U.S. EPA today strengthened the federal public health standard for nitrogen dioxide (NO2) pollution, a limit that has been in place for nearly four decades.

The final rule introduces a new one-hour maximum standard for NO2 at 100 parts per billion (ppb), a level that EPA says will protect millions of Americans from peak short-term exposures. The agency is also retaining the existing annual standard of 53 ppb.

"This new one-hour standard is designed to protect the air we breathe and reduce health threats for millions of Americans," said EPA Administrator Lisa Jackson. "For the first time ever, we are working to prevent short-term exposures in high-risk NO2 zones like urban communities and areas near roadways."

The new standard also establishes new monitoring requirements to measure NO2 levels near major roads. Cities with at least 500,000 residents must have monitors near roadways, and larger cities and areas with major roads will have additional monitors. Cities with at least 1 million residents will continue with communitywide monitoring.

The revision marks the first time EPA has updated the national health-based NO2 standard in nearly four decades. The Clean Air Act requires the agency to set national standards for six "criteria" pollutants, including NO2, and to periodically review those standards. EPA has reviewed the health-based NO2 standards twice since the standard was first proposed in 1971 but both times chose not to revise the standards.

Short-term exposures to NO2, which occur primarily near major roads, have been linked to impaired lung function and increased respiratory infections, especially in people with asthma, according to EPA. NO2 forms from the emissions of cars, trucks and buses, off-highway equipment and power plants.

EPA's final rule falls within recommendations by EPA's scientific advisers but does not go far enough for some environmentalists and public health advocates.

The agency's Clean Air Scientific Advisory Committee (CASAC) recommended setting one-hour limits at or below 100 ppb. EPA's staff scientists recommended a one-hour maximum standard between 50 and 200 ppb, with strong support for a level at or below 100 ppb.

But groups including Clean Air Watch and the American Lung Association called for EPA to set a one-hour daily maximum standard of 50 ppb or lower to protect vulnerable groups.

"This standard is a step forward for public health protection, but it is also a missed opportunity to do something better for the breathing public," said Clean Air Watch President Frank O'Donnell. "It suggests EPA may also do the bare minimum on other pending standards, including ozone."

The American Petroleum Institute, which urged EPA to forgo a one-hour standard, expressed concerns that the short-term NO2 limit is based on a faulty scientific record.

"There is no significant evidence that the short-term NO2 standard established today by the Administrator is necessary to protect public health," API said in a statement. "EPA is over-regulating this air quality standard for political -- not health -- reasons."

EPA expects to identify areas in compliance with the standard based on the existing monitoring network by January 2012. New monitors must begin operating by Jan. 1, 2013. The agency intends to redesignate whether areas meet the new standard when three years of data are available from the new monitoring network.

The final rule will take effect 60 days after publication in the Federal Register.