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Tropospheric Ozone, the Polluter

This section provides a brief overview of ground-level ozone (i.e., tropospheric ozone) where it is largely considered a pollutant. The general concepts found in this section include the following:

  • Ozone is a naturally occurring gas found in the troposphere and other parts of the atmosphere.

  • Tropospheric ozone is often called "bad" ozone because it can damage living tissue and break down certain materials.

  • Concentrations of ozone are not uniform in the troposphere.

  • Longer exposure to ozone will increase the negative effects.

  • The amount of ozone present in the troposphere varies from day to day and from place to place.

This section includes two classroom activities.


Ozone occurs naturally at ground-level in low concentrations. The two major sources of natural ground-level ozone are hydrocarbons, which are released by plants and soil, and small amounts of stratospheric ozone, which occasionally migrate down to the earth's surface. Neither of these sources contributes enough ozone to be considered a threat to the health of humans or the environment.

But the ozone that is a byproduct of certain human activities does become a problem at ground level and this is what we think of as 'bad' ozone. With increasing populations, more automobiles, and more industry, there's more ozone in the lower atmosphere. Since 1900 the amount of ozone near the earth's surface has more than doubled. Unlike most other air pollutants, ozone is not directly emitted from any one source. Tropospheric ozone is formed by the interaction of sunlight, particularly ultraviolet light, with hydrocarbons and nitrogen oxides, which are emitted by automobiles, gasoline vapors, fossil fuel power plants, refineries, and certain other industries.

In urban areas in the Northern Hemisphere, high ozone levels usually occur during the warm, sunny summer months (from May through September). Typically, ozone levels reach their peak in mid to late afternoon, after the sun has had time to react fully with the exhaust fumes from the morning rush hours. A hot, sunny, still day is the perfect environment for ozone pollution production. In early evening, the sunlight's intensity decreases and the photochemical production process that forms ground level ozone begins to subside.

Negative Impacts of Tropospheric Ozone

While stratospheric ozone shields us from ultraviolet radiation, in the troposphere this irritating, reactive molecule damages forests and crops; destroys nylon, rubber, and other materials; and injures or destroys living tissue. It is a particular threat to people who exercise outdoors or who already have respiratory problems.

Ozone affects plants in several ways. High concentrations of ozone cause plants to close their stomata. These are the cells on the underside of the plant that allow carbon dioxide and water to diffuse into the plant tissue. This slows down photosynthesis and plant growth. Ozone may also enter the plants through the stomata and directly damage internal cells.

Rubber, textile dyes, fibers, and certain paints may be weakened or damaged by exposure to ozone. Some elastic materials can become brittle and crack, while paints and fabric dyes may fade more quickly.

When ozone pollution reaches high levels, pollution alerts are issued urging people with respiratory problems to take extra precautions or to remain indoors. Smog can damage respiratory tissues through inhalation. Ozone has been linked to tissue decay, the promotion of scar tissue formation, and cell damage by oxidation. It can impair an athlete's performance, create more frequent attacks for individuals with asthma, cause eye irritation, chest pain, coughing, nausea, headaches and chest congestion and discomfort. It can worsen heart disease, bronchitis, and emphysema.

So why can't we take all of this "bad" ozone and blast it up into the stratosphere? The answer lies in the vast quantities needed and ozone's instability in the dynamic atmosphere. Ozone molecules don't last very long, with or without human intervention. The vehicle necessary to transport such enormous amounts of ozone into the stratosphere does not exist, and, if it did, it would require so much fuel that the resulting pollution might undo any positive effect. Rather than seek such grandiose solutions, we need to decrease the production of those chemicals that break down ozone in the stratosphere and help create ozone in the troposphere.

The dual ozone problems—pollution or smog in the troposphere and depletion of the ozone layer in the stratosphere—are indeed very different. But the problems have common ties in that they both are related to air pollutants that come from industry, transportation, and other human activities.

Concluding Thoughts

The majority of U.S. citizens live in areas that are impacted by tropospheric ozone pollution. They are familiar with "smog-alerts," local government pleas to reduce vehicle traffic, and news reports about cities that have failed to meet EPA standards for ozone pollution levels. As you work through these activities with your class, you can easily connect the instructional activities to your students' surroundings by collecting and discussing news reports about smog issues in your own or nearby metropolitan areas. You may further wish to engage students in a discussion (or perhaps even a research project) about the impact of private automobiles on ozone levels and the prospects for alternative-fuel vehicles to reduce vehicle emissions and tropospheric ozone.


The following activities will help your students better understand the concepts covered in this section.

To proceed, either click on Activities in the menu at the top or click on another unit to switch units.