Measure E2: Daily Air Quality

Introduction: Common Air Pollutants |
Exceedances of Short-Term Air Quality Standards |
Daily Air Quality | Long-Term Exposure to Criteria Air Pollutants |
Hazardous Air Pollutants

The AQI is based on measurements of up to five of the six air quality criteria pollutants (carbon monoxide, ground-level ozone, nitrogen dioxide, particulate matter, and sulfur dioxide). Lead is not included in the AQI. The specific pollutants considered in the AQI for each metropolitan area depend on which pollutants are monitored in that area. Each pollutant concentration is given a value on a scale that is related to the air quality standards for that pollutant. An AQI value of 100 for a criteria pollutant generally corresponds to the short-term National Ambient Air Quality Standard for that pollutant, and is the level EPA has set to protect public health for a single day. Above this level, pollutant-specific health advisories are issued. The daily AQI is based on the pollutant with the highest index value on the scale that day. It does not add up values for more than one pollutant. Therefore, it does not reflect the possible effects of simultaneous exposure to high levels of multiple pollutants.

EPA has divided the AQI scale into categories. Air quality is considered “good” if the AQI is between 0 and 50, posing little or no risk. Air quality is considered “moderate” if the AQI is between 51 and 100. Some pollutants at this level may present a moderate health concern for a small number of individuals. Moreover, such a level may pose health risks if maintained over many days. Air quality is considered “unhealthy for sensitive groups” if the AQI is between 101 and 150. Members of sensitive groups such as children may experience health effects, but the general population is unlikely to be affected. Air quality is considered “unhealthy” if the AQI is between 151 and 200. The general population may begin to experience health effects, and members of sensitive groups may experience more serious health effects.

Measure E2 is based on the reported AQI for counties of the United States. (Not all counties have air quality monitoring stations.) This measure was developed by reviewing the air quality designation for each day for each county and weighting the daily designations by the number of children living in each county. The overall measure reports the percentage of children’s days of exposure considered to be of good, moderate, or unhealthy air quality.

The advantage of this approach, compared with that used in measure E1, is that it provides a sense of the intensity of pollution over the course of a year. This method provides data on the air quality category for each day, rather than simply reporting whether a county ever exceeds any standard for any pollutant. However, the method has some limitations. The AQI is based on the single pollutant with the highest value for each day; it does not reflect any combined effect of multiple pollutants. It reflects only short-term, daily pollution burdens. It does not include lead. The approach is influenced by the frequency of measurements. Because the AQI is reported daily, pollutants that are measured daily—such as ozone—will appear to have more effect than those that are measured less frequently, such as PM-10, which typically is measured every six days. Also, the AQI is not well-suited for reporting concentrations of nitrogen dioxide, because this pollutant does not have a short-term standard.

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• The percentage of days that were designated as having “unhealthy” air quality (including days that were unhealthy for everyone as well as those that were unhealthy for sensitive groups) decreased between 1990 and 1999, dropping from 3 percent in 1990 to less than 1 percent in 1999. The percentage of days with “moderate” air quality remained around 20 percent between 1990 and 1999, although an upward trend is suggested by the fact that the percentage of moderate air quality days was higher in 1999 than for any other year in this analysis. As the percentage of either unhealthy or good air days decreases, the percentage of moderate days would be expected to increase.
• The coverage of monitoring for this measure, in terms of area and percentage of days monitored, was largely unchanged between 1990 and 1999. Approximately 30 percent of children’s days of exposure to air pollutants were not monitored. This percentage includes days for which no AQI was reported in counties where the AQI is sometimes reported, as well as counties in which the AQI is not reported at all. On days that were monitored, in many cases only one or a few pollutants were monitored.

Related Measures:

• Respiratory Illnesses

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