A smog star is a straightforward label that tells you how much ground-level ozone pollution a region produces, based on how often it exceeds federal air quality standards

The smog star system ranks U.S. counties and metropolitan areas on a scale of one to five stars, with five stars meaning the worst air quality for ozone pollution. The system was created by the American Lung Association and appears in their annual "State of the Air" report. A county gets more stars the more days per year it fails to meet the National Ambient Air Quality Standard (NAAQS) for ground-level ozone — the federal limit set by the Environmental Protection Agency.

Ground-level ozone forms when sunlight reacts with nitrogen oxides and volatile organic compounds, mostly from cars, power plants, and factories. It is not the same as the ozone layer high in the atmosphere that protects you from UV radiation. Breathing ground-level ozone damages your lungs, especially during exercise or outdoor work. The smog star tells you whether your area has a chronic ozone problem or just occasional bad days.

The rating is purely informational — it does not trigger automatic regulations or fines. However, counties with more stars are required by the EPA to develop plans to reduce ozone, and they may face restrictions on new industrial permits or highway funding if they do not improve. For you as a resident, the star rating is a signal of whether you should limit outdoor activity on high-ozone days and whether your area's air quality is getting better or worse year to year.

Key Takeaways

  • A five-star smog rating means a county exceeds the federal ozone standard more than 10 days per year; one star means fewer than 2 days per year.
  • Ground-level ozone is created by sunlight reacting with pollution from vehicles and industry, not related to the protective ozone layer in the upper atmosphere.
  • The American Lung Association publishes smog star ratings in their annual report, and you can look up your county or metro area by name.
  • Counties with higher star ratings must submit air quality improvement plans to the EPA, though the rating itself does not when ready change local rules.
  • Smog star ratings change year to year based on actual ozone measurements, so a county's rating can improve or worsen depending on pollution trends.

How the Star Scale Works

The American Lung Association assigns stars based on the number of days a county recorded ozone concentrations above the EPA's standard. The EPA standard for ground-level ozone is 70 parts per billion (ppb) averaged over an eight-hour period. On any day the ozone level exceeds this, the county records a violation day.

The star assignments break down roughly as follows: one star means the area had fewer than 2 violation days in the three-year period studied; two stars means 2 to 4 days; three stars means 5 to 10 days; four stars means 11 to 20 days; and five stars means more than 20 days. The exact thresholds can shift slightly year to year as the EPA updates its standard, but the principle remains the same — more violation days mean more stars and worse air quality.

The rating is based on a rolling three-year average, not a single year. This smooths out one unusually bad summer and gives a more realistic picture of whether an area has a persistent problem or just had a bad stretch. A county might have a five-star year followed by a two-star year if pollution sources shut down or weather patterns change.

Why Ozone Pollution Matters for Health

Ground-level ozone is a respiratory irritant. When you breathe it in, it inflames the airways in your lungs and can reduce how much oxygen your body absorbs. People with asthma, emphysema, or other chronic lung disease are most vulnerable, but even healthy people can experience coughing, chest pain, or shortness of breath on high-ozone days.

Children are at higher risk because their lungs are still developing and they spend more time outdoors playing. Outdoor workers — construction crews, landscapers, mail carriers — face higher exposure during their shifts. Older adults and people with heart disease are also more susceptible to ozone's effects, because the inflammation can stress the cardiovascular system.

The EPA set the ozone standard at 70 ppb based on research showing that this level is the threshold below which most people do not experience measurable harm. Areas that regularly exceed this level have a public health problem, which is why the EPA requires them to develop reduction plans. The smog star tells you whether your area is above or below this threshold on a typical basis.

What Causes High Smog Star Ratings

Ozone forms through a chemical reaction between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. Nitrogen oxides come mainly from vehicle exhaust and power plant emissions. Volatile organic compounds come from gasoline vapors, paint, solvents, and industrial processes. On hot, sunny days with little wind, these pollutants accumulate and react, creating ozone.

Geography plays a large role. Areas in valleys or near coasts can trap pollutants, making ozone worse. The South and Southwest tend to have higher smog star ratings because of heat and sunlight. Areas downwind of major industrial regions can receive ozone pollution from upwind sources, even if they have few local emission sources themselves.

Population density and vehicle traffic are strong predictors of high ratings. Large metropolitan areas with millions of commuters typically have higher smog stars than rural counties. However, some rural areas in the South and West also have high ratings due to geography and natural emissions from vegetation that react with pollution.

How to Find Your Area's Smog Star Rating

The American Lung Association publishes smog star ratings in their annual "State of the Air" report, released each spring. You can visit their website and search by county name, city, or state. The report shows your area's current star rating, how it compares to the previous year, and which specific monitoring stations recorded the ozone data.

