EPA's Report on the Environment: External Review Draft
Percent of Days with Air Quality Index Values Greater Than 100
Note to reviewers of this draft revised ROE: This indicator reflects data through 2009. EPA anticipates updating this indicator in 2014.
The Air Quality Index (AQI) provides information on pollutant concentrations of ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. Formerly known as the Pollutant Standard Index, the nationally uniform AQI is used by state and local agencies for reporting daily air quality and air quality related health advisories to the public.
In 1999, the AQI was updated to reflect the latest science on air pollution health effects and to make it more appropriate for use in contemporary news media (U.S. EPA, 2003a). It also serves as a basis for community-based programs that encourage the public to take action to reduce air pollution on days when levels are projected to be of concern. The index has been adopted by many other countries (e.g., Mexico, Singapore, Taiwan) to provide the public with information on air quality.
The AQI is based on pollutant concentration data measured by the State and Local Air Monitoring Stations network and by other special purpose monitors. For purposes of this indicator, AQI data are presented for metropolitan areas known as Core-Based Statistical Areas (CBSAs), which are defined by the Office of Management and Budget. For most pollutants in the index, the concentration is converted into index values between 0 and 500, “normalized” so that an index value of 100 represents the short-term, health-based standard for that pollutant as established by EPA (U.S. EPA, 1999). The higher the index value, the greater the level of air pollution and health risk. An index value of 500 reflects a risk of imminent and substantial endangerment of public health. The level of the pollutant with the highest index value is reported as the AQI level for that day. An AQI value greater than 100 means that at least one criteria pollutant has reached levels at which people in sensitive groups may experience health effects. A complete description of how AQI values are calculated and what they represent is documented in many publications (e.g., U.S. EPA, 2003b).
This indicator is based on the percent of days across 87 metropolitan areas (CBSAs with 500,000 people or more) during the year that recorded an AQI greater than 100 at one or more monitoring sites. While the AQI indicator is calculated from ambient concentration data for criteria pollutants, this indicator’s trends should not be expected to mirror the trends in the other ambient concentration indicators, due to the differing spatial coverage of monitoring stations across the various indicators.
The percent of days with AQI greater than 100 was calculated in two steps. First, for each year, the total number of days with AQI above 100 in each of the 87 metropolitan areas (CBSAs) was summed in order to get a national total. Then, the national total was divided by the total number of days in the annual sample (365 × 87, or 31,755 days) to obtain the percentage of days with AQI above 100 in a year. Note that this calculation will understate the actual percentage of days with AQI above 100 for pollutants that are not measured daily (e.g., PM2.5).
Data are presented for 1990 through 2009. However, because meteorology can strongly influence AQI values in a given year, the change in AQI over time is evaluated by comparing the 3-year average observation at the beginning of the period of record (i.e., 1990-1992) to the 3-year average at the end (i.e., 2007-2009). Comparing 3-year averages reduces the potential for biases introduced by years with unique meteorological conditions. The air quality data that go into the index consist of daily (24-hour) measurements for PM10 and PM2.5 and continuous (1-hour) measurements for CO, NO2, ozone, and SO2. Lead measurements do not factor into the AQI. AQI values in this indicator are based on the current NAAQS, which include the 2010 standards for NO2 and SO2. Of the pollutants considered, four (ozone, SO2, NO2, and PM) account for most instances with AQI values greater than 100.
What the Data Show
AQI Based on All Criteria Pollutants (Except Lead)
The percent of days with AQI greater than 100 in 87 large metropolitan areas based on all criteria pollutants (except lead) decreased from 13.2 over the 1990-1992 time frame to 4.8 over the 2007-2009 time frame (Exhibit 1). The AQI data based on all criteria pollutants are not directly comparable over this time frame, because PM2.5 measurements were first included in the index in 1999. For this reason, the indicator also presents AQI trends based strictly on ozone and PM2.5 measurements.
The percent of days with AQI greater than 100 based on all criteria pollutants are higher than those reported in previous releases of this indicator because this version of the indicator applies the new 2010 1-hour NAAQS for SO2 (75 ppb) and NO2 (100 ppb) retroactively when calculating AQI. The previous releases of this indicator were based on the NAAQS for these pollutants that were available at the time.
AQI Based on Ozone Only
For a nearly identical subset of metropolitan areas, the percent of days with AQI values greater than 100 due to ozone levels alone (based on the 2008 NAAQS) decreased from 7.5 over the 1990-1992 time frame to 3.3 over the 2007-2009 time frame (Exhibit 1).
AQI Based on PM2.5 Only
In the 1999-2001 period, the percent of days with AQI greater than 100 due to PM2.5 concentrations was 2.1. This percentage decreased in subsequent years, falling to 0.8 percent for the 2007-2009 period.
AQI in the EPA Regions Based on All Criteria Pollutants (Except Lead)
Trends in AQI based on all criteria pollutants (except lead) between 1990 and 2009 varied across the ten EPA Regions (Exhibit 2). For all ten Regions, the percent of days with AQI greater than 100 in 2009 was lower than that in 1990, though substantial year-to-year variability occurred. However, as noted above, the AQI values for 1990 and 2009 are not directly comparable, because PM2.5 measurements did not factor into the AQI prior to 1999.
The AQI does not address hazardous air pollutants.
Air quality can vary across a single metropolitan area. In assigning a single number for each pollutant in each area, the AQI does not reflect this potential variation.
The data for this indicator are limited to metropolitan areas (CBSAs) comprising urban and suburban areas with populations greater than 500,000. Thus, this indicator does not reflect metropolitan areas smaller than 500,000 or rural areas.
The AQI does not show which pollutants are causing the days with an AQI of more than 100, or distinguish between days with AQI slightly above 100 and days with much higher AQI.
This composite AQI indicator does not show which specific metropolitan areas, or how many metropolitan areas, have problems—a specific number of days could reflect a few areas with persistent problems or many areas with occasional problems.
- This indicator only covers the days on which ambient monitoring occurred. Because PM2.5 is not sampled daily in some areas, the data presented in this indicator may understate the actual number of days on which AQI values were greater than 100 due to PM2.5 concentrations. Although ozone is not sampled throughout the year, the percent of days with AQI greater than 100 is believed to be accurate because monitoring occurs throughout the summer, when ozone concentrations are typically highest.
Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, based on AQI values computed from ambient air monitoring data for criteria pollutants found in EPA’s Air Quality System (U.S. EPA, 2010). Spreadsheets with the processed AQI data for the metropolitan areas considered in this indicator are publicly available (http://www.epa.gov/air/airtrends/aqi_info.html)through calendar year 2008; data for 2009 were compiled from queries of the Air Quality System. This indicator aggregates the processed AQI data nationally and by EPA Region.
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