The United States on Friday tightened restrictions on emissions of soot from industry and vehicles by 20 percent, predicting that the regulations would avert thousands of deaths.

The Environmental Protection Agency, in its first major announcement since President Barack Obama's re-election, ordered stricter rules on so-called fine particle pollution that can easily pass into people's lungs.

"Families across the country will benefit from the simple fact of being able to breath cleaner air," Lisa Jackson, the head of the agency, told reporters on a conference call.

"More children will be able to go outside and play with their friends without fear of triggering an asthma attack," she said.

The agency said the new standards on diesel vehicles and equipment would prevent up to 40,000 premature deaths and 4.7 million days of sick time at work by 2030.

The agency said implementing the regulations will require anywhere from $53 million to $350 million, but estimated that the cost would be outweighed by health benefits of between $4 billion and $9 billion a year.

The agency set a yearly limit on fine particle pollution of no more than 12 micrograms per cubic mete, down from the current limit of 15 micrograms.

Jackson said that 66 of the more than 3,000 counties in the United States are believed not to be in compliance with the new standard, but that only seven counties -- all in California -- were not on track to meet the rules by 2020.

The Environmental Protection Agency is a major target of conservative Republicans, who oppose its efforts to fight climate change by ordering reductions of greenhouse gas emissions from power plants.

Republican Representative Darrell Issa, a frequent critic of the Obama administration, said that the regulation "will inhibit economic growth for businesses and communities."

"In this struggling economy, we must avoid overly burdensome and costly new rules that would encumber American businesses," Issa, chairman of the House Oversight and Government Reform Committee, said in a statement.

Michael Halpern of the Union of Concerned Scientists, a pressure group that urges policies based on scientific research, was upbeat about the agency's soot rules.

"It could be that the Obama administration's backbone is growing stronger now that we're past the election. Hopefully this means that the administration will side with the science on other issues, too," he wrote in a blog.

Jackson said the agency was responding to a court-ordered deadline to revisit its standards set under the landmark 1963 Clean Air Act.