The EPA also maintains real-time and historical air quality data on its AirNow website. You can enter your zip code and see current ozone levels, forecasts for the next few days, and historical trends. AirNow uses a different color-coded scale (green, yellow, orange, red, purple) for daily air quality, which is separate from the smog star system but covers the same pollutant.

Many local news stations and weather apps also report daily ozone levels and issue air quality alerts on high-pollution days. If your area has a high smog star rating, you may see these alerts frequently during summer months, which is when ozone is worst.

What Happens When an Area Gets a High Smog Star Rating

Counties with high smog star ratings are classified by the EPA as "nonattainment areas" for ozone. This classification triggers several requirements. The county must develop a State Implementation Plan (SIP) that outlines how it will reduce ozone to meet the federal standard. The plan typically includes measures like stricter emissions limits on industry, vehicle inspection programs, and restrictions on outdoor burning.

Nonattainment areas may also face restrictions on new industrial development. Companies seeking to build factories or power plants in these areas must offset their emissions by reducing pollution elsewhere, or they may be denied permits entirely. This can slow economic development but is intended to prevent ozone from getting worse.

Federal highway funding can also be withheld from nonattainment areas that do not make progress on their air quality plans. This creates pressure on state and local governments to invest in public transit, electric vehicles, and other pollution-reduction strategies. However, the smog star rating itself does not automatically trigger fines or penalties — it is the underlying EPA classification that drives regulatory action.

How Smog Star Ratings Change Over Time

Smog star ratings are recalculated each year based on the most recent three-year average of ozone violation days. An area's rating can improve if pollution sources are reduced, if regulations become stricter, or if weather patterns become less favorable for ozone formation. Conversely, ratings can worsen if emissions increase or if several unusually hot, sunny summers occur in a row.

Some areas have seen steady improvement over decades. The Los Angeles area, historically one of the worst in the nation, has reduced ozone significantly since the 1970s through vehicle emission standards, industrial regulations, and cleaner fuels — even though the population and vehicle traffic have grown. Other areas have seen little change or have worsened as development and traffic have increased faster than pollution controls.

The EPA periodically tightens the ozone standard itself, which can cause previously compliant areas to be reclassified as nonattainment. This happened in 2015 when the EPA lowered the standard from 75 ppb to 70 ppb, causing dozens of additional counties to be classified as nonattainment areas and receive higher smog star ratings.

What You Can Do in a High Smog Star Area

On days when ozone is forecast to be high, limit outdoor activity, especially strenuous exercise. Check the daily air quality forecast on AirNow or your local news before planning outdoor work or recreation. If you have asthma or another lung condition, keep your rescue inhaler with you and consider staying indoors on high-ozone days.

Reduce your own emissions by driving less, carpooling, or using public transit on high-ozone days. Avoid using gas-powered lawn equipment, grills, and other small engines during peak ozone hours (typically afternoon and early evening). These actions have a small individual impact but collectively help reduce the pollution that forms ozone.

Support local air quality initiatives. Many areas with high smog star ratings have programs to retrofit older vehicles, plant trees, or improve public transit. Participating in or advocating for these programs can help your area reduce its smog star rating over time.

Frequently Asked Questions

Is a smog star rating the same as the daily air quality index?

No. The smog star is an annual rating based on how many days a year ozone exceeds the federal standard. The daily air quality index (AQI) is a real-time measurement of current pollution levels. An area with a five-star rating has many bad ozone days per year, but on any given day the AQI could be green (good) or red (unhealthy) depending on current conditions.

Can I move to avoid high ozone areas?

You can, but ozone is a regional problem. Areas with one or two stars tend to be rural, smaller cities, or regions with less traffic and industry. However, even low-star areas can have occasional high-ozone days, especially in summer. If you have severe asthma or lung disease, consulting a doctor about where to live may be worth considering.

Does the smog star rating affect property values?

High smog star ratings can affect property values in some markets, particularly for families with children or people with respiratory conditions. However, many other factors — schools, jobs, crime, housing supply — typically matter more. Real estate listings do not always mention air quality, so you may need to research it separately.

How long does it take for an area to improve its smog star rating?

Improvement depends on how aggressively an area reduces emissions and how much pollution comes from outside sources. Some areas have improved by one or two stars over five to ten years through vehicle standards and industrial controls. Others have seen little change despite regulations because geography or upwind pollution limits progress.

What is the difference between smog and ozone?

Smog is a general term for air pollution that reduces visibility. Ozone is one component of smog. Smog can also include particulate matter, nitrogen dioxide, and other pollutants. The smog star specifically measures ozone, not all types of smog.