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Air Quality Update, 2017


I last looked at Missouri air quality data through the year 2016. This post begins a series to update the information through 2017. First will come an introduction to the Air Quality Index (AQI) criterion pollutants, then 2 posts on AQI trends over the years, then a post on which are the most important pollutants, and finally, a post on why air quality is important for human health.

Figure 1. The St. Louis Cathedral viewed from the Park Plaza on Black Tuesday (11/28/1939). Source: St. Louis Post-Dispatch.

Missouri has a notorious role in the annals of air quality, for 2 reasons. First, on November 28, 1939, a temperature inversion trapped pollutants in St. Louis; a thick cloud of dark smoke blanketed the city, blotting out the sun. The day came to be known as “Black Tuesday,” and it was one of the worst air quality events in recorded history. Figure 1 at right shows a view that day of the St. Louis Cathedral from (I think) the Park Plaza. More photos are available by searching on Google Images for “Black Tuesday St. Louis.” Second, St. Louis was one of 6 cities included in a 1993 study that conclusively showed a relationship between air pollution and mortality. St. Louis was the second most polluted city in that study. (Dockery et al, 1993)

Since then, many steps have been taken to reduce air pollution, and air quality has improved dramatically. Has the trend continued, or has the trend begun to backslide?

Since the 1980s the EPA has gathered air quality data from cities and counties in Missouri and maintained it in a national database. The following posts look at yearly data from 2003-2017. In addition, to give a longer term perspective, they include data for 1983 and 1993.

Figure 2. Counties for which the EPA reports air quality. Data source: Environmental Protection Agency.

The EPA data now includes 24 counties. In some of them, however, air quality has not been measured for the entire period. Figure 2 is map showing the locations of the 24 counties. They can be gathered into three groups: a group along the Mississippi River, a group in the Kansas City-St. Joseph Area, and a widely dispersed group that does not fall into either of the other two groups.

The EPA constructs an Air Quality Index (AQI) based on measurements of 6 criterion pollutants: particulates smaller than 2.5 micrometers particulates between 2.5 and 10 micrometers, ozone, carbon monoxide, nitrous oxide, and sulphur dioxide.

Particulates are tiny particles of matter that float around in the atmosphere. When we breathe, we inhale them, and if there are too many of them, they cause lung damage. There are 2 sizes: inhalable coarse particles have diameters between 2.5 and 10.0 micrometers, while fine particles have diameters less than 2.5 micrometers. How small is that? The diameter of a human hair is about 70 micrometers, so they are roughly 1/30 the width of a human hair. Figure 3 illustrates the size difference – these are really tiny particles. Recent evidence suggests that fine particles cause serious health problems; they get deep into the lungs, sometimes even getting into the bloodstream. (EPA 2015)

Figure 3. Size difference between human hair and PM2.5 particle.

Ozone is a highly corrosive form of oxygen. High in the atmosphere, we need ozone in order to absorb ultra-violet radiation. But at ground levels, it is corrosive to plants and animals, and too much of it can cause lung damage.

Sulphur dioxide smells like rotten eggs. Too much of it causes lung damage, and it also reacts with water vapor in the atmosphere to form sulphuric acid, one of the main ingredients of acid rain. A series of posts I wrote on background air pollution shows that background levels of sulphur dioxide have decreased over the last 30 years. However, concentrations of it can still build up and affect public health near emission sources.

Nitrous oxide is corrosive and reacts with ozone and sunlight to form smog. It is also one of the main causes of acid rain. Background levels in the atmosphere have decreased, but it, too, can build up locally near emission sources.

Perhaps the most important air pollutant of all, carbon dioxide, is not one of the criterion pollutants. It is not included in the AQI, and is not included in the discussion in the following posts. Carbon dioxide is the primary cause of climate change. I have written extensively on climate change in this blog, and interested readers can consult those posts by clicking on “Climate Change” at the top of the page or by looking for specific titles in the Table of Contents.

The biggest sources of air pollution are power plants, industrial facilities, and cars. These tend to concentrate in urban areas, but air quality can be a concern anywhere; some of Missouri’s air quality monitoring stations are located near rural lead smelters, for instance. Indeed, in my posts about the largest GHG emitting facilities in Missouri (here), I discovered that 7 out of 10 were located in rural areas.

In addition, weather plays an important role in air quality. On some days, weather patterns allow pollution to disperse, but on others they trap it, causing air quality to worsen. Hot, sunny summer days are of particular concern, although unhealthy air quality can happen any time. Black Tuesday was in November, after all.

The EPA has established maximum levels of each pollutant, and reports the number of days on which there are violations. The EPA also combines the pollutants into an overall Air Quality Index, or AQI, in order to represent the overall healthfulness of the air. The AQI is a number, but it does not have an obvious meaning. Suppose the median AQI is 75 – what does that mean? So the EPA has created six broad AQI ranges: Good, Moderate, Unhealthy for Sensitive Individuals, Unhealthy, Very Unhealthy, and Hazardous. The EPA reports a yearly AQI number and the number of days in which the AQI falls in each range.

In the following posts, I will update Missouri’s AQI, then the specific pollutants that seem to cause repeated problems.


Dockery, Douglas W., Arden Pope III, Xiping Xu, John D. Spengler, James H Ware, Martha E. Fay, Benjamin G Ferris, and Frank E. Speizer. 1993. “An Association Between Air Pollution and Mortality in Six U.S. Cities.” The New England Journal of Medicine, 329 (4), pp. 1753-1759.

Environmental Protection Agency. Air Quality Index Report. This is a data portal operated by the EPA. Data downloaded on 7/31/2017 from http://www.epa.gov/airdata/ad_rep_aqi.html.

Environmental Protection Agency. 2015. Particulate Matter: Basic Information. Viewed online 3/23/2017 at https://www.epa.gov/pm-pollution.

St. Louis Post-Dispatch. Look Back: Smoky St. Louis. This is a gallery of photos concerning the 1930s smog problem in St. Louis. Photo purchased online from http://stltoday.mycapture.com/mycapture/folder.asp?event=896392&CategoryID=23105.

Wikipedia. 1939 St. Louis Smog. Viewed 11/6/15 at https://en.wikipedia.org/wiki/1939_St._Louis_smog.


  1. […] the progress Missouri has made in improving its ambient air quality several times, most recently here. We often ignore indoor air quality when we discuss air pollution, however. I don’t know how you […]

  2. […] by the EPA. In general, air quality has improved significantly. (The most recent series starts here.) The Air Quality Index monitoring program in Missouri focuses on large metropolitan areas or […]

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