The Air Quality Index (AQI) and symptoms of carbon monoxide poisoning

Here are a couple of short topics that I often have trouble squeezing into a class lecture.

Air quality index

A large metropolitan area like Tucson and Pima County is required to continuously measure concentrations of several air pollutants.  You can read more about air quality monitoring done by the Pima County Department of Environmental Quality here.   A photograph of one of the Tucson area monitoring sites (click here to see a map of all 18 monitoring sites) is shown below.

monitoring site at Corona de Tucson (source)
The main pollutants being monitored are shown below (see p. 8 in the ClassNotes). 




The concentration of lead in the air has decreased significantly since lead was removed from gasoline (the following quote is from a Wikipedia article on gasoline: "In the US, standards to phase out leaded gasoline were first implemented in 1973 ..... In 1995, leaded fuel accounted for only 0.6% of total gasoline sales ...... From 1 January 1996, the Clean Air Act banned the sale of leaded fuel for use in on-road vehicles. Possession and use of leaded gasoline in a regular on-road vehicle now carries a maximum $10,000 fine in the US.")

In Tucson, carbon monoxide, ozone, and particulate matter are of primary concern and measurements are reported online.  If I were to tell you that the measured carbon monoxide concentration yesterday was 4.5 ppm (averaged over an 8 hour time period) would you be able to tell me whether that was high or low, hazardous or not?  Most people wouldn't be able to answer that question.  Many people wouldn't know that ppm stands for "parts per million".  Those are units of concentration (4.5 CO molecules mixed in with 1 million air molecules). 

Rather than reporting the actual measured values, an Air Quality Index value is reported instead.    The AQI is the ratio of the measured to accepted concentrations multiplied by 100%.



The acceptable levels are known as the National Ambient Air Quality Standards (NAAQS).  The numbers are different depending on the particular pollutant.

The NAAQS values for carbon monoxide are:
9 ppm     (average value over an 8 hour period)
35 ppm     (average over a 1 hour period)    

So we can compute the AQI for a measured CO concentration of 4.5 ppm.        



The air quality in this case would be acceptable.  Air becomes unhealthy when the AQI value exceeds 100%.  It's somewhat like computing your percentage grade on a quiz.  You divide the points earned by the total number of points possible and multiply by 100%.  100% on a quiz is good, an air quality index value of 100% is bad.

Current Air Quality Index values (click on Current Measurements than on Air Index Now at the left of the screen)

Symptoms of carbon monoxide poisoning

Carbon monoxide is a serious hazard indoors where it can build to much higher levels than would ever be found outdoors.  This next link is to a newspaper article describing an incident at Virginia Tech (that occurred near the beginning of the school year in 2007).   Carbon monoxide from a malfunctioning hot water heater sickened 23 Virginia Tech students in an apartment complex.  The CO concentration is thought to have reached 500 ppm.  You can get an idea of what kinds of symptoms and effects that concentrations this high would cause from the figure in the middle of p. 9 in the photocopied ClassNotes.





The effects of CO depend on what concentration you exposed to and the duration of the exposure.  In this case we'll follow the arrows from lower left to the upper right of the figure.  The arrows represent a concentration of about 500 ppm.  Beginning at lower left we see that we wouldn't experience any symptoms with an exposure to even 500 ppm for just a few minutes.  Note also the NAAQS values near the bottom of the graph.  Beginning at about 1 hour exposure the arrows cross from the lower green half  to the upper yellow and orange half of the graph.  Beginning at 1 hour you would experience headache, fatigue, dizziness, nausea.  The symptoms would worsen if the exposure lasted for a few hours: throbbing headache, nausea, convulsions, and collapse.  The 500 ppm line comes very close to coma and death part of the graph.   At Virginia Tech several students were found unconscious and a few had difficulty breathing on their own but were resuscitated; they very nearly died.

Carbon monoxide alarms are relatively inexpensive (~$50) and are available at most hardware stores.  I've got one in my house to protect me and my cats.  They will monitor CO concentrations indoors and warn you when concentrations reach hazardous levels.   Indoors CO is produced by gas furnaces and water heaters that are either operating improperly or aren't being properly vented to the outdoors.  A few hundred people are killed indoors by carbon monoxide every year in the United States.  An operating carbon monoxide alarm probably saved the lives of the 6 Tucson residents in December 2010.  You can learn more about carbon monoxide hazards and risk prevention at the Consumer Product Safety Commission web page.