Ozone (O3) is a gas composed of three oxygen atoms which you cannot see or smell and is commonly referred to as smog. 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 in the presence of heat and sunlight.
Pollutants called nitrogen oxides (NOx) and volatile organic compounds (VOCs) are Ozone precursors. They are the ingredients that come together to form smog. See the smog (Ozone) equation below. 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 the EPA's description of Ozone. Here is CARB's description.