Ozone

Ozone (O3) is a highly reactive gas that is a form of oxygen. Ozone is not directly emitted from any source, but rather is a secondary pollutant formed when hydrocarbon vapors and nitrogen oxides combine in heat and sunlight.
Ground-level ozone in the lower atmosphere (troposphere) is not the same as the natural protective layer of ozone in the upper atmosphere (stratosphere). Ozone in the upper atmosphere protects us from the sun’s ultraviolet rays; ozone in the lower atmosphere harms us.

Ozone reacts chemically ("oxidizes") with internal body tissues that it comes in contact with, such as those in the lungs. Exposure can lead to shortness of breath, chest pain, wheezing & coughing. Long-term, repeated exposure to high ozone levels can reduce lung function, inflame the lung lining & increase respiratory discomfort.

Ozone levels tend to rise between May and October when higher temperatures, increased sunlight, & stagnant atmospheric conditions cause the chemical transformation of air pollutants into ozone.