This webpage is for students taking the "Houston CAN" lesson about ozone air pollution.

HCAN Logo Image


GOAL: Understand what causes ozone pollution and how we can track it using the Houston Clean Air Network.


We begin with an explanation of air pollution and ozone. For more information, check out Air Pollution 101.


Houston Clean Air Network


To get started with the lesson, click on



The Houston CAN map shows Houston with a colorful overlay of ozone levels across the region right now.

Click on various spots on the map.

What do you see when you click on the map?



HCAN Slider


The slider moves up and down the Air Quality Index and explains what each value means. You can also move the slider yourself.

What does the Air Quality Index say about one of the values you found?



To learn more about the Air Quality Index, click on the “?” in the top right corner of the map.




At what level of the Air Quality Index do you think you might need to limit your exposure to ozone pollution?



Now it’s time to search for an address. Click on the field that says “Set Time/Place.” Enter an address and write it down so you can remember it.

HCAN  2015-08-23 Leeland



What is the ozone level at that address right now?

What does the Air Quality Index say about ozone at that level?

If you were exposed to ozone at that level, what should you do?




On different days and times, the ozone value will be different. Click on “Set Time/Place” and enter a new date and time. After the new date and time have loaded, reenter the address you used before.


HCAN 2015-08-11 Leeland


Did the ozone value change?

What was the new ozone value?



High Ozone Days in Houston


Now let’s look at some high ozone days. August was a particularly bad month for ozone in 2015. Use “Set Time/Place” to check out some dates in August.

You can also view animations of ozone plumes moving through the city. Once you’ve selected a date, click “Play” to animate.

Here are a few dates in August with high ozone.

August 9, 2015

HCAN 2015-08-09








August 11, 2015

HCAN 2015-08-11



August 12, 2015

HCAN 2015-08-12








August 13, 2015

HCAN 2015-08-13_3



What did you notice about ozone levels on these days?

What was the highest ozone level you saw?

What was the recommended action at that level?




Possible Causes of Ozone Days


Remember the lesson on ozone formation. What do we need in the atmosphere to form ozone?



Ozone Formation 6



We can use tools from the Texas Commission on Environmental Quality (TCEQ) to find out if any of these chemicals were emitted on high ozone days. 

The TCEQ keeps records of all major pollution events. You can look up a specific date, time, and location and see if there were any pollution events then.


To get started, click on the TCEQ Air Emissions Event Report Database.





Let’s look at emissions event in August. Click on “I just want to search by date and county or region.

TCEQ AEER Date Search

For “Event began on or after this date” enter: August 1, 2015.

For “Event began on or before this date” enter: August 1, 2015.

Our region is Harris County, so select “HARRIS” from the drop down menu for County.

When you’re all done, click “Submit.”


Having trouble getting it all entered? Click this link to go straight to the the search results.



The TCEQ Air Emissions Event Report Database will search for any pollution events on the dates and County we entered.


 What do you see?

TCEQ AEER August Harris Results

Click on the “Tracking Number” to learn more about an individual emissions event. The tracking number links to an Air Emissions Event Report. It lists the time, date, location, and duration of a pollution event. It also lists a possible cause for the event, any corrective action taken, and the method used to estimate the amount of pollution emitted.

Here is an example of an Air Emissions Event Report.

TCEQ AEER Shell Oil Deer Park 8-04


Select any individual Air Emissions Event Report and answer the following questions:


Which Air Emissions Event Report did you select?

When did this pollution event occur?

What caused this event?



The Air Emissions Event Report also lists sources of emissions and the pollutants emitted.

What are some of the pollutants emitted in this event?



As we learned in our lesson on ozone formation, Volatile Organic Compounds (VOCs) are one precursor to ozone pollution. You can see a list of VOCs here.

Are any of the chemicals listed in the Emissions Event Report Volatile Organic Compounds?

List the name and amount of any Volatile Organic Compounds you see in the report.



Remember, it takes nitrogen oxides, volatile organic compounds, and sunlight to form ozone. We almost always have plenty of nitrogen oxides in the air in Houston. On sunny days, we have plenty of sunlight. VOCs are often all that’s missing for ozone to begin forming in Houston.

Many of our industrial facilities in Houston are large sources of VOCs.


Is is possible that some of the emissions events in August contributed to high ozone days?


Is it possible that this?



Caused this?

HCAN 2015-08-12


For more information about how Houston CAN protect itself from ozone pollution, visit or