The U.S. Environmental Protection Agency has unveiled a proposal to toughen national air-quality standards for soot, a move that could bring major public-health benefits as well as a political backlash from Republicans and industrial lobbyists.

The proposed rule deals with fine-particle soot, or floating flecks of organic ash less than 2.5 microns in diameter — small enough to penetrate deeply into the lungs and heart. These tiny particles are emitted by a wide range of sources, from wood-burning stoves and wildfires to diesel engines, factories and power plants. Once inhaled, they can easily burrow through lung tissue and enter the bloodstream, potentially causing health problems such as asthma, bronchitis and irregular heartbeat. Breathing this type of particulate matter, known as "PM2.5," prematurely kills tens of thousands of Americans every year, according to the EPA.

The proposal would cut the annual PM2.5 exposure standard from 15 microns (µm) per cubic meter of air to between 12 and 13 µm, EPA assistant administrator Gina McCarthy said Friday in a conference call with reporters. After a nine-week public comment period, the agency will then issue a final ruling by Dec. 14.

Friday's update came under court order from U.S. District Court Judge Robert Wilkins, part of a years-long legal battle over how much PM pollution is safe for Americans to breathe. The Clean Air Act requires the EPA to review air-quality rules every five years and update them based on scientific advice, but in 2006 the Bush administration ignored its science advisers and opted to leave the annual standard at 15 µm. A coalition of states and environmental groups sued the EPA, and in 2009 a federal appeals court ordered the agency to update its soot standards.

Last fall, however, the Obama administration tried to delay the next rule change until 2013, citing "the abundance of new scientific evidence concerning the potential health and welfare effects of PM pollution." Eleven states sued the EPA in District Court, and on June 6 Wilkins ordered the agency to propose a new rule this week.

The resulting proposal represents a "major victory for public health," according to a statement issued Friday by New York Attorney General Eric Schneiderman, who led the 11-state lawsuit. "Every day, soot pollution threatens the health of more than one-third of our nation's population, particularly our most vulnerable — children, the elderly and the sick," Schneiderman said. "With this settlement, the health of over 100 million American will no longer be ignored, and the years of delay in revising our nation's current lax soot standards will end."

Environmentalists widely shared Schneiderman's enthusiasm; John Walke of the Natural Resources Defense Council, for one, called the proposal a "tremendous win for our health and environment." Some industrial advocates, however, argued its costs will outweigh its benefits. "EPA's proposal could substantially increase costs to states, municipalities, businesses and ultimately consumers without justified benefits," Howard Feldman of the American Petroleum Institute said in a statement. "We are concerned that it could come at a significant economic cost and lost investments and limit our ability to produce the energy our nation needs."

Many Republicans have invoked similar arguments about the cost of soot regulations, raising the prospect of election-season debates over particulate pollution — possibly one reason why the Obama administration tried to delay the proposal until next year. In a June 6 letter to EPA Administrator Lisa Jackson, top Republicans on the House Energy and Commerce Committee warned that "stringent standards that seek to control a ubiquitous and naturally occurring pollutant will likely be costly and have significant regulatory and other implications."

While the Clean Air Act prohibits the EPA from considering economic factors when making public-health decisions, McCarthy insists the new soot rule will have a minimal economic impact. In fact, due to previous rules for other air pollutants, she says 99 percent of U.S. counties already comply with the soot proposal.

For more information about soot, smog and other air-quality issues, see this detailed overview of the EPA's proposed rule and the related links from MNN below.

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Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

EPA proposes stricter soot standards
The agency says its proposed rule could save tens of thousands of lives every year, but critics warn of economic burdens for some U.S. industries.