Ozone: The Good and Bad of Ozone

Origins and Characteristics of Stratospheric and Ground-Level Ozone

NOx pollution contributes to unhealthy smog over cities.
NOx pollution contributes to ozone formation and unhealthy smog over cities. Cultura RM/Justin Borucki/Getty Images

Essentially, ozone (O3) is an unstable and highly reactive form of oxygen. The ozone molecule is made up of three oxygen atoms that are bound together, whereas the oxygen we breathe (O2) contains only two oxygen atoms.

From a human perspective, ozone is both helpful and harmful, both good and bad.

The Benefits of Good Ozone

Small concentrations of ozone occur naturally in the stratosphere, which is part of the Earth’s upper atmosphere.

At that level, ozone helps to protect life on Earth by absorbing ultraviolet radiation from the sun, particularly UVB radiation that can cause skin cancer and cataracts, damage crops, and destroy some types of marine life.

The Origin of Good Ozone

Ozone is created in the stratosphere when ultraviolet light from the sun splits an oxygen molecule into two single oxygen atoms. Each of those oxygen atoms then binds with an oxygen molecule to form an ozone molecule.

Depletion of stratospheric ozone poses serious health risks for humans and environmental hazards for the planet, and many nations have banned or limited the use of chemicals, including CFC, that contribute to ozone depletion.

The Origin of Bad Ozone

Ozone is also found much nearer the ground, in the troposphere, the lowest level of Earth’s atmosphere. Unlike the ozone that occurs naturally in the stratosphere, tropospheric ozone is man-made, an indirect result of air pollution created by automobile exhaust and emissions from factories and power plants.

When gasoline and coal are burned, nitrogen oxide gases (NOx) and volatile organic compounds (VOC) are released into the air. During the warm, sunny days of spring, summer and early fall, NOx and VOC are more likely to combine with oxygen and form ozone. During those seasons, high concentrations of ozone are often formed during the heat of the afternoon and early evening (as a component of smog), and are likely to dissipate later in the evening as the air cools.

Does ozone pose a significant risk to our climate? Not really - ozone does have a small role to play in global climate change, but the majority of the risks are elsewhere.

The Risks of Bad Ozone

The man-made ozone that forms in the troposphere is extremely toxic and corrosive. People who inhale ozone during repeated exposure may permanently damage their lungs or suffer from respiratory infections. Ozone exposure may reduce lung function or aggravate existing respiratory conditions such as asthma, emphysema or bronchitis. Ozone may also cause chest pain, coughing, throat irritation or congestion.

The adverse health effects of ground-level ozone are particularly dangerous for people who work, exercise, or spend a lot of time outdoors during warm weather. Seniors and children are also at greater risk than the rest of the population, because people in both age groups are more likely to have reduced or not fully formed lung capacity.

In addition to human health effects, ground-level ozone is also hard on plants and animals, damaging ecosystems and leading to reduced crop and forest yields. In the United States alone, for example, ground-level ozone accounts for an estimated $500 million in reduced crop production annually.

Ground-level ozone also kills many seedlings and damages foliage, making trees more susceptible to diseases, pests and harsh weather.

No Place is Completely Safe from Ground-Level Ozone

Ground-level ozone pollution is often considered an urban problem, because it is formed primarily in urban and suburban areas. Nevertheless, ground-level ozone also finds its way to rural areas, carried hundreds of miles by the wind or forming as a result of auto emissions or other sources of air pollution in those areas.


Edited by Frederic Beaudry.