Why is ground-level ozone harmful even though stratospheric ozone protects Earth from ultraviolet radiation?
The distinction between stratospheric and ground-level ozone lies in their location and the chemical processes that create them. While both consist of the same molecule (), their impact on life is vastly different depending on where they exist in the atmosphere.12
The Protective Stratospheric Ozone
The ozone layer resides in the stratosphere, roughly 15 to 35 km above Earth’s surface. It is formed naturally through the interaction of solar radiation with oxygen molecules. This layer is vital to life because it acts as a shield, absorbing harmful ultraviolet radiation that would otherwise cause severe damage to living organisms, including skin cancer in humans.3124
The Harmful Ground-Level Ozone
In contrast, ground-level ozone is a secondary pollutant and a primary component of smog. It is not emitted directly but forms when pollutants—specifically nitrogen oxides and volatile organic compounds—undergo photochemical reactions in the presence of strong sunlight and heat.132
Because ozone is a highly reactive and toxic gas, its presence at ground level is hazardous. It acts as a powerful oxidizing agent that can irritate the eyes and mucous membranes, posing significant risks to human health and the environment.2