Ground-Level Ozone (O3)

Ground-level ozone is not emitted directly, it forms when nitrogen oxides (NOx) from vehicle exhaust and industrial emissions react with volatile organic compounds (VOCs) in sunlight. This makes it primarily a summer pollutant, peaking on hot, sunny, stagnant days. It is the dominant driver of unhealthy AQI days in many eastern US counties.

Ozone irritates the respiratory system, can trigger asthma attacks, and reduces lung function. Children, elderly adults, and people with respiratory conditions are most at risk. The EPA's AQI threshold for "Unhealthy for Sensitive Groups" is 71 ppb (parts per billion) for an 8-hour average.

On PlainAirData county pages, check the primary pollutant breakdown to see what proportion of your county's unhealthy days are ozone-driven versus PM2.5-driven.

Fine Particulate Matter (PM2.5)

PM2.5 refers to particles with diameter under 2.5 micrometers, small enough to penetrate deep into the lungs and enter the bloodstream. Sources include combustion (vehicles, power plants, wildfires), industrial processes, and secondary formation from gaseous pollutants. Wildfire smoke is overwhelmingly a PM2.5 problem.

Long-term PM2.5 exposure is associated with cardiovascular disease, lung cancer, and premature death. Short-term spikes during wildfire events can trigger heart attacks and strokes in vulnerable populations. The EPA's AQI threshold for "Unhealthy for Sensitive Groups" is 35.5 μg/m³ for a 24-hour average.

Coarse Particulate Matter (PM10)

PM10 includes particles up to 10 micrometers, larger than PM2.5 and typically from construction dust, road dust, and agricultural operations. PM10 is less of a health concern than PM2.5 because the larger particles are filtered by the nose and throat before reaching the deep lungs. It is the dominant pollutant in arid, dusty regions.

Carbon Monoxide, Sulfur Dioxide, and Nitrogen Dioxide

Carbon monoxide (CO) comes primarily from vehicle exhaust and is most concerning in enclosed or poorly ventilated areas. Sulfur dioxide (SO2) comes from burning fossil fuels containing sulfur, particularly coal-fired power plants. Nitrogen dioxide (NO2) comes from vehicle exhaust and power generation. All three have declined significantly in most US areas due to Clean Air Act regulations.

These three pollutants rarely drive AQI above the Good range in most US counties today. When they do, it typically indicates a local industrial source or unusual traffic congestion. PlainAirData's county data shows the pollutant breakdown so you can see which pollutants affect your area most.

How to Read PlainAirData's Pollutant Data

• County pages show which pollutant drives the most unhealthy days in each county
• AQI guide, explains the full 0-500 scale and what each level means
• Health effects guide, detailed health impacts of air pollution exposure
• Common pollutants guide, deeper dive into pollutant sources and chemistry