Ozone (O₃)
Also known as: Ozone
What is Ozone?
Ground-level ozone is not emitted directly into the air but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. This is different from the protective ozone layer in the upper atmosphere.
Sources
Major sources of NOx and VOCs include emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapors, and chemical solvents. Ozone levels are typically highest during hot, sunny weather when these reactions occur most readily.
Health Effects
Breathing ozone can trigger chest pain, coughing, throat irritation, and congestion. It can worsen bronchitis, emphysema, and asthma, reduce lung function, and inflame the linings of the lungs. Repeated exposure may permanently scar lung tissue.
Protecting Yourself
The most reliable way to reduce your exposure is to monitor the Air Quality Index before spending extended time outdoors. On days when the AQI for Ozone is elevated, consider moving strenuous exercise indoors and keeping windows closed if outdoor air is worse than indoor air.
Sensitive groups — including children, older adults, and people with asthma or heart disease — should take extra precautions at lower AQI thresholds than the general public. High-quality air purifiers with HEPA and activated carbon filters can significantly reduce indoor concentrations. You can track current Ozone levels for any US city using the search on this site, which pulls directly from EPA AirNow monitoring stations.
EPA Standard
Annual average standard set by the EPA to protect public health.
AQI Scale
US Cities Most Affected by Ozone
Cities with the most days where Ozone was the primary pollutant driving AQI.