Ozone (O3) is a gas composed of three oxygen atoms. Ozone found at ground level is a major component of smog, and it can damage lung tissue and cause other health problems. Ozone is generally not released directly into the air. Rather, it forms by a chemical reaction between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of heat and sunlight.  Ozone levels are highest on hot summer days when there is little wind. Ground level ozone, or ozone pollution, should not be confused with stratospheric ozone, located high up in the atmosphere, which helps to block harmful ultraviolet rays. Here is EPA's description of ozone. Here is CARB's description.