Ground-Level Ozone (Smog) Trends
Ground-level ozone is the dominant pollutant in 452 of 1,020 monitored US cities. Unlike the protective ozone layer in the upper atmosphere, ground-level ozone is a harmful pollutant that forms when nitrogen oxides and volatile organic compounds react in sunlight — the main ingredient in smog.
How Ozone Forms
Ground-level ozone is not emitted directly. It forms through a photochemical reaction: nitrogen oxides (NOx) from vehicles and power plants combine with volatile organic compounds (VOCs) from fuels, solvents, and natural sources in the presence of sunlight. This is why ozone levels are highest on hot, sunny summer afternoons — and why Sun Belt cities dominate ozone concerns.
The EPA's ozone standard is 70 parts per billion (ppb) averaged over 8 hours. Many US metro areas exceed this standard during summer months, triggering Ozone Action Days that warn vulnerable populations to limit outdoor activity.
Cities Where Ozone Is Getting Worse
Rising temperatures from climate change are making ozone worse. Hotter summers mean more photochemical reactions, producing more smog even as vehicle emissions per car have declined. 30 cities show worsening ozone-driven AQI trends.
| City | State | 5yr Avg AQI | Grade | Trend |
|---|---|---|---|---|
| Delta, Colorado | CO | 42 | C | +4.1/yr |
| Johnson, Wyoming | WY | 40 | C | +4.0/yr |
| Archuleta, Colorado | CO | 39 | C | +3.0/yr |
| Maricopa, Arizona | AZ | 90 | F | +2.9/yr |
| SONORA, Country Of Mexico | MX | 44 | D | +2.3/yr |
| Winnebago, Illinois | IL | 48 | D | +1.9/yr |
| Neosho, Kansas | KS | 48 | D | +1.8/yr |
| Horry, South Carolina | SC | 37 | C | +1.8/yr |
| Inyo, California | CA | 57 | F | +1.7/yr |
| Sumner, Kansas | KS | 47 | C | +1.6/yr |
| York, South Carolina | SC | 44 | C | +1.4/yr |
| Baker, Florida | FL | 40 | C | +1.4/yr |
| Scott, Minnesota | MN | 41 | C | +1.4/yr |
| Butler, Ohio | OH | 50 | D | +1.4/yr |
| Johnson, Kansas | KS | 44 | C | +1.3/yr |
| Ward, North Dakota | ND | 36 | C | +1.3/yr |
| Bernalillo, New Mexico | NM | 59 | D | +1.3/yr |
| Chaffee, Colorado | CO | 53 | C | +1.2/yr |
| Kane, Illinois | IL | 43 | C | +1.2/yr |
| Siskiyou, California | CA | 41 | D | +1.2/yr |
Ozone and Climate Change
Climate scientists project that ground-level ozone will worsen significantly as global temperatures rise. A 2-3 degree Fahrenheit increase in average summer temperature could increase ozone concentrations by 5-10% in many US cities. This means that even as we reduce vehicle and industrial emissions, temperature-driven ozone formation could offset those gains.
Health Effects of Ozone Exposure
Ozone irritates the respiratory system, causing coughing, throat irritation, chest pain, and shortness of breath. Repeated exposure reduces lung function over time and can aggravate asthma, bronchitis, and emphysema. Children and outdoor workers are especially vulnerable because they spend more time breathing heavily in outdoor air.
Frequently Asked Questions
Ozone levels peak on hot, sunny summer afternoons (typically 1-7 PM). This is because ozone formation requires sunlight and heat. Winter ozone levels are much lower in most cities. Coastal cities may see slightly different patterns due to sea breeze effects.
Ground-level ozone is the primary component of smog. When you see a hazy, yellowish-brown layer over a city on a hot day, that is ozone-rich smog. Ozone is invisible on its own, but it reacts with other pollutants to create visible haze.
Ozone is a gas formed by chemical reactions in sunlight, while PM2.5 consists of solid or liquid particles. Ozone dominates in 452 US cities (typically sunny, hot metros), while PM2.5 dominates in the rest (often affected by wildfire smoke, industry, or traffic). Both are harmful but affect the body differently.
When AQI is above 100 due to ozone, sensitive groups (people with asthma, children, older adults) should reduce prolonged outdoor exertion. Above 150, everyone should limit outdoor exercise. If you run or cycle outdoors, exercise in the morning when ozone levels are lowest.
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