Pollutant Guide
Ground-Level Ozone
By Jason Curtis · 3 min read · Updated 2026-05-21
What it is
Ozone (O3) is a gas made of three oxygen atoms. The ozone layer 10 to 30 miles up in the stratosphere is the "good" ozone that blocks UV radiation. At ground level, the same molecule is a pollutant and the main ingredient in summertime smog.
Ground-level ozone is not emitted directly. It forms when sunlight bakes a mix of nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the air, which is why ozone peaks on hot, sunny afternoons.
Where it comes from
The precursors come from vehicle exhaust, power plants, refineries, chemical plants, gas stations, paints and solvents, and gas-powered lawn equipment. Even trees release VOCs (isoprene from oaks, for example), which combine with NOx from cars to make ozone in some suburban areas.
Ozone often blows downwind of its source region. A still summer day in the Northeast can transport ozone hundreds of miles from Midwestern power plants. Wildfires also push ozone formation through NOx and VOC emissions.
Health effects
Ozone is a strong oxidant. It irritates the airways even in healthy adults, causing coughing, chest tightness, throat burn, and shortness of breath, especially during exercise. People with asthma have more attacks, and rescue inhaler use rises on high-ozone days.
Long-term exposure to elevated ozone is linked to reduced lung function in children, new asthma cases, and increased respiratory mortality. Outdoor workers, athletes, children, and older adults face the highest exposure and risk.
How it's measured and typical levels
Ozone is measured in parts per billion (ppb). The standard metric is the maximum 8-hour rolling average during the day, since health effects scale with both concentration and duration.
Key benchmarks:
- WHO peak-season 8-hour guideline: 60 µg/m³ (about 30 ppb)
- US EPA NAAQS (8-hour): 70 ppb, based on the annual fourth-highest daily max averaged over 3 years
- EPA Air Quality Index "Unhealthy for Sensitive Groups": starts at 71 ppb (8-hour)
- AQI "Unhealthy": 86 ppb and above
Clean background air runs around 20 to 40 ppb. Urban afternoons in summer often hit 60 to 90 ppb. The worst US ozone events (Los Angeles basin, parts of Texas) still occasionally exceed 120 ppb.
What you can do
Check the daily AQI in summer. When ozone is forecast above 70 ppb, shift outdoor exercise to early morning, when levels are lowest. Children and people with asthma should limit strenuous outdoor activity on high-ozone days.
Indoor ozone is typically 20 to 80% of outdoor levels. Closing windows and running an air conditioner with the fresh-air damper closed helps. Standard HEPA filters do not capture ozone; activated carbon filters do. Avoid "ionic" or ozone-generating air cleaners; they add the pollutant you are trying to avoid.
To reduce your contribution, refuel cars after sunset on hot days, keep tires inflated, and use electric lawn equipment instead of gas.
Sources
This article is for educational purposes only. Canairy does not provide medical advice, diagnosis, or treatment. Talk to a qualified health professional about your specific situation.