Ozone forms readily when sunlight strikes oxygen, so it is naturally present in our oxygen-rich atmosphere. In the upper atmosphere, ultraviolet light from the sun strikes the two atoms of molecular oxygen (O2) and splits them into single atoms. Those individual atoms each combine with another molecule of two-atom oxygen to make a three-atom molecule, which is ozone (O3). Most of this ozone converts readily back to oxygen, but a small amount of ozone persists.
Another ozone-forming process takes place in the lower atmosphere, where chemicals emitted from industrial processes, vehicle exhaust, and other products of fossil fuel combustion form ozone through a more complex series of chemical reactions. As concentrations of these pollutants involved in ozone formation (precursors of ozone) increase with industrialization and vehicle traffic, ozone concentrations increase at ground level where we breathe.
When we breathe air unpolluted by human activities, we usually take in about 10 to 15 parts of ozone per billion parts of air (10-15 ppb). However, pollution from human activities has elevated levels of the ozone we breathe. Surface ozone measurements have increased considerably since the onset of the Industrial Revolution, possibly by as much as 100-200 percent at some locations over the past century. (Fishman et al. 1999) Spring and summer ozone concentrations in many regions reach above 125 ppb during episodes of excessive heat and stagnant air. The U.S. Environmental Protection Agency (EPA) has established 80 ppb of ozone exposure over eight hours as the National Ambient Air Quality Standard, but recognizes the possibility that ozone exposure at lower levels over several years can significantly impair human health, especially the health of children.
Scientists liken the effect of overexposure to ozone to premature aging in our lungs. Ground-level ozone can adversely affect everyone. Ironically, people living in many rural areas suffer from ozone overexposure more than many people living in cities. This is because ozone levels are generally higher downwind of ozone precursor sources, at distances of hundreds or even thousands of kilometers, so ozone concentrations in rural areas can be higher than in urban areas. The high levels of ozone that are now common downwind of cities adversely affect the productivity of some crops and even wild plant species. Additionally, overexposure to ozone can compound effects from other conditions that stress plants, such as drought. Reactions involving ozone also cause deterioration of electronic devices and materials such as rubber, plastics, outdoor paints, photographic papers, and fabrics.
Ozone plays other roles the Earth’s climate system. It is a minor greenhouse gas and contributes to climate change. It’s also involved in chemical reactions that produce the hydroxyl radical (OH), which plays a critical role in the atmosphere’s ability to cleanse itself of many chemicals we consider pollutants. At the same time, the hydroxyl radical itself is necessary for ozone formation in the lower atmosphere.
Ozone chemistry in sunlight (photochemistry) is one of the most complex phenomena in the natural world. Ozone’s effects on the systems of the Earth are no less complex. They challenge our understanding, and at the same time, they are very important for us to understand. Ozone affects human well-being and the health of agriculture and ecosystems.
The Ozone We Breathe
Highways of a Global Traveler: Tracking Tropospheric Ozone