Key facts
- Ground-level ozone is usually formed in the atmosphere rather than emitted directly.
- Sunlight drives reactions between nitrogen oxides and volatile organic compounds.
- Ozone can irritate and inflame airways and aggravate asthma.
- WHO’s 2021 guideline includes 100 µg/m³ for the daily maximum 8-hour mean.
“Good” ozone and “bad” ozone
Ozone in the stratosphere helps shield Earth from harmful ultraviolet radiation. Ground-level, or tropospheric, ozone is different: it is breathed directly and can harm people, vegetation and ecosystems.
How ground-level ozone forms
Ozone forms through photochemical reactions involving nitrogen oxides and volatile organic compounds. Important precursor sources include vehicles, power plants, industrial boilers, refineries, solvents and other combustion or chemical activities.
Warm, sunny and stagnant conditions often favor ozone formation, but ozone and its precursors can also travel, so elevated concentrations are not limited to a source’s immediate surroundings.
Health effects and sensitive groups
EPA states that ozone can cause coughing, throat irritation, difficulty breathing deeply, airway inflammation and worsening of diseases such as asthma. Children, older adults, people with asthma and people active outdoors are among the groups at greater risk.
WHO ozone guideline values
| Averaging period | WHO 2021 guideline |
|---|---|
| Daily maximum 8-hour mean | 100 µg/m³ |
| Peak-season average | 60 µg/m³ |
Frequently asked questions
Is ozone emitted directly by cars?
Usually not. Vehicles emit precursor pollutants that can react in sunlight to form ozone.
Why can ozone be high on a sunny day?
Sunlight accelerates the chemical reactions that form ground-level ozone from nitrogen oxides and volatile organic compounds.
Does the ozone layer cause city smog?
No. Protective stratospheric ozone and harmful ground-level ozone occur in different parts of the atmosphere and have different consequences.
Sources
This reference was prepared from the following primary public-health and regulatory sources:
- WHO Global Air Quality GuidelinesWorld Health Organization
- Ground-level Ozone BasicsUnited States Environmental Protection Agency
- Health Effects of Ozone PollutionUnited States Environmental Protection Agency