	Introduction

On January 19, 2010, the Environmental Protection Agency (EPA) proposed revisions to the National Ambient Air Quality Standards (NAAQS) for ozone. Final standards were expected to be issued by the end of August 2010, but EPA has delayed promulgation, first to October, then to December, and now to July 2011. Primary NAAQS are standards, "the attainment and maintenance of which in the judgment of the [EPA] Administrator ... are requisite to protect the public health," with "an adequate margin of safety." Depending on the severity of the pollution, ozone nonattainment areas have anywhere from 3 to 20 years to actually attain the standard. Thus, establishment or revision of a NAAQS sets in motion a long and complicated implementation process that has far-reaching impacts for public health, for sources of pollution in numerous economic sectors, and for states and local governments. The Ozone Standard

The ozone standard affects a larger percentage of the population than any of the other NAAQS. As of September 2010, 39% of the U.S. population lived in areas designated nonattainment under the 1997 standard for ozone, 119 million people in all. Since the standard has been strengthened as a result of a review completed in 2008 and the Administrator is now proposing a further strengthening, more areas will be affected, and those already considered nonattainment may have to impose more stringent emission controls. The Primary Standard

The primary (health-based) standard promulgated in 1997 was set at 0.08 parts per million (ppm), averaged over an 8-hour period. The panel unanimously recommended a range of 0.060 to 0.070 ppm (60 to 70 parts per billion) for the primary 8-hour standard. At 0.070 ppm, 515 counties (76% of those with monitors) exceeded the standard. Given the trend toward cleaner air in recent years, and regulations on both mobile and stationary sources that have taken effect in the intervening years, the agency expects the number of counties exceeding the standard to be less than indicated by these projections. (For a discussion, see archived CRS Report RL34057, Ozone Air Quality Standards: EPA's March 2008 Revision .) As shown in Figure 4 , nonattainment with the proposed secondary standard could be widespread: based on the latest available (2006-2008) data at the time of proposal, 196 counties would be nonattainment if the secondary standard were set at 15 ppm-hours, and 579 counties (86% of all counties with monitors) would be nonattainment under a 7 ppm-hours standard. Most stationary and mobile sources are considered to be contributors to ozone pollution. Issues

The major issues raised by the proposed standards concern whether the Administrator has made appropriate choices (i.e., whether her choices for the primary and secondary standards are backed by the scientific studies). This mismatch could support a case for stronger federal controls on the sources of ozone precursors or a reexamination of the attainment deadlines. Another issue arises from a close inspection of EPA's maps: whether the current monitoring network is adequate to detect violations of a more stringent standard. Only 675 of the nation's 3,000 counties have ozone monitors in place. With as many as 650 of them (96%) showing violations of the most stringent proposed standard, using current data, how confident is the agency that the 2,350 counties without monitors would all be in attainment? That proposal would require that each state operate at least three ozone monitors in non-urban areas.
On December 8, 2010, the Environmental Protection Agency (EPA) announced that it will delay issuing revised ambient air quality standards for ozone until July 2011 so that it can consider further recommendations from an independent panel of scientific advisers. The agency proposed changes to the National Ambient Air Quality Standards (NAAQS) for ozone on January 19, 2010, with an expected promulgation date of August 2010. The December announcement marks the third time that the agency has postponed issuing the revised standards.
NAAQS are standards for outdoor (ambient) air that are intended to protect public health and welfare from harmful concentrations of pollution. By changing the standard, EPA would be concluding that protecting public health and welfare requires lower concentrations of ozone pollution than it previously judged to be safe. Under the January 2010 proposed standards, as many as 96% of the counties that currently monitor ozone might need to take action to reduce emissions. The proposal would also, for the first time, set a separate standard for public welfare, the principal effect of which would be to call attention to the negative effects of ozone on forests and agricultural productivity.
The ozone standard affects a large percentage of the population: as of September 2010, 119 million people (nearly 40% of the U.S. population) lived in areas classified "nonattainment" for the primary ozone NAAQS. As a result of the standard's strengthening, more areas would likely be affected, and those already considered nonattainment may have to impose more stringent emission controls.
The proposed revision would lower the primary (health-based) standard from 0.075 parts per million—75 parts per billion (ppb)—averaged over 8 hours to somewhere in the range of 70 to 60 ppb averaged over the same time. Using the most recent three years of monitoring data, 515 counties (76% of all counties with ozone monitors) would violate the new standard at 70 ppb; 650 counties (96% of those with monitors) would be in nonattainment if the standard is set at 60 ppb. By comparison, only 85 counties have monitors showing exceedance of the currently implemented 1997 standard. (The counties that might exceed the proposed standard are shown in Figure 3 of this report.)
The proposed standards, now expected to be finalized in July 2011, will set in motion a long and complicated implementation process that has far-reaching impacts. The first step, designation of nonattainment areas, is expected to take place within a year of the new standards' promulgation; the areas so designated would then have 3 to 20 years to reach attainment.
The proposed standards raise a number of issues, including whether they should lead to stronger federal controls on the sources that contribute to ozone pollution. Current federal standards for cars, trucks, nonroad vehicles and engines, power plants, and other stationary pollution sources are not strong enough to bring many areas into attainment, thus requiring local pollution control measures in many cases. EPA, the states, and Congress may also wish to consider whether the current monitoring network is adequate to detect violations of a more stringent standard. Only 675 of the nation's 3,000 counties have ozone monitors in place.
This report discusses the standard-setting process, the specifics of the new standard, and issues raised by the Administrator's choice; and it describes the steps that will follow EPA's promulgation.