What’s with the not-so-witty “high ozone, carpool tomorrow” messages on the highway?

Living in Phoenix we are all familiar with high ozone alert days.  We have all seen the not-as-witty-as-usual air quality messages on the highway message boards asking us to carpool the next day.  But what is ozone; why does the American Lung Association give Maricopa county a grade F on its State of the Air Report?

High pollution advisory sign on the 101

Ozone is O3 or three oxygen atoms bonded together. A bond is broken when there is an energy source, such as light, electricity or heat. The usual reaction to the discovery that ozone is oxygen is usually, “Oxygen is good, right?”, afterall it is what we breathe.  However, the oxygen we breathe is O2 or two oxygen atoms bonded together.  That one little atom makes a really big difference!  Our bodies cannot use ozone the way our body uses O2 because ozone is the wrong size and has very different properties.  For starters, ozone is a highly reactive oxidant. An oxidative molecule is one that wants to react or break its bonds, we call this a chemical reaction.  If energy (heat, light, electricity) is available ozone will take it and become something else, giving off a by-product as a result of the reaction. When we breathe in ozone it causes many negative side effects because it wants to react with our bodies, such as wheezing and coughing, shortness of breath, lung tissue inflammation and asthma attacks.  In plants if affects photosynthesis rates and has been found to decrease crop yield. Ever notice an increase in respiratory irritation/illness when we have stretches of high pollution days? This is due to the oxidative nature of the molecule and the fact that cells are sensitive to very small amounts of it (0.1 ppm). This video helps you visualize what that means: https://ed.ted.com/lessons/how-to-visualize-one-part-per-million-kim-preshoff-the-ted-ed-community In fact, we are so sensitive to it, we can smell it in very low concentrations.  Everyone knows what rain “smells like”, coming from the midwest this smells triggers a euphoric nostalgia in me. However, very few know that what you are smelling is ozone.  Humans can smell a teaspoon of ozone in the area of an olympic sized swimming pool. As high energy electrical charges are released as a storm comes in it forms ozone.  

Ozone is created when UV rays react with oxygen in the air.

The ozone layer is, at its closest, 6 miles above us.  When ozone is ground level and able to be breathed in, it becomes a dangerous pollutant to all life, including plants. Plants make life possible on Earth by providing O2  and glucose, two essential components for life. Low level ozone, or tropospheric ozone is a secondary pollutant. This means it is not directly emitted from a source, instead it is the result of chemical reactions between primary pollutants emitted directly from car exhaust and industry and UV rays.  Hydrocarbons, or fossil fuels, and nitrogen oxides (NOX) given off by cars and industry, use the energy in the UV rays to rearrange themselves into ozone. A by-product of this reaction is smog, often seen just at the horizon in Phoenix. Energy hungry ozone also absorbs heat bouncing off Earth and traps it, this classifies ozone as a greenhouse gas. This is why you are asked to carpool on poor air quality days, to reduce the amount of total car emissions, and therefore, help keep ozone levels down.

Phoenix is the perfect place for high ozone because of three major factors: climate, geographic location and population.  The more people you have, the more car exhaust and industry you have. Therefore, just generally there is higher ozone in urban areas than in less populated areas, however, the geography of our beautiful city compounds this.  The Valley of the Sun is just that, a valley. Think of Phoenix as the bottom of the bowl, where you mix your ingredients. The mountains around us are like the sides of the bowl, keeping the contents of the bowl, in the bowl.  Cloud cover in Phoenix acts as the lid to the bowl, trapping the contents of the bowl. Ozone gets trapped in Phoenix and the cloudy skies of monsoon trap the ozone. When the monsoon rains finally come, they help to “clean” the air of the ozone, dissolving it and pulling it down to be stored within earth.  The amazing Phoenix winds also help to clear the ozone from the city. According to U.S. Climate Data, Arizona has 3835 hours of sunshine in a year. That is an unusually high amount of available UV rays to help create ozone. Our beautiful blue skies and sunshine compound our ozone production. Add that to our population size and our geography and you have the perfect environment for ozone to form and stay.  But there is still one more factor, the climate coupled with the urban heat island effect seen in all cities, generates an enormous amount of heat. Remember that energy hungry ozone traps heat. Cities are already naturally warmer than the surrounding environment because of all the heat absorbing concrete and steel, but Phoenix is unique because of the desert environment it was created in. Phoenix is the 6th hottest city in the world according to the World Atlas.  Now add in a higher than average amount of ozone that is trapping even more heat. Ozone levels rise as much as 20% when it is hot.  

On high pollution days in Phoenix, smog can be seen on the horizon.

When one takes all this into consideration Phoenix seems like an unsafe, perhaps illogical city to live in.  Luckily we do have a system in place that allows us to make choices to limit our production of ozone and our exposure to ozone when it is high.  This is where the Air Quality Index (AQI) from the Environmental Protection Agency (EPA) comes in. The EPA has created air quality guidelines for ozone, and other air pollutants, and made this information available to the public through the AQI.  The information contained in the Air Quality Index has been gathered through controlled, replicated scientific study. Learn more about the AQI and how air quality is assessed and categorized here: https://www.airnow.gov/index.cfm?action=aqibasics.aqi. When the message board says “carpool tomorrow” it is based on the AQI and recommendations of the EPA, which are both based on good, solid science.  Consider carpooling, taking the bus or the light rail. To avoid exposure, limit time outside. Personally, I don’t let my small children outside to play on high ozone days, we find things to entertain us inside.  What does worry me is this – I, like many Phoenicians, did not grow up here, therefore I have not been exposed to high levels of ozone my whole life, my children will be. Hopefully, with everyone’s help, my childrens’ lives will see ever decreasing levels of ozone in the city of their birth and it not be something they are exposed to for the entirety of their lives.  So next time you see an alert for a high ozone day ask a friend to carpool…seriously, do it.

Author’s Note: This was just released August 13, 2019

Air Pollution May Be As Harmful To Your Lungs As Smoking Cigarettes, Study Finds

For More Information:

Author: Shelby Maguire

Shelby earned a bachelor's degree at Lake Erie College in biology and a master's degree in education at Ursuline College, both in her home state of Ohio. She currently lives in Phoenix, AZ where she was a high school science teacher for 10 years. She left the classroom to run ACE Recycling with her husband, but is an advocate for education and a life-long learner. She is passionate about science and works to educate the general public about science, specifically environmental issues. She is a self-proclaimed science nerd who loves research. She and her husband John, have three small children.

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