Continue your usual outdoor activities.
PM₂.₅ consists of very small airborne particles. Sources include fuel burning in vehicles, heating and industry, as well as fires. Some particles form when gases react in the atmosphere. Their small size allows them to reach deep into the lungs.
Ground-level ozone (O₃) forms when sunlight drives reactions involving nitrogen oxides and volatile organic compounds. Levels often rise in sunny weather, and pollution can be transported away from its sources. Ground-level ozone is harmful; it is different from the protective ozone layer high in the atmosphere.