Ozone is a life-saving shield in the upper atmosphere but a toxic pollutant at ground level.

Ozone (O3) exists in two distinct layers of the atmosphere with opposite effects on human health. The 'good' ozone layer resides in the stratosphere, roughly 10 to 50 kilometers above Earth's surface. This layer absorbs 97% to 99% of the Sun's medium-frequency ultraviolet light, specifically UV-B rays. Without this protection, life on land would be impossible due to severe DNA damage and cellular mutations.In contrast, tropospheric or ground-level ozone is a secondary pollutant created by chemical reactions. It forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight. Major sources of these precursors include vehicle exhaust, power plants, and chemical solvents. This process is most intense during hot, sunny days when the air is stagnant, leading to high concentrations of smog in urban areas.Ground-level ozone is a powerful oxidant that can damage lung tissue upon inhalation. The Environmental Protection Agency (EPA) monitors these levels under the Clean Air Act, setting National Ambient Air Quality Standards to protect public health. Chronic exposure is linked to permanent lung scarring and increased rates of premature death. This dual role is why atmospheric scientists use the mnemonic 'good up high, bad nearby' to describe the molecule.