How Bad is Chicago’s Air?

With air pollution, averages don’t tell us a whole lot. We need to look at specific locations to see if air quality is really improving. Let’s examine Chicago (Cook County). As the center of a big ‘ol historical industrial area, you’d expect Chicago to have some of the worst air in the country.

Cook County	1998	2007
Maximum AQI	159	182
90th Percentile	87	93
Median	50	54
Number of Days when Air Quality was…
Good	186	154
Moderate	156	182
Unhealthy for Sensitive Groups	22	23
Unhealthy	1	2
Number of Days When Main Pollutant Was:
CO	32	0
Ozone	128	120
Sulfur Dioxide	15	1
PM 2.5: Particulate matter < 2.5 micrometers	147	222
PM 10: Particulate matter < 10 micrometers	43	22

(Source: Illinois 1998 and Source: Illinois 2008)

You can probably spin these numbers however you like. On the surface, Chicago had fewer Good days and more Unhealthy days in 2007 than 1998. The primary cause would seem to be more days with lots of Small Particulates (< 2.5) in the air. On the other hand, there were half as many days with large particulates (the < 10 micrometers kind that you can choke on) and far fewer days where Sulfur Dioxide or Carbon Monoxide were the primary pollutant. Of course, it makes you wonder how much of those particulates come from chocolate?

FWIW, in 2007 Chicago had the sixteenth worst air in the United States.

AQI Statistics

Maximum: The highest daily AQI value in the year. The highest possible AQI value is 500 (i.e., you live inside a steel mill.)

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 Air Quality was…

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.