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Ozone Is the Most Important Air Pollutant

In several previous posts, I have discussed the air quality data maintained in the EPA’s Air Quality System Data Mart. The EPA calculates an Air Quality Index (AQI) that combines the various pollutants into an index intended to represent overall air quality. But the EPA also identifies how many days each pollutant was the main one.

I identified 19 Missouri counties for which AQI data was available in more than 6 of the 12 years for which I downloaded data (1983, 1993, 2003-2012). The main air pollutant varies among them. For instance, in 2012, the main pollutant in Taney County was ozone on all 214 days monitored. However, in Iron County, sulfur dioxide was the main pollutant on all 351 days monitored.

Main Pollutant ChartDespite the fact that it might not apply to all locales, I thought it might be useful to explore how the most important pollutant has changed over time. The method I used to do this is described below. The graph at right shows percentage of days each pollutant is the main pollutant (for all 19 counties combined). In the graph, CO = carbon monoxide, NOX= nitrogen oxides, O3 = ozone, SO2 = sulfur dioxide, PM2.5 = inhalable particulates smaller than 2.5 micrometres, and PM10 = inhalable particulates between 2.5 and 10 micrometres.

PM2.5 was the most important zero percent of the time in 1983 and 1993, but increased in importance markedly thereafter. I believe this occurred because of changes in national standards and monitoring practices–the current standard for PM2.5 was established in 1997. EPA is in the process of changing this standard.

Sulfur dioxide has declined significantly in its importance. Where once it was the most important pollutant on almost half of all monitored days, now it is most important on less than 10%. Carbon monoxide has also declined in importance. In 1983, it was most important on almost 20% of days. Since 2003, however, it has not been the most important pollutant on any monitored day in any county in Missouri.

On the other hand, PM2.5 and ozone have each increased in importance. Although it would not be true in every location, across the state ozone is now the most important pollutant on approximately 50% of all days.


Air Data, EPA, http://www.epa.gov/airquality/airdata.

For PM2.5 standard: Overview of EPA’s Proposal to Revise the Air Quality Standards for Particle Pollution (Particulate Matter), EPA, http://www.epa.gov/pm/pdfs/PMNAAQSProposalOVERVIEW61512UPDATED.pdf.

Method used to construct the graph:

For each county, the EPA reports the number of days each year a pollutant is the main one. For each pollutant, I summed the values across the 19 counties, yielding the total number of county-days for which each pollutant was most important (call this number “A”). I summed these, yielding the total number of county-days for which importance estimates were made (call this number “B”). Then, I divided A by B, and multiplied by 100, yielding the percentage amounts used in the graph.

Fewer Unhealthy Air Days

Unhealthy Mississippi ChartIn my previous two posts, I have discussed the air quality data maintained in the EPA’s Air Quality System Data Mart, and I have reported on the number of days with good air quality. It is one thing to ask whether a county’s air quality is good, and another to ask if it is so bad that it is unhealthy. This post focuses on the number of days with unhealthy air.

I downloaded data going back 10 years (2012-1993), and for a longer term perspective, for 1993 and 1983. Nineteen Missouri counties have been monitored for more than 6 of those 12 years. I put them in three groups: a group along the Mississippi River north and south of St. Louis, a group in the Kansas City-St. Joseph region, and a widely dispersed “other” group.

The EPA data distinguishes four levels of unhealthy air: Unhealthy for Sensitive Individuals, Unhealthy, Very Unhealthy, and Hazardous. No Missouri county is reported to have Very Unhealthy or Hazardous air quality for any of the years I studied. The graphs on the right show the percent of monitored days for which air quality was either Unhealthy for Sensitivie Individuals, or Unhealthy.

Unhealth KC ChartThe top graph is for counties along the Mississippi. It shows that since 1983, there are far fewer unhealthy air days. The lone exception is for Jefferson County, where the number of unhealthy air days has increased: there, the number of unhealthy air quality days runs somewhere between 1 in 6 and 1 in 4. In all counties the number ticked up in 2012.

The second graph is for counties in the Kansas City-St. Joseph area. As we saw in the graphs for good air quality (previous post), the region started off with better air than did the Mississippi counties. Jackson County started with zero days of unhealthy air in 1983, and less than 2% in 1993. This seems to be an unusual finding, and one wonders if it is an error. The overall trend here seems to be flat, although the counties tend to show the same uptick in 2012 as the counties along the Mississippi.

The third graph is for Other counties. Iron County had a significant number of unhealthy air days in 1983-2003. After that, however, the number of unhealthy air quality days plummeted, I don’t know why. For all of the counties, the percentage of unhealthy air days is quite low, although it upticked slightly in 2012.

Unhealthy Outstate ChartThe uptick seen in 2012 for virtually all counties is probably related to the drought and record heat experienced around the state in that year. (See here for my post on it.)

