What does this ruling mean to you? CNN's William Hudson says:
Despite federal limits on emissions of mercury from other sources, such as waste incinerators, there have been no limits on coal-fired power plants, which the EPA says constitute the single largest source of mercury emissions.
"As a mom, I'm especially excited to know that millions of mothers and babies will now be protected from mercury poisoning," said Mary Anne Hitt, director of Beyond Coal Campaign, a clean energy advocacy group.
"We all teach our kids the simple rule that if you make a mess you should clean it up - and now polluters will have to follow that same rule," she wrote in an e-mail conversation.
"Mothers around the country who have been worried about mercury pollution causing learning disabilities and other problems for their kids will be able to sleep easier tonight."
Health experts have known for a long time that mercury causes damage to developing fetuses, with long-term effects on the child.
Methylmercury, found in fish and shellfish, can harm a child's thinking, language, fine motor skills, memory, attention, and visual spatial skills when exposed in the womb.
One study estimates that for each part per million of mercury found in a mother's hair -- a common way of testing for mercury exposure -- her child loses approximately 0.18 IQ points.
Outbreaks of methylmercury poisoning have resulted in some children being born with severe disabilities, even when their mothers did not show signs of nervous system damage. But adults are at risk for mercury poisoning too; symptoms can include impairment of vision, speech, hearing and walking.