After signing an executive order Tuesday that rolls back federal environmental regulations, President Donald Trump said he wants to "end the war on coal" and increase production of "clean coal — really clean coal." But as one expert told ATTN:, what Trump really wants isn't real.

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Twitter users took note of the remark and responded in chorus.

"It's a complete lie," Liz Perera, climate policy director at the environmental organization Sierra Club, told ATTN:. "The idea of clean coal, first off, it's a misnomer. It doesn't exist. There's no such thing as clean coal."

"The technologies that are out there that they try to call 'clean coal' — some of it is just regular coal that is [processed through] a newer type of plant, but still has all of the pollution associated with it," Perera said. "The other type of thing they call 'clean coal' is carbon capture-and-storage, where they spend a billion dollars to build a plant that's not economical and never will be economical and then strap a bunch of pipes on it to inject the carbon dioxide into the ground." And once underground, there's no guarantee the carbon dioxide will stay there, forever.

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Coal is relatively cheap to produce, but it's the dirtiest fossil fuel to burn because it emits a toxic mix of sulfur, carbon dioxide, ammonia, soot, nitrogen compounds, and some metals such as mercury. About a third of all "energy-related" greenhouse gas emissions in the U.S. are from coal, Scientific American reported.

The dangers of burning coal are well known. Coal emissions contribute to acid rain, harming plant life and water systems. A 2010 study conducted by the Clean Air Task Force determined that emissions from coal-fired plants lead to about 13,000 premature deaths and 20,000 heart attacks in the U.S. each year.

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President Barack Obama cited the health and environmental dangers of coal when he implemented the Clean Power Plan in 2015. The plan was designed to gradually reduce the country's dependence on dirty fossil fuels and transitioning to relatively cleaner alternatives such as natural gas and renewable energy sources like wind and solar. The Environmental Protection Agency predicted that the legislation would prevent 1,500 to 3,600 premature deaths, 90,000 asthma attacks among children, and 1,700 heart attacks annually by 2030.

Trump's dismantling of the Clean Power Plan means his administration is now responsible for the public health consequences, Perera argued.

"I would really put that at his feet," she said. "He's causing people to die, really. It's a serious toll."

The White House did not respond to a request for comment.