The Trump administration on Tuesday proposed relaxing pollution standards for power plants nationwide, a move that could slow the decline of US carbon emissions and lead to hundreds more premature deaths and thousands of asthma attacks and missed school days.

The Environmental Protection Agency's Affordable Clean Energy rule, which President Trump planned to tout at a roundtable meeting in Charleston, West Virginia, on Tuesday evening, represents the administration's most ambitious proposal to bolster the nation's coal industry.

Although it probably would have a modest impact on curbing carbon dioxide emissions in the power sector, it could potentially increase human health risks from other pollutants.

The measure, which would replace an Obama-era rule that set strict carbon dioxide limits for each state and encouraged the shuttering of coal plants, is likely to widen the environmental policy divide between red and blue states. Officials in conservative states may allow utilities to extend the life of coal-fired units, while those in liberal states are likely to keep pressing for steep reductions in greenhouse gas emissions.

"We're pleased that the proposed rule recognizes and respects the environmental and energy needs of each state," said John Mura, communications director for Kentucky's Energy and Environment Cabinet.

His state has made progress in cutting emissions, but 79 percent of its electricity still comes from coal.

Several Democratic leaders said Trump's proposed rollback is irresponsible.

"This is a declaration of war against America and all of humanity - it will not stand," California Gov. Jerry Brown (D) said in a statement.

"Truth and common sense will triumph over Trump's insanity."

One of the measure's biggest effects could be on public health, because it would allow coal-fired plants to run longer if they became more efficient, which could increase the total amount of soot and smog-forming pollutants they emit.

The EPA projects that the rule could lead to 470 to 1,400 premature deaths each year compared with the Obama-era rule.

"There is nothing clean or affordable about this rule," Carter Roberts, president and chief executive of the World Wildlife Fund, an advocacy group, said in a statement.

"Coal plants emit more carbon pollution than all other sources. They're often more expensive than natural gas, solar, or wind energy. And worst of all, they impose severe public health costs on us all."

But Bill Wehrum, who heads EPA's air and radiation office, said that the proposal would provide companies with an incentive to update their operations and that the agency has other policies that reduce traditional pollutants.

"What we're dealing with here are greenhouse gases," Wehrum said.

"We have abundant legal authority to deal with those other pollutants directly, and we have aggressive programs in place that directly target emissions of those pollutants."

The agency estimates that the power sector's greenhouse gas emissions would continue to decline in coming years, but almost entirely as a result of market pressures.

By 2030, carbon dioxide levels would be 0.7 percent to 1.5 percent lower than they would have without any regulation in place, it said, a reduction of 12 million to 24 million metric tons.

By contrast, the Obama rule would have cut 67 million metric tons compared to business as usual over the same time frame.

Many states are pressing ahead to close coal plants and expand the use of natural gas and renewable energy. Craig W. Butler, director of Ohio's Environmental Protection Agency, said his state has retired 9,200 megawatts of coal power since 2010, largely because of Ohio's robust natural gas reserves.

"There's been a culling in the market in Ohio, just because of market forces," Butler said. "The plants we have remaining are much more efficient."

Environmentalists and several Democratic attorneys general said they would challenge the proposed rule in court because it would delegate too much authority to the states.

"This isn't how the Clean Air Act is supposed to work," said Earthjustice counsel Howard Fox, noting that the federal government traditionally sets specific pollution targets.

"States have leeway about how to achieve the reduction, but not whether to achieve it."

Industry officials praised the administration for the plan, which EPA estimates would save the power sector US$400 million a year in compliance costs compared with the rule it would replace.

A key component of the rule would allow plants to run longer if they can convert coal to electricity more efficiently, without having to install new controls to curb fine particulate matter, or soot, as well as sulfur dioxide and nitrogen oxides. Fine particulates are linked to heart and lung disease, while the other two pollutants can cause respiratory illnesses.

EPA estimates that by 2030, the measure could cause 48,000 new cases of exacerbated asthma, and at least 21,000 new missed days of school a year.

"The cost of failing to act is enormous - from increases in lung disease and heavy metal poisoning to the many health harms from climate change that doctors are already seeing among their patients," said Mona Sarfaty, director of the Medical Society Consortium on Climate and Health.

About 40 percent of the nation's coal fleet has either been retired or slated for retirement since 2010, according to the American Coalition for Clean Coal Electricity. The Sierra Club says that 270 of the 523 coal plants it targeted in 2010 are either shut down or scheduled to be.

Electric utilities with coal generation are encouraged by the EPA's proposal. John McManus, senior vice president of environmental services at American Electric Power, one of the nation's largest electric utilities, said in an interview that the proposal could keep some coal plants operating that would have had to close otherwise. Although he added, "Nothing lasts forever."

The Obama-era rule, dubbed the Clean Power Plan, has never taken effect because the Supreme Court stayed it after more than two dozen Republican attorneys general and industry groups said the EPA exceeded its legal authority.

Trevor Houser, an energy analyst with the Rhodium Group, said the US power sector was already on track to meet the Obama administration's target of cutting emissions 32 percent by 2030.

But he added the United States is falling short of the broader climate goal it pledged in the 2015 Paris Agreement, to reduce its greenhouse gas emissions between 26 percent and 28 percent compared to 2005 levels by the end of the next decade.

"The US is not on track to meet its commitments under the Paris Agreement, the best chance that we have for averting catastrophic climate change," said Houser, who served in the State Department under President Barack Obama.

"This is a missed opportunity to accelerate progress in the power sector in a way that would deliver on those commitments."

Dino Grandoni contributed to this report.

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