Monitoring for carbon monoxide (CO) in the Treasure Valley began in 1977. Violations of the health-based standard for CO occurred every winter from 1977 through 1986. As a result of these high levels of CO, northern Ada County was designated a CO nonattainment area by the U.S. Environmental Protection Agency (EPA).

The primary source of CO emissions in Ada County is incomplete combustion of gasoline by motor vehicles, accounting for an estimated 69% of annual emissions in 1995 and 75 percent of 1995-96 winter emissions. Wood burning, outdoor burning, lawn and garden equipment (summer only), industry, and other types of fossil fuel burning equipment make up the majority of the rest of the CO emissions in this area.

Motor vehicle tailpipe emissions have been reduced through measures such as the Ada County Vehicle Inspection and Maintenance Program, federal new vehicle emissions standards, efforts to increase alternative transportation options, and improvements in traffic flow in downtown Boise. Wood burning and outdoor burning controls, designed to address particulate matter problems, have also had some impact on CO emissions. No exceedances of CO have occurred since 1991.

To address northern Ada County's nonattainment classification, DEQ developed a CO air quality improvement plan for northern Ada County. The plan includes a commitment to continue monitoring CO levels and assurances that existing measures to control CO emissions, such as the vehicle emissions testing program in Ada County, will remain in effect. The plan also includes contingency measures that will be activated if CO levels reach specified conditions.

The plan was prepared to meet EPA requirements to support redesignation of the area to attainment status. It was approved by EPA in December 2002, putting the area into attainment for CO.