Ground-Level Ozone
Ground-level ozone is the main component of smog and one of the most common air pollutants in the world. Unlike the protective ozone layer high in the stratosphere, ground-level ozone is harmful to breathe.
How Ozone Forms
Ground-level ozone is not emitted directly from any source. It forms through photochemical reactions when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in sunlight. NOx comes primarily from vehicle exhaust and power plants; VOCs come from vehicles, solvents, and industrial processes.
Why ozone is worst on hot sunny days: Heat and sunlight accelerate the chemical reactions that produce ozone. Calm wind conditions also allow ozone to accumulate near ground level rather than dispersing.
Ozone Health Effects
- Irritates the respiratory system, causing coughing and throat irritation
- Reduces lung function — makes breathing harder during exercise
- Inflames and damages the cells lining the lungs
- Aggravates asthma, emphysema, and chronic bronchitis
- Long-term exposure linked to development of asthma in children
- Can cause chest tightness and pain when taking a deep breath
Ozone AQI Levels
| AQI | Category | Ozone (8-hour, ppm) |
|---|---|---|
| 0–50 | Good | 0–0.054 |
| 51–100 | Moderate | 0.055–0.070 |
| 101–150 | Unhealthy for Sensitive Groups | 0.071–0.085 |
| 151–200 | Unhealthy | 0.086–0.105 |
| 201–300 | Very Unhealthy | 0.106–0.200 |
Ozone Season
Ozone levels are highest during summer months when sunlight is intense and temperatures are elevated. In many cities, the highest ozone concentrations occur in the afternoon (typically 2–8 pm local time) when photochemical reactions have had the whole day to build up.
In the northern hemisphere, ozone season typically runs May through September. Coastal cities with sea breezes tend to have lower ozone; inland cities surrounded by mountains (like Los Angeles or Mexico City) often trap ozone at dangerous levels.
Who is Most at Risk?
- Children — their lungs are still developing and they spend more time outdoors
- Older adults — respiratory systems are more vulnerable
- People with asthma or other lung diseases
- People who are active outdoors — joggers, cyclists, outdoor workers
- People with certain genetic susceptibilities
Protection Tips on High-Ozone Days
- Exercise outdoors in the early morning (before 10 am) when ozone is lowest
- Avoid strenuous outdoor activity when AQI ozone is above 100
- Check the AQI forecast the night before if you plan outdoor exercise
- Keep car windows up and use air conditioning on hot smoggy days
- Reduce driving — less NOx from vehicles means less ozone formation
- Avoid refueling your car and using gas-powered lawn equipment on ozone alert days
Ozone vs. PM2.5
Ozone and PM2.5 are the two pollutants most responsible for poor AQI readings worldwide. They often occur together in urban areas but peak at different times: PM2.5 is often worst in morning rush hour and winter months, while ozone peaks in summer afternoons. In many global cities (Beijing, Delhi, Jakarta), PM2.5 is the dominant driver; in cities like Los Angeles and Mexico City, ozone is often the primary concern.