Bad news for the Bush administration: A federal appeals court on Friday struck down a U.S. EPA rule that would have let coal-fired power plants trade the right to emit mercury, a neurotoxin that contaminates waterways, accumulates in fish, and has been linked to nerve and brain damage, particularly in children. Environmentalists and public health advocates, among others, had wanted every coal plant to have to reduce its emissions of mercury, but in 2005 the Bush admin opted for a cap-and-trade system that would let dirtier plants buy the right to pollute from cleaner ones. (The cap-and-trade approach is widely accepted as a good way to tackle greenhouse-gas emissions because they affect the atmosphere as a whole; in contrast, mercury pollution has severe local effects, so communities near plants with high mercury emissions would get the short end of the stick.) The court said the EPA violated the Clean Air Act in 2005 when it implemented the rules, which exempted power plants from strict emissions controls. The ruling came in response to a lawsuit filed by a number of states, enviros, and public health groups; they’re all cheering today’s ruling as a major victory.