HEALTH EFFECTS OF CARBON MONOXIDE

Carbon monoxide (CO) is a colorless, odorless, poisonous gas produced by incomplete burning of carbon in fuel. The health threat from CO is most serious for those who suffer from cardiovascular disease, particularly those with angina and peripheral vascular disease. The National Ambient Air Quality Standard (NAAQS) for CO is set at 9.5 ppm (parts per million) for an eight-hour average, this level is not to be exceeded more than once per year. This health based standard is intended to protect those most sensitive to the effects of CO exposure.

Extremely high concentrations (above 4000 ppm) of CO can develop indoors or inside motor vehicles due to faulty heating systems or exhaust leaks. At these concentrations, exposure to CO can cause unconsciousness or death unless the victim is removed from the source and provided with immediate medical care. These high exposures develop only indoors or in confined spaces. Outdoor exposures to CO are much lower. Outdoor concentrations above 20 ppm are rare. The effects of exposure to elevated outdoor concentrations of CO are subtle even among susceptible individuals.

VIOLATIONS OF AIR QUALITY STANDARD

A community is considered to be in compliance with the federal air quality standard if one or fewer violations of the standard are measured in each of two consecutive calendar year periods. Anchorage is now on the verge of compliance with federal air quality standards for CO. Anchorage was violation-free in calendar year 1995. In 1996, however, CO concentrations exceeded federal standards three times.

Anchorage carbon monoxide pollution levels have declined dramatically since the mid-1980's. In the early 1980's, CO concentrations in Anchorage violated federal standards as many as 52 days per year. During the past few years, CO levels have exceeded standards only a few days each year. CO levels are measured at five air quality monitoring stations scattered around Anchorage. Between 1983 and 1996, CO concentrations at the air quality monitoring stations declined by 50 to 60 percent.

WHY HAS CO AIR QUALITY IMPROVED SO DRAMATICALLY IN ANCHORAGE?

In 1981, the U.S. passed a new tougher emission standard for motor vehicles. Motor vehicle emissions have since declined and continue to decline further as newer model cars and trucks with more efficient air pollution emission control systems replace older less air pollution control efficient vehicles. The Anchorage Vehicle Inspection and Maintenance (I/M) Program as well as the ethanol-blended fuel requirement have also contributed to air quality improvements in Anchorage. These programs reduced emissions of CO by an estimated 30 tons per day in 1995 and 1996.

WEATHER PLAYS AN IMPORTANT ROLE IN CO AIR QUALITY

Carbon monoxide pollution levels are highest in the winter. Strong temperature inversions and cold air near the ground overlain by warm air above trap air pollution near ground level. A temperature inversion occurs when cold air near the ground is overlain by warm air above. Many Anchorage residents are familiar with this winter phenomena. During an inversion, temperatures on the Anchorage hillside can be 10 to 20 degrees Fahrenheit warmer than those down in the bowl.

Very strong and persistent temperature inversions can develop at high northern latitudes because of long winter nights and weak daytime sunlight. For this reason, Anchorage and Fairbanks have had special difficulties in meeting air quality standards for carbon monoxide.

SOURCES OF CO EMISSIONS IN ANCHORAGE

An estimated total of 135 tons of CO are emitted during a typical weekday in the winter by all sources in the Anchorage bowl. About 84% of this CO is produced by cars and trucks. Most of the CO generated by motor vehicles is emitted during the first few minutes of operation. CO emissions are highest then because the engine is cold, the engine is operating rich, and the catalytic converter has not yet reached the operating temperature (about 600° F) necessary to convert carbon monoxide to carbon dioxide.

Other, less significant sources of CO are aircraft, fireplaces, and woodstoves. Very little CO comes from power generation or industrial sources in Anchorage.

ANCHORAGE CO AIR POLLUTION LEVELS COMPARED WITH OTHER CITIES

Although levels of CO have declined, Anchorage has relatively high levels of carbon monoxide pollution compared with most other cities. Our cold climate and strong temperature inversions exacerbate the carbon monoxide problem. Because of the severe climate, Fairbanks also has relatively high CO concentrations. Most cities in the lower 48 states have achieved or will soon achieve compliance with the federal CO standard.

Anchorage and Fairbanks are among only ten cities in the U.S. that remain out-of-compliance with the federal air quality standard for CO. Spokane, Washington is the only other city in the Pacific Northwest or Alaska.