This website was developed by undergraduate biology researchers working with Dr. Karen Bernd at Davidson College.

Ozone (O3)


What is Ozone?

Ozone (O3) is a colorless, odorless gas comprised of three oxygen atoms, making it a reactive oxgyen species (ROS).  Depending on where it is located in the atmosphere, ozone can have both beneficial and deleterious effects on your health (see figure 1).

Stratospheric Ozone
In the stratosphere, ozone acts as a protective barrier for the earth by absorbing the majority of the sun’s ultra violet rays.  This type of ozone is produced naturally and has yearly cycles where an ozone 'hole' grows and shrinks but is slowly being degraded by man-made chemicals called ozone-depleting substances (ODS) (EPA, 2009).

Tropospheric Ozone (Ground-level Ozone)
While ozone may be helpful up high in the stratosphere, ground-level ozone has the potential to damage our lung epithelial cells through cell death or alterations to cellular processes (EPA, 1999; EPA 2003).  This oxidative damage causes irritation and inflammation of the respiratory tract, which can lead to increased susceptibility to allergens, pollutants and infections (EPA, 1999). It also contributes to the oxidation and degradation of everything from plastic park benches to metal sky scrapers.

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Figure 1. Diagram showing the difference between "good" and "bad" ozone is just location

Where does ground level ozone come from?

Ground level ozone is the primary component of smog and is generated from chemical reactions between UV rays (sunlight), heat, oxides of nitrogen (NOx) and volatile organic compounds (VOCs). These NOx and VOCs are emitted into the atmosphere through exhaust from the incomplete burning of fuel in automobiles, trucks, aircrafts, ships and industrial facilities, as well as several natural sources (EPA, 1999).

Ozone levels tend to increase during the day an peak in the early evening. Levels also tend to be higher in summer months. This is because reactions that cause ground level ozone require sunlight's UV rays and the heat of the day to have time to work on theprimary pollutants, NOx and VOC. During the night fewer primary pollutants are made (less driving, industry) and air currents dilute the ozone formed during that day. This is why you may have been told it is better to exercise outside in the morning or why the air may just not 'breathe well' if you are out in an urban area around 5pm in July vs December.

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Figure 2. Diagram showing the generation of ground level ozone.

Why We Care

With more than 100 million people in the United States living in areas that fail the clean air quality standards set by the U.S. Environmental Protection Agency (EPA), ozone exposure remains an important topic of interest (Chuang et al., 2009; Federal Registrar, 2008).  Studies have shown links between oxidative stress and health complications such as asthma, chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis and acute respiratory distress syndrome (ARDS) (Biswas &Rahman, 2009; EPA, 1999; Chuang et al., 2009). Even at low levels, chronic exposure to ozone has been shown to cause strutural injury to the lung and chronic inflammation (Van Bree et al., 2001). To learn more about these diseases, check out our Lungs Page.

Understanding the air around us

As part of the Clean Air Act, the EPA is required to set National Ambient Air Quality Standards to monitor concentrations of certain air pollutants that are considered to harmful to the environment or health of the general public (EPA, 2010). To learn more about learn more about about these standards, check out the EPA’s webpage.

As an indicator of a location’s daily air quality and based on these standards, the EPA determines an Air Quality Index (AQI). The AQI is calculated for five major air pollutants that are regulated under the Clean Air Act:  Ground-level ozone, particle pollution (also known as particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide (AIRnow, 2010).  Of these five pollutants, ground-level ozone and airborne particles pose the greatest threat to the general public’s health in the United States (AIRnow, 2010).

The AQI is the color coded scale that you may see reported in the news. It is set to a scale of 0 to 500 and is divided by relative health impact into the six color categories.

Air Quality Index Levels of Health Concern

Numerical Value



0 to 50

Air quality is considered satisfactory, and air pollution poses little or no risk


51 to 100

Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution.

Unhealthy for Sensitive Groups

101 to 150

Members of sensitive groups may experience health effects. The general public is not likely to be affected.


151 to 200

Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects.

Very Unhealthy

201 to 300

Health alert: everyone may experience more serious health effects


301 to 500

Health warnings of emergency conditions. The entire population is more likely to be affected.

Table contents from:
Table 1. THe EPA's Air Quality Index (AQI) and what the different categories mean.

To find out what the air quality in your area is and how it ranks compared to other cities, check out the American Lung Association's 2010 State of the Air results.

Send comments, concerns or questions to Dr. Karen Bernd