Ozone is a form of oxygen gas found high in the atmosphere that protects Earth from harmful ultraviolet rays, but at ground level it can be a pollutant.
From Greek *ozein* meaning “to smell,” because ozone has a sharp, fresh smell, especially after lightning storms. The gas was identified in the 19th century.
High up, ozone is a hero shielding us from UV radiation; near the ground, it can irritate lungs and damage plants. It’s the same molecule, O₃, proving that in chemistry, location can change a substance from helpful to harmful.
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