What Do Carbon Monoxide Levels Mean?

Each winter we hear advisories being broadcast over local radio stations about current and projected carbon monoxide levels and the resulting air quality. One may hear in Fairbanks, for instance, the announcement that "Carbon monoxide levels for an eight-hour average may exceed 15 parts per million. Air quality will be poor." Just what does this mean, anyway?

Although different municipalities may have different criteria on which to base estimates of air quality, most are governed by Environmental Protection Agency (EPA) standards. Fairbanks North Star Borough Environmental Services, for instance, uses the term "good" to describe air with less than 9 parts per million (ppm) of carbon monoxide (CO), "fair" to describe air with between 9 and 15 ppm, and "poor" to describe air with greater than 15 ppm CO.

Carbon monoxide is just one of literally thousands of chemical pollutants being thrown into the air from automobiles, power plants and other man-made sources. Air pollutants are legally classified into seven categories, one of which is CO. Because CO is the easiest to measure, it can be used to judge the general air quality, under the working assumption that other pollutant concentrations will fluctuate similarly.

A locality is legally defined by EPA as being out of compliance with clean air standards if the average CO concentration over any one hour period during an entire year exceeds 35 ppm or if it exceeds 9 ppm over an eight-hour average.

For comparison, undiluted cigarette smoke contains about 30,000 ppm of CO, undiluted warm car exhaust about 7,000 ppm, and the chimney of a home wood fire about 5,000 ppm. Clean countryside air contains about 0.02 ppm of CO. The smoke from one pack of cigarettes, if distributed uniformly throughout an average sized house, could result in a CO concentration of up to 14 ppm.

An average healthy person at sea level is just barely affected by prolonged exposure to concentrations of 9 ppm, but the presence of other pollutants aggravates the situation, and respiratory and cardiac problems pose an increased risk. Chronic exposure to high concentrations of CO (30 to 100 ppm), such as in a poorly vented garage, can lead to long-term deterioration of the cardiovascular system.

Once it has been introduced, it takes several hours for the blood system to cleanse itself of CO. Headaches are a common early symptom of CO poisoning, but these can be easily mistaken as being due to other causes. Deaths due to CO most commonly occur when one is in an enclosed space for several hours where the concentration is on the order of several hundred ppm. Carbon monoxide in rooms with unvented kerosene space heaters will vary between 0.5 and 50 ppm depending on the air exchange rate. An average concentration would be about 1 ppm with one air exchange per hour in the room.

In order to dilute the CO concentration produced by a car burning one gallon of gasoline to a level below 10 ppm, the exhaust would have to be mixed into 10,000 cubic yards of air. This is readily accomplished in the open country, but in cities with dense concentrations of automobiles and tall buildings to block air circulation, the problem intensifies. Several Alaskan cities experience even more aggravated conditions due to natural temperature inversions which trap the pollutants and prevent them from being dispersed. Pollution conditions can be worse on clear days than during ice-fog episodes. Living 5 to 10 stories above the street, or in a building with a good rooftop air intake can significantly reduce the pollution concentration inside the building, as compared to street level or the first two stories.

Although the automobile is to blame for most of the CO problem, chimney smoke from wood stoves is beginning to compete with it as the most objectionable source of air pollution in some suburban areas. In this case, it is not just the CO that matters, but other gases and particulates as well.