An arm of the federal Centers for Disease Control and Prevention loosened its mercury contamination standard yesterday, proclaiming it safe for people to consume three times as much mercury as the EPA advises. State health officials often use the standard to calculate advisories on how much fish people can safely eat. Environmentalists were joined by Sen. Patrick Leahy (D-Vt.) in protesting the relaxation of the standard, saying the fishing industry would use it to argue against strong fish advisories and power plants would use it to argue against regulation of mercury emissions.