Federal environmental regulators on Thursday ordered BP to switch to less-toxic dispersants to break up oil gushing into the Gulf of Mexico, amid fears that the chemical now being sprayed over the sea and injected deep underwater could harm marine life. Under the directive from the Environmental Protection Agency, BP has 24 hours to identify a less-toxic and potentially more effective option -- and must start using it within three days. Although the version BP has been using -- two formulations of the brand Corexit -- is on EPA's list of approved dispersants, there are 16 others, many of which fared better in toxicity and effectiveness tests. 'BP is using this dispersant in unprecedented volumes and, last week, began using it underwater at the source of the leak -- a procedure that has never been tried before,' the EPA said in a statement. 'Because of its use in unprecedented volumes and because much is unknown about the underwater use of dispersants, EPA wants to ensure BP is using the least toxic product authorized for use.' The dispersants are detergent-like substances designed to help break down oil into tiny droplets that can be consumed by naturally occurring microbes. The EPA initially allowed BP to spray the dispersant over oil slicks on the surface of the Gulf -- during good weather and water conditions. On May 15, the agency allowed BP to inject the chemicals deep underwater, directly at the site of the gushing oil. At the time, EPA Administrator Lisa Jackson described the decision to allow the unprecedented technique as a trade-off between the harm of allowing oil to accumulate and the possible damage to marine life from the detergent-like substance. In the month since the April 20 explosion of the Deepwater Horizon rig, an estimated 655,000 gallons of the Corexit oil dispersant have been sprayed on the sea or at the leak -- with an estimated 55,000 gallons being delivered underwater. Corexit is made by the Sugar Land, Texas-based Energy Services Division of Nalco Co. According to test results logged with the EPA, at least 14 alternative dispersants were less toxic in fish, and seven were less toxic in shrimp. The move was cheered by environmentalists and lawmakers worried that the substance could be stored in the organs and tissues of marine life. Rep. Ed Markey, D-Mass., speculated that the deep-sea delivery of Corexit was contributing to the possible development of large underwater oil plumes. Markey said EPA's directive addressed 'concerns that the dispersant BP chose to use is more toxic than other available formulations.' 'The effect of long-term use of dispersants on the marine ecosystem has not been extensively studied,' Markey added, 'and we need to act with the utmost caution.' BP's dispersant of choice was a hot topic during a House Transportation and Infrastructure hearing Wednesday, as lawmakers repeatedly challenged the company's decision to go with Corexit. Several questioned whether corporate ties between BP and Nalco prompted the choice. Nalco's board of directors includes Daniel Sanders, the former president of ExxonMobil, and Rodney Chase, a nearly four-decade veteran of BP. But Nalco representatives noted that the estimated $40 million in Corexit sales is only a small portion -- about 1 percent -- of the company's expected 2010 revenues. Under EPA's previous orders, BP has been monitoring dispersant use in the Gulf. According to the EPA, 'toxicity data does not indicate any significant affects on aquatic life' and the smaller size of oil droplets 'is a good indication that, so far, the dispersant is effective.' 'We are gratified that the EPA has acknowledged that the use of Nalco's dispersants has been effective and has had no undue impact on the marine environment,' Nalco said in a statement Thursday. Under the EPA order, BP has to search for alternative dispersants -- but can alert federal regulators if it is not 'able to identify alternative dispersant products' that are readily available. BP is able to consider what stockpiles are available and how much time it would begin to manufacture and ship a replacement dispersant. (Jennifer A. Dlouhy can be reached at 202-263-6400 or at the e-mail address jdlouhy(a)hearstdc.com) 