The new limits - which are presented as a range - will likely put hundreds more counties nationwide in violation, a designation that will require them to find additional ways to clamp down on pollution or face government sanctions, most likely the loss of federal highway dollars.
The tighter standards will cost tens of billions of dollars to implement, but will ultimately save billions in avoided emergency room visits, premature deaths, and missed work and school days, the EPA said.
The proposed range was what scientists had recommended during the Bush administration. However, former President George W. Bush personally intervened and set the standard above what was advised after protests from electric utilities and other industries. The Bush standard was still stricter than the previous smog standard set in 1997.
EPA Administrator Lisa Jackson said in a statement Thursday that science, this time around, had been followed.
"EPA is stepping up to protect Americans from one of the most persistent and widespread pollutants we face," Jackson said. "Using the best science to strengthen these standards is long overdue action that will help millions of Americans breathe easier and live healthier."
The Obama administration last year had indicated it planned to scrap the Bush smog limits, when it asked a federal judge to stay a lawsuit challenging the March 2008 standards brought by 11 states and environmental groups.
Smog is a respiratory irritant that has been linked to asthma attacks and other respiratory illnesses. It is formed when emissions from burning gasoline, power and chemical plants, refineries and other factories mix in sunlight.
While smog has been a long-term problem in parts of Texas, California, and along the northeast Coast, the new standards could affect counties in Idaho, Nevada, Oregon, the Dakotas, Kansas, Minnesota and Iowa for the first time based on EPA data.
Environmentalists immediately endorsed the decision. "If EPA follows through, it will mean significantly cleaner air and better health protection," said Frank O'Donnell, president of the advocacy group Clean Air Watch.
Representatives of the oil and gas industry, which said they have already invested $175 billion toward environmental improvements, were quick to say the proposal lacked "scientific justification."
"There is absolutely no basis for EPA to propose changing the ozone standards promulgated by the EPA Administrator in 2008," the American Petroleum Institute said in a statement. "To do so is an obvious politicization of the air quality standard setting process that could mean unnecessary energy cost increases, job losses and less domestic oil and natural gas development and energy security."
The EPA proposal presents a range for the allowable concentration of ground-level ozone, the main ingredient in smog, from 60 parts per billion to 70 parts per billion. That's equivalent to 60 to 70 tennis balls in an Olympic-sized swimming pool full of a billion tennis balls. EPA will select a specific figure within that range later this year.
The Bush standard adopted in 2008 was 75 parts per billion. Since 1997, it had been 84 parts per billion.
The stricter limit comes with additional costs, from $19 billion up to $90 billion a year by 2020, according to EPA. The Bush administration had put the cost to industry and drivers to meet its standard at between $7.6 billion to $8.5 billion a year.
Counties and states will have up to 20 years to meet the new limits, depending on how severely they are out of compliance. They will have to submit plans for meeting the new limits by end of 2013 or early 2014.