Smog is not one pollutant — it is a soup. In many cities from Los Angeles to Milan to Beijing, the summer culprit is often ground-level ozone: three oxygen atoms that form when sunlight cooks vehicle and industrial emissions. It is different from the ozone layer that protects Earth; at street level, it irritates lungs.
How photochemical smog forms
Nitrogen oxides (NO₂) and volatile organic compounds from traffic and solvents react in strong sunlight. Ozone builds through the morning, often peaking between 2 p.m. and 6 p.m. on hot, still days. PM2.5 may look fine while ozone pushes AQI into yellow or orange — which is why checking only "particle pollution" misses part of the picture.
Health effects and who is at risk
Ozone causes chest tightness, cough, and reduced lung function — even in healthy athletes. Children, older adults, and people with asthma feel it first. Unlike particles you can sometimes see, ozone is invisible; the sunny haze you see is often particles plus humidity, not ozone alone.
- Schedule runs and sports before 10 a.m. on hot summer days.
- Avoid heavy exertion outdoors when ozone AQI is high.
- Do not assume sea breeze cities are immune — inland suburbs still spike.
- Air conditioning filters some indoor ozone; open windows at ozone peak imports it.
Smog seasons around the world
Northern hemisphere summers bring ozone season to Europe and North America. Tropical cities can see year-round photochemical smog when heat and traffic combine. Winter smog in many Asian megacities is more particle-dominated — different season, same need to read which pollutant drives the index.
Use hourly forecasts: morning green can turn afternoon orange on the same cloudless day.
See which pollutant drives today's AQI — particles, ozone, or both — on Aether for your city.
Check live air quality on Aether