Ozone (O3) - Ground level
Ground-level (tropospheric) ozone is formed when nitrogen oxides and volatile organic compounds react in sunlight. It is harmful to lungs at the ground - distinct from the protective stratospheric ozone layer.
Reviewed by Hayden Williams. Last reviewed 2026-05-01. Unit: ppb.
Ozone is a strong oxidant. At ground level it inflames airways and reduces lung function within hours of exposure. Production peaks on hot, sunny afternoons in urban areas and downwind suburbs. Unlike PM, ozone is rarely emitted directly from a single source - it forms in the atmosphere from precursors emitted by traffic, industry, and solvents.
Health effects
Triggers asthma exacerbation, reduces lung function, inflames airways, worsens COPD. Acute exposure is linked to increased emergency room visits and reduced exercise tolerance.
Acute ozone exposure causes detectable lung function reductions in healthy young adults at concentrations below 80 ppb (WHO 2021 guideline). Chronic exposure is associated with increased respiratory mortality, slowed lung development in children, and elevated risk of asthma incidence. Outdoor exercise during high-ozone afternoons substantially increases inhaled dose.
Vulnerable groups
Children (active outdoors, developing lungs), people with asthma or COPD, outdoor workers, athletes training in summer, elderly with cardiopulmonary disease.
Common sources
- Vehicle exhaust (NOx and unburned hydrocarbons)
- Industrial emissions (refineries, chemical plants)
- Solvents and paints (VOC precursors)
- Hot sunny weather amplifies photochemical formation
- Wildfires (precursor injection plus direct ozone)
- Long-range transport from upwind regions
Regional context
Highest summer ozone in the US Sun Belt and California Central Valley, Southern Europe (Po Valley, Spain), Mediterranean cities, North African coasts, Northern China during hot months, and increasingly in Australia during heatwaves. Cool, cloudy regions rarely exceed WHO guidelines except during regional smog events.
Regulatory thresholds
How O3 is measured
Ultraviolet photometry is the reference method - a UV beam dimmed by ozone is measured continuously. Most regulatory monitors use this technique. Low-cost electrochemical sensors are available but require frequent recalibration.
How to reduce your exposure
- Reschedule outdoor exercise to morning or evening (peak ozone is 2-6 PM)
- Stay indoors during high-ozone alerts when AQI > 150
- Indoor levels are typically much lower because ozone reacts with surfaces - simply being indoors helps
- Drive less during ozone alerts; avoid filling the gas tank during the day
- Use low-VOC paints and solvents
Trends
Despite NOx reductions in developed economies, peak summer ozone has been stubborn or rising in some regions because climate change drives more frequent heatwaves and stagnant air. Asia's ozone is rising as vehicle fleets grow faster than emissions controls. EU and US Sun Belt cities still routinely exceed WHO 2021 guidelines.
Cities where O3 matters most
Live readings for cities historically affected by elevated O3:
Frequently asked
Is ground-level ozone the same as the ozone layer?
No. Stratospheric ozone (10-50 km up) protects the planet from UV radiation. Ground-level (tropospheric) ozone is a harmful pollutant formed when NOx and VOCs react in sunlight. They are chemically the same molecule but in completely different roles.
When is ozone worst?
Hot, sunny summer afternoons. Peak typically 2-6 PM in urban and downwind suburban areas. Ozone is usually lowest in the early morning before photochemistry kicks in.
Why is ozone bad indoors not a problem?
Ozone is highly reactive and breaks down quickly when it contacts surfaces. Indoor ozone levels are typically 20-70% of outdoor concentrations. Air conditioners with closed windows reduce indoor ozone further.
Does ozone cause asthma?
Long-term ozone exposure is linked to new-onset asthma, particularly in children playing outdoors in summer. Acute exposure during peak hours triggers attacks in people with existing asthma.
Sources + further reading
Track O3 live across 217 cities on atmos.today. See methodology for how readings are sourced and calculated.