High ambient carbon monoxide (CO) levels are almost exclusively related to car and truck exhaust. All nine monitoring sites in our jurisdiction are located in areas of traffic congestion. These include the central business districts of our largest cities, and other high traffic areas such as shopping malls. As with ozone, all carbon monoxide monitoring in our jurisdiction is performed by the Washington State Department of Ecology.
    Trends. Over the last 10 years, and more dramatically over the past 18 years, carbon monoxide levels have dropped significantly. Cleaner operating motor vehicles, the State motor vehicle inspection program and better formulated gasoline are the main explanations for this decrease. The winter of 1996-97

marked the end of the oxygenated fuel program. Since 1992, ethyl alcohol had been added to gasoline from November through February for the purpose of reducing carbon monoxide vehicle emissions. The program was detemined to be unnecessary to maintain compliance with the CO standard.
    All monitoring sites have shown attainment with the federal standard since 1991. EPA recognized our region in October 1996 as being in attainment for carbon monoxide.