Ozone (O3)
A reactive gas formed at ground level when sunlight triggers chemical reactions between nitrogen oxides and volatile organic compounds.
Detailed Explanation
Ground-level ozone is not emitted directly into the air. Instead, it forms through a photochemical reaction when nitrogen oxides (NOx) from vehicle exhaust and industrial emissions react with volatile organic compounds (VOCs) in the presence of sunlight. This is why ozone levels typically peak during hot, sunny afternoons in summer months. Ozone is the primary ingredient in smog and is distinct from stratospheric ozone, which protects Earth from ultraviolet radiation. At ground level, ozone is harmful, it irritates the respiratory system, aggravates asthma, reduces lung function, and causes chest pain and coughing. Long-term exposure is linked to the development of asthma in children and may contribute to premature death. Ozone is the dominant pollutant in many Sun Belt cities including Los Angeles, Phoenix, and Houston. Climate change is expected to worsen ozone pollution because higher temperatures accelerate the chemical reactions that produce it. AirHistory data shows ozone is the dominant pollutant in approximately 55% of tracked US cities, making it the most common primary pollutant nationally.
Related Terms
Smog
Visible air pollution, primarily composed of ground-level ozone and fine particles, that reduces visibility and harms health.
Volatile Organic Compounds (VOCs)
Carbon-containing chemicals that easily evaporate at room temperature and contribute to ozone formation and direct health effects.
Nitrogen Dioxide (NO2)
A reddish-brown gas produced primarily by vehicle engines and power plants that contributes to smog and respiratory problems.
Criteria Pollutants
The six common air pollutants regulated by the EPA under the Clean Air Act: ozone, PM2.5, PM10, CO, NO2, and SO2.
Frequently Asked Questions
A reactive gas formed at ground level when sunlight triggers chemical reactions between nitrogen oxides and volatile organic compounds.
Ground-level ozone is not emitted directly into the air. Instead, it forms through a photochemical reaction when nitrogen oxides (NOx) from vehicle exhaust and industrial emissions react with volatile organic compounds (VOCs) in the presence of sunlight. This is why ozone levels typically peak during hot, sunny afternoons in summer months.
this entity is one of the U.S. air quality and pollution monitoring concepts that recurs across this site. The definition above is the technical answer; the paragraphs below add the practical context for how the concept connects to the the EPA Air Quality System (AQS) data behind every per-entity page on the site.
In the the EPA Air Quality System (AQS) data, this concept shapes one or more of the fields that drive the per-entity grades and rankings on this site. The methodology page describes which fields feed into which output; this glossary entry documents the underlying term.