About the Air Quality Index Summary Report

What does the report tell me?

The AirData Air Quality Index Summary Report displays an annual summary of Air Quality Index (AQI) values for counties or metropolitan statistical areas. Air Quality Index is an approximate indicator of overall air quality, because it takes into account all of the criteria air pollutants measured within a geographic area. Although AQI includes all available pollutant measurements, you should be aware that many areas have monitoring stations for some, but not all, of the pollutants.

Each row of the AQI Summary Report lists summary values for one year for one county or metropolitan statistical area (MSA). The summary values include both qualitative measures (days of the year having "good" air quality, for example) and descriptive statistics (median AQI value, for example). Each row also includes a link to a chart of the daily AQI values summarized in that row.

What does the report look like?

The example shows the two types of AQI information in the report -- AQI statistics, including columns for the annual maximum, 90th percentile, and median of daily AQI values; and number of days per year that each criteria air pollutant predominated in determining the AQI value. Each row gives annual AQI values for a county (or an MSA).

How can I customize the report?

Sort Order

You can use the SORT buttons in each report column to change the order of rows in the report. The default sort order is median AQI value, from highest to lowest.

AirData automatically adds maximum AQI value as a secondary sort key for most report columns. If a report includes rows with identical values for the primary sort key, which you choose with the SORT buttons, those rows are arranged in descending order by AQI maximum value.

Detail or Summary

This report always displays annual summary AQI values. Omitting optional report columns does not change the level of summarization. A row of the report always displays data for a single county or MSA, for a single year. You may see a chart of detailed data (AQI daily values) by clicking the "See chart" hypertext link at the end of each row of the report.

Because of the report organization, one row is indistinguishable from another if you omit columns that identify geographic area (county / MSA name, county / MSA code). Although you may omit both of these columns, it is generally not a useful thing to do.

Type of Summary

AQI values may be summarized at two geographic scales: county or metropolitan statistical area (MSA). Data from all monitoring sites within a geographic area - a county or MSA - are combined to obtain the summary. Because MSAs are composed of counties, an MSA summary generally includes results from more monitoring sites than a county summary.

If you select MSA summary type, the report includes AQI data for every MSA that is wholly or partly within the geographic area you chose for the report. For example, the MSA for the city of St. Louis, MO, includes counties in both Missouri and Illinois. AQI data for the entire St. Louis MSA would be included in a report for the state of Missouri or for the state of Illinois.

What do the report columns mean?

The AQI Summary Report includes the columns listed below. You may omit columns by deselecting the checkbox beside their names. At least one column must be selected to generate a report.

Row #

Sequence number of report rows (lines). Sequence numbers are not associated with particular rows; they simply enumerate the rows of a report from first to last. Thus, choosing an alternate sort order for a report would change the sequence numbers associated with particular rows.

# Days with AQI

Number of days in the year having an Air Quality Index value. This is the number of days on which measurements from any monitoring site in the county or MSA were reported to the AQS database.

Number of Days when Air Quality was...

These columns indicate how the daily AQI values for a county or MSA were distributed among four broad categories of air quality:

Good: Number of days in the year having an AQI value 0 through 50.
Moderate: Number of days in the year having and AQI value 51 through 100.
Unhealthy for Sensitive Groups: Number of days in the year having an AQI value 101 through 150.
Unhealthy: Number of days in the year having an AQI value 151 or higher. This includes the AQI categories unhealthy, very unhealthy, and hazardous. Very few locations (about 0.3% of counties) have any days in the very unhealthy or hazardous categories.

AQI Statistics

These columns provide simple statistical measures of the AQI values for a county or MSA:

Maximum: The highest daily AQI value in the year.

The highest possible AQI value is 500. Rarely, a pollutant concentration exceeds the level equivalent to AQI 500. In these instances, AQI value is given as 501 to indicate "greater than 500."
90th percentile: 90 percent of daily AQI values during the year were less than or equal to the 90th percentile value.
Median: Half of daily AQI values during the year were less than or equal to the median value, and half equaled or exceeded it.

Number of Days when AQI pollutant was...

A daily index value is calculated for each air pollutant measured. The highest of those index values is the AQI value, and the pollutant responsible for the highest index value is the "Main Pollutant." The criteria pollutants used to calculate AQI are:

CO - Carbon monoxide
NO2 - Nitrogen dioxide
O3 - Ozone
SO2 - Sulfur dioxide
PM2.5 - Particulate matter smaller than 2.5 micrometers
PM10 - Particulate matter smaller than 10 micrometers

These columns give the number of days each pollutant measured was the main pollutant. A blank column indicates a pollutant not measured in the county or MSA.

Year

Calendar year to which the AQI values pertain.

The report includes the following two columns if you select county summary type.

County: Name of the county for which AQI values are reported.
County Code: The 5-character FIPS code of the county for which AQI values are reported, consisting of 2-digit state code and 3-digit county code. FIPS is the acronym for Federal Information Processing Standards, which defines codes used in most U.S. government information systems.

The report includes the following two columns if you select MSA summary type.

MSA: Name of the Metropolitan Statistical Area for which AQI values are reported.
MSA Code: The 4-digit code of the Metropolitan Statistical Area for which AQI values are reported.

State: Postal abbreviation for the state or territory in which the county or MSA is located. [ Details ]
Region: EPA region number in which the county or MSA is located. There are ten EPA regions. [ Details ]
AQI Daily Values: Clicking the See chart hypertext link displays a chart (plot) of the daily Air Quality Index values summarized in the report row. A chart shows each day's index value and main pollutant.