Tropospheric (ground-level) ozone is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC). Whether in its pure form or mixed with other chemicals, ozone can be harmful to health.
When inhaled, ozone can damage the lungs. Relatively low amounts of ozone can cause chest pain, coughing, shortness of breath and throat irritation. Breathing high levels of ozone can worsen asthma symptoms. Ozone contributes to what we typically experience as “smog” or haze.
More information: U.S. EPA