Ozone (O3) is a form of oxygen with three atoms per molecule. In the upper atmosphere (stratosphere), ozone forms a protective layer shielding Earth from harmful ultraviolet radiation, and its depletion led to international agreements banning ozone-destroying chemicals. At ground level, however, ozone is a harmful air pollutant and key component of smog, formed when sunlight reacts with vehicle emissions and industrial pollution. Ground-level ozone irritates respiratory systems and worsens conditions like asthma. Municipalities address ground-level ozone through emissions controls, air quality monitoring, and smog advisories warning residents to limit outdoor activity on high-pollution days.