On January 6, 2010 the EPA rolled out a proposal to reduce the 8-hour primary standard for tropospheric (ground level) ozone from its current level of 0.075 ppm to somewhere between 0.060 ppm and 0.070 ppm. This reduction has the potential for many US airsheds to be declared as nonattainment with the new standard (including many in the Rocky Mountain West). Such a declaration will require state regulatory agencies to initiate implementation plans to bring affected airsheds back into attainment. This presentation will summarize the current status of and reasoning for EPA’s proposed reduction to the primary ozone standard and identify those airsheds that will likely be declared as nonattainment once the final rule is published. In addition, this presentation will provide an overview of EPA’s proposed reduction to the secondary standard for tropospheric ozone and describe the new weighting scale to be applied to ozone concentrations to validate compliance.