Ground-Level Ozone (O₃)

Ground-level ozone is not emitted directly, it forms when nitrogen oxides (NOx) from vehicle exhaust and volatile organic compounds (VOCs) from industrial sources react in the presence of sunlight. This makes ozone a warm-weather pollutant, peaking on hot summer afternoons when sunlight is most intense.

Ozone irritates the respiratory system, worsens asthma, reduces lung function, and can cause chest pain and coughing even in healthy adults during moderate exercise. Children, outdoor workers, and people with asthma are most vulnerable. The EPA's current standard is 70 parts per billion (ppb) averaged over 8 hours.

On PlainAir, check the "Ozone Days" column for any county to see how many days ozone was the primary pollutant driving the AQI.

PM2.5, Fine Particulate Matter

PM2.5 refers to particles 2.5 micrometers or smaller in diameter, roughly 30 times smaller than the width of a human hair. These particles are produced by combustion: vehicle engines, power plants, industrial processes, residential wood burning, and wildfires. Because they are so small, PM2.5 particles bypass the body's natural defenses and penetrate deep into the lungs, where they can enter the bloodstream.

Long-term exposure to elevated PM2.5 is associated with cardiovascular disease, lung cancer, and premature death. Short-term spikes, especially during wildfire smoke events, trigger immediate respiratory symptoms and can be dangerous for people with heart or lung conditions. The EPA tightened the annual PM2.5 standard in 2024 from 12 µg/m³ to 9 µg/m³.

PM10, Coarse Particulate Matter

PM10 includes all particles 10 micrometers or smaller, which means it encompasses PM2.5 as well as larger particles like dust, pollen, and mold spores. Major sources include unpaved roads, construction sites, agricultural operations, and wind-blown dust. PM10 is most prevalent in arid and semi-arid regions of the western US.

While PM10 is less dangerous than PM2.5 (larger particles are filtered by the nose and throat), it still aggravates respiratory conditions. PM10 drives AQI in fewer US locations than ozone or PM2.5, primarily in desert regions and areas with heavy construction or agriculture.

Nitrogen Dioxide (NO₂)

NO₂ is produced primarily by vehicle exhaust and power plant emissions. It is a reddish-brown gas with a sharp odor that irritates the airways. NO₂ is also a precursor to ozone formation, it helps create ground-level ozone through photochemical reactions.

Concentrations are highest near busy roads and in urban centers. Studies have linked roadway-adjacent NO₂ exposure to increased asthma rates in children and reduced lung development. NO₂ rarely drives the overall AQI in most US locations, but it contributes to the chemical reactions that produce ozone.

Carbon Monoxide (CO)

CO is a colorless, odorless gas produced by incomplete combustion of fossil fuels. Vehicle exhaust is the primary outdoor source. CO reduces the blood's ability to carry oxygen, which at high concentrations can cause dizziness, confusion, and even death. Outdoor CO levels have dropped dramatically since the 1970s thanks to catalytic converters and cleaner fuels.

CO seldom drives AQI in modern US cities. It remains primarily an indoor hazard from malfunctioning furnaces, generators, and vehicles running in enclosed spaces. PlainAir tracks CO days for completeness, but ozone and PM2.5 are far more relevant for outdoor air quality assessment.

Comparing Pollutants Across Regions

The dominant pollutant varies by geography and season. Western states see more PM2.5 from wildfires and PM10 from dust. Southeastern and mid-Atlantic states experience more ozone days due to heat, humidity, and vehicle emissions. Industrial corridors may have elevated NO₂. PlainAir's county pages show the breakdown of pollutant days so you can see exactly what drives your area's air quality.

Compare regions using our state summaries or drill into metro-level data for the areas you care about.

Frequently Asked Questions