I started this series of posts remembering The Day the Sun Didn’t Shine, November 28, 1939 in St. Louis. Compared to that, our air quality has clearly improved. Even since 1983, the number of unhealthy air days seems to have significantly decreased. However, the trend with good air quality days is not so marked. A significant portion of the state, especially in the large metropolitan areas, is still breathing air that is less than good on 3-5 out of every 10 monitored days.

Added 10/30/13:

Since posting the above, I have discovered that the Department of Natural Resources operated an air pollution monitoring site in Glover, MO. In 2003, the Glover Lead Smelter closed, and perhaps this accounts for the sudden improvement in sulfur dioxide levels in Iron County.


Air Data, EPA, http://www.epa.gov/airquality/airdata.

On Some Days, Air Quality is Good

The Air Quality System Data Mart maintained by the EPA contains data on the air quality of a number of Missouri counties going back to the early 1980s. I downloaded data going back 10 years (2012-1993), and for a longer term perspective, for 1993 and 1983.

Good Mississippi ChartNineteen counties have been monitored for more than 6 of those 12 years. I put them in three groups: a group along the Mississippi River north and south of St. Louis, a group in the Kansas City-St. Joseph region, and a widely dispersed “other” group.

For a fuller discussion of the data maintained by the EPA, see my previous post Is Air Quality Better or Worse?

The graphs at right show the percent of monitored days on which the Air Quality Index was in the Good Range. The top graph is for the Mississippi counties, the middle one is for the Kansas City-St. Joseph counties, and the bottom one is for the Other counties.

There is a higher percentage of good air days in the Other counties. This is, perhaps, not surprising, as these counties tend to be more rural in character, and they lack the very large metropolitan areas in the other two groups.

Good KC ChartThe counties along the Mississippi seem to fall into a group of four counties that has more good air quality days, and a group of three that has fewer. The latter group includes St. Louis City and St. Louis County, plus Jefferson County. Air quality in these three was good in less than 60% of the monitored days. It is difficult to infer trends over time, but the general trend seems to be towards more good air quality days. In particular, St. Louis City, St. Louis County, and St Charles County all experienced significant increases in the number of good air quality days between 1983 and 1993. Jefferson County, on the other hand, experienced a large decrease in the number of good air quality days.

In the Kansas City-St. Joseph area, the counties generally started off in 1983 with a higher percentage of good air quality days. However, the trend seems to be towards fewer good air quality days. Jackson, Cass, and Buchanan Counties now have less than 70% good air quality days. The drop in air quality experienced by Buchanan County in 2008 stands out, and I do not know its cause.

Good Outstate ChartThe Other counties are distributed widely around the state. All had more than 50% good air quality days throughout the entire period, and a significant fraction above 70%. The trend here appears to be variable, with some counties experiencing more good air quality days, some fewer.

It is important to know whether air quality is good, but it is also important to know if it is bad enough to be unhealthy. I will report on that in my next post.


Air Data, EPA, http://www.epa.gov/airquality/airdata/

Is Air Quality Better or Worse?

Missouri has a notorious role in the annals of air quality. During the first part of the 20th Century smoke pollution had been a problem in St. Louis. In 1939 it was particularly bad, however. On November 28, 1939, a temperature inversion trapped pollutants; a thick cloud of dark smoke blanketed St. Louis, blotting out the sun. The day came to be known as “Black Tuesday,” or “The Day the Sun Didn’t Shine.” You can view photos by searching for “black tuesday St. Louis” on Goggle Images. It was one of the worst air quality events in recorded history.

Since then, many steps have been taken to reduce air pollution, and Black Tuesday events don’t occur any more. But how have we been doing recently? Is our air quality still improving, or has it peaked and started to backslide?

I haven’t been able to find a report that summarizes air quality data for the state. Since the 1980s, however, the EPA has gathered air quality data from cities and counties in Missouri and maintained it in a national database, and this data can be used to construct an analysis.

I downloaded data going back 10 years, from 2012 to 2003. In addition, to give a longer term perspective, I downloaded data for 1993 and 1983.

AQI CountiesNineteen counties have been monitored for more than 6 of the 12 years for which I downloaded data. A map showing their locations is at right. 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.

Air quality tends to be a function of a number of pollutants that are emitted into the atmosphere: particulates, ozone, carbon monoxide, nitrous oxide, and sulphur dioxide are five of the most important.

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. The small ones (less than 2.5 micrometers – 100 times thinner than a human hair) are the most dangerous.

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, too much of it can cause lung damage.

Nitrogen oxides react with ozone and sunlight to form smog, and they irritate the lungs when breathed.

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, the main ingredient of acid rain.

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. 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, but 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. EPA also combines small particulate and ozone levels into an 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 EPA has created six broad AQI ranges: Good, Moderate, Unhealthy for Sensitive Individuals, Unhealthy, Very Unhealthy, and Hazardous. EPA reports a yearly AQI number and the number of days in which the AQI falls in each range.

