This polygon shapefile represents nonattainment areas for carbon monoxide (CO) in EPA Region 9, including California, Nevada and Arizona, for 2011. Nonattainment areas are geographic areas which have not met National Ambient Air Quality Standards (NAAQS) for carbon monoxide air pollution. These standards are part of the Clean Air Act. See Code of Federal Regulations, Section 40 Part 81, for detailed information about Ambient Air Quality Standards.This data set is specifically designed for use with ArcGIS and related software. Data may be used for reference to boundaries of attainment areas in Region 9.