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Carbon Monoxide


Carbon monoxide (CO) is a colorless and odorless gas that can be lethal in high concentrations.

The National Ambient Air Quality Standards (NAAQS) for CO is 9 parts per million (ppm) on an eight-hour average or 35 ppm on a one-hour averaging period. CO occurs naturally by the decomposition of organic matter. The primary man-made source of CO is the incomplete combustion of fuels such as gasoline. CO forms when there is insufficient oxygen present for complete combustion. For Utah's nonattainment areas, 98% of CO emissions are from vehicles, wood burning and miscellaneous non-road mobile sources. Figure 4 summarizes CO emissions for the counties that contain the defined nonattainment and maintenance areas.

The concentration of CO in the ambient air depends primarily on local weather conditions and the number of automobiles in the area. High levels of CO can have acute health effects on humans by reducing the supply of oxygen in the bloodstream. Normally, blood cells transport oxygen to, and remove carbon dioxide from, every cell in the body. The blood cells are more attracted to CO than to oxygen. Therefore, exposure to high levels of CO results in oxygen deprivation to various parts of the body. CO exposure can aggravate existing conditions such as heart and lung diseases. At high levels, CO exposure can be fatal. Nationally, a few hundred fatalities a year occur due to high concentrations of CO, usually in poorly ventilated buildings, idling parked cars with faulty exhaust systems and residential fires. The danger from CO is greatest in unborn and newborn infants, the elderly, and those suffering from chronic illnesses.

Since mobile emissions are the primary source of CO, the control strategies are directed toward motor vehicles. Strategies include vehicle inspection and maintenance programs and transportation control measures such as transit improvements and ridesharing programs.

Carbon Monoxide Standard Status

There have been no violations of the CO standard in Utah since 1993. Provo was officially re-designated to attainment on January 3, 2006. Salt Lake City was re-designated to attainment on January 21, 1999 and Ogden was re-designated on March 9, 2001. The maintenance plans for Salt Lake City and Ogden were updated and approved by EPA in 2005.

Carbon Monoxide Trends

CO maximum values tend to occur during wintertime inversions. In order to establish trends, we compare network maximum values to the frequency of wintertime inversions (an inversion is defined as a day with clearing index < 250). CO pollution levels have been decreasing over the past 15-20 years despite the fact that the number of inversion days has shown no downward trend. For example, the number of inversion days in 1999-2005 is comparable to the number of inversion days in 1984-1987, yet the pollution levels in 1999-2005 are much lower than in the 1980's. This indicates that emissions changes and controls have improved air quality in the Wasatch Front.

The 8-hour CO NAAQS (9 ppm) allows one exceedance of the standard per year. The calculation of the network maximum value (as shown in the plots) and the calculation of the NAAQS for each pollutant are not identical and differences between these measures should be noted.