Key: PM-10 = airborne particulates of less than 10 microns; PM-2.5 = airborne particulates of less than 2.5 microns.
Notes: Data for 2008 and later years may not be comparable to previous years due to changes in methodology. Indices are calculated using data on highway vehicle emissions only. Particulate matters include PM without condensibles.
Source: U.S. Department of Transportation, Research and Innovative Technology Administration, Bureau of Transportation Statistics, National Transportation Statistics, tables 4-45 through 4-50, available at www.bts.gov as of October 2013.