estimate suggests the well leaked 4.9m barrels of oil before being capped last month.

The estimate would make it the largest accidental oil spill（漏油，浮油） in world history.

According by the new estimate prepared by the government's Flow Rate Technical Group, BP was able to collect roughly 800,000 barrels of oil from the well in total.

The well initially⁴ leaked about 62,000 barrels of oil per day, higher than any previous estimate of the flow. But as the well exhausted⁵ itself, the flow slowed to about 53,000 barrels per day.

The spill totaled 205.8 million gallons.

The flow ended 15 July, when BP put a temporary cap on the well. BP hopes to begin its attempt to seal the well permanently⁶ with a "static kill" procedure on Tuesday.

If the new estimates are correct, the spill would be great than the 1979 Ixtoc I leak in the Gulf of Mexico, which gushed⁷ 3.3 million barrels. Only the intentional⁸ release of an estimated eight million barrels of oil into the Gulf by Iraqi troops during the Gulf War in 1991 was greater.

Environmental trade-offs

BP has been using dispersant（分散剂） to break up the spilled oil, with the substance acting⁹ in a similar way to washing-up liquid.

But amid concerns over dispersants' long-term environmental impact, the Obama administration in May issued an order limiting their use.

The study by the Environmental Protection Agency, released on Monday, showed the dispersant widely used in response to the oil spill in the Gulf, called Corexit 9500A, was "generally no more or less toxic than the other available alternatives".

"The results indicate that the dispersant-oil mixtures are generally no more toxic to test species than oil alone," Paul Anastas, the EPA's assistant administrator¹⁰ for research and development, told reporters.

In a statement, EPA chief Lisa Jackson said: "We have said all along that the use of dispersant presents environmental trade-offs, which is why we took steps to ensure other response efforts were prioritised（优先） above dispersant use and to dramatically cut dispersant use."

The agency also said the use of dispersants had nearly halted since the well was capped on 15 July.

On Sunday, the Washington Post reported that the US Coast Guard had granted BP 74 requests to use dispersants over 54 days on the Gulf surface and underwater, in spite of an administration order to limit their use.

The paper found that the Coast Guard had granted every request to use the chemicals, although in some cases it had reduced the amount that could be deployed¹¹.

In a letter released on Saturday, Democratic Congressman¹² Edward Markey, chairman of the House energy and environment sub-committee, wrote that instead of complying with the EPA's instructions, "BP often carpet-bombed the ocean with these chemicals and the Coast Guard allowed them to do it".

Last week, BP reported a record $17bn (£11bn) loss, having set aside $32bn to cover the costs of the spill - the worst in US history.