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Vehicle age distribution is an important consideration when evaluating the effects of exhaust emissions. To quantify CO idle data from motor vehicles of different model year groupings and vehicle classifications, the Colorado Department of Health (CDH) reviewed emissions data from The Colorado Automobile Inspection and Readjustment (A.I.R.) Program. Colorado's A.I.R. Program is presently in its 9th year and is considered a mature I/M (inspection maintenance) Program. (1) Program data from December 1989, January 1990 and February 1990 were studied to determine the mean % CO by vehicle class and model year group. Classes include: light duty, heavy duty and mixed light and heavy duty (fleet) vehicles. Model year groups include: 1974 and older, 1975-1980, 1981-1982, and 1983 and newer model year vehicles. The 10% highest CO emitting vehicles (by CO emission concentration) in these categories were also evaluated.