Corexit EC9500A and Corexit EC9527A were the principal variants.[136] The two formulations are neither the least toxic, nor the most effective, among EPA's approved dispersants, but BP said it chose to use Corexit because it was available the week of the rig explosion.[137][138] On 19 May, the EPA gave BP 24 hours to choose less toxic alternatives to Corexit from the National Contingency Plan Product Schedule, and begin applying them within 72 hours of EPA approval or provide a detailed reasoning why no approved products met the standards.[139][140] On 20 May, BP determined that none of the alternative products met all three criteria of availability, non-toxicity and effectiveness.[141] On 24 May, EPA Administrator Lisa P. Jackson ordered EPA to conduct its own evaluation of alternatives and ordered BP to reduce dispersant use by 75%.[142][143][144] BP reduced Corexit use by 25,689 to 23,250 US gal (97,240 to 88,010 L) per day, a 9% decline.[145] On 2 August 2010, the EPA said dispersants did no more harm to the environment than the oil and that they stopped a large amount of oil from reaching the coast by breaking it down faster.[132] However, some independent scientists and EPA's own experts continue to voice concerns about the approach.[146]