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


1939 St. Louis Smog, Wikipedia, http://en.wikipedia.org/wiki/1939_St._Louis_smog.

Air Quality Index Report, http://wwwepagov/airdata/ad_rep_aqihtml.

What Are the Six Common Air Pollutants? EPA, http://www.epa.gov/airquality/urbanair.

Toxic Releases in Missouri

Many industrial processes require the use of toxic substances. These substances must be properly handled to prevent harm to people, land, and water. During the 1970s and early 1980s concerns grew about how toxic substances were being handled. For instance, tons of toxic waste were discovered dumped in the Love Canal neighborhood of Niagara Falls. Oil containing dioxin was sprayed on the streets of Times Beach, Missouri, turning it into a ghost town; people can’t live there to this day. Concerns came to a head in 1984, when a malfunction at a chemical plant in Bhopal, India released a cloud of poisonous gas that killed more than 3,000 people overnight, and 15,000-20,000 eventually (5-7 times as many as were killed in the 9/11 attacks). Shortly thereafter, a serious release of toxic gas occurred in Institute, West Virginia. In response, Congress passed the Emergency Planning and Community Right-to-Know Act in 1986, and the Pollution Prevention Act in 1990. These laws require facilities to report releases, transfers, and waste management activities of toxic materials.

The Toxics Release Inventory (TRI) program of the EPA gathers this information and makes it available to the public on their website. In addition, they publish annual fact sheets, including fact sheets for each of the 50 states. The TRI data does not cover all toxic materials and all facilities, but it does cover an important set of them.

After being used, toxic substances are recycled, used to generate electricity, injected into wells, stored, landfilled, emitted into the air, discharged into surface water, and spread over the land. They can be handled either on-site or off-site. Determining whether any of these activities represent a potential hazard to people, land, or water is complex. One cannot simply assume, for instance, that on-site means safe. On the other hand, one cannot assume that emission or discharge of the substance means that there is toxic exposure. The statistics in the TRI are only a starting point, and many factors must be taken into consideration when analyzing TRI data.

MO Tox Rel 2002-2011Current TRI fact sheets for Missouri cover the years 2002-2011. In the graph at right, the blue line shows total releases and disposal of toxic materials in Missouri over that time period. In 2007, EPA changed the minimum reporting threshold from 2,000 to 5,000 pounds, suggesting that comparisons between years may not be valid. Over the entire time period, toxic releases and disposal decreased by 4%. While there does not appear to be a significant discontinuity in the data following 2007, the data shows a rapid increase until 2005, followed by a rapid decrease. The percentage differences are very large: between 2002 and 2004 the increase was 56% From 2004-2011, the decrease was 38%. Such large , rapid changes may, indeed, reflect changes in data gathering rather than actual changes in the amount of toxic materials.

It might be possible to trace the data-gathering changes made by EPA and do and “apple-to-apples” comparison. Such a study would be welcome.

Because toxic materials are most frequently used in manufacturing, I thought that it might be interesting to compare toxics to the level of manufacturing. I cannot find a time series that shows the output of the manufacturing sector in Missouri from 2002-2011. I have found, however, a time series that tracks manufacturing employment. It is shown as the red line on the graph. Manufacturing employment in Missouri declined rather steadily between 2001 and 2011, by 24%. The shape of the red line is not a good fit to the shape of the blue line, however. While a decline in manufacturing may account for some of the decline in toxic materials between 2005 and 2011, it does not explain the increase between 2002 and 2005.

Source: Factors to Consider When Using Toxics Release inventory Data, EPA.

Source: Factors to Consider When Using Toxics Release inventory Data, EPA.

The second graph is taken from Factors to Consider When Using Toxics Release Inventory Data, published by the EPA. The graph shows total toxic releases and disposal for the United States (the columns) and the number of reporting facilities (the red line). The overall shape of the national data is similar to the Missouri data, except that the peak occurred 5 years earlier. The text underneath the graph explains various changes in the TRI data that are marked on the graph.


Information on the Bhopal disaster comes from various sources, including:

“Bhopal Disaster,” Encyclopedia Britannica, http://www.britannica.com/EBchecked/topic/1257131/Bhopal-disaster.

“Bhopal Disaster,” Wikipedia, http://en.wikipedia.org/wiki/Bhopal_disaster.

Government of Madhya Pradesh Bhopal Gas Tragedy Relief and Rehabilitation Department, http://www.mp.gov.in/bgtrrdmp/relief.htm.

For United States toxic releases:

State Fact Sheet, TRI Explorer, for 2002-2011, http://iaspub.epa.gov/triexplorer/tri_factsheet_search.searchfactsheet.

Manufacturing Employment in Missouri, FRED Economic Data, Federal Reserve Bank of St. Louis, http://research.stlouisfed.org/fred2/graph/?s[1][id]=MOMFGN

Factors to Consider When Using Toxics Release Inventory Data, EPA, 7/24/2012, http://www.epa.gov/tri/tridata/index.html.

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