The U.S. Environmental Protection Agency (EPA) recently proposed revising the national air quality standard for ozone, the key pollutant in smog and regional haze. EPA acknowledges that the rule will cost billions of dollars each year to implement. Industry fears the additional regulatory costs, and environmentalists assert the rule does not restrict ozone enough. Select jurisdictions with high levels of background ozone, particularly Western states, will find it more difficult to reach attainment, and current mechanisms to account for naturally-occurring ozone spikes may prove inadequate. Republican leaders in the House and Senate have signaled their intention to oppose the rule in legislation. The EPA must navigate these complex issues as it prepares to finalize the rule by October 1, 2015.