Air Quality Glossary
Plain-language definitions of 30 air quality terms, pollutants, measurement methods, health effects, regulations, and the metrics behind AirHistory's Air Quality Grades.
Pollutants
PM2.5 (Fine Particulate Matter)
Airborne particles smaller than 2.5 micrometers in diameter, small enough to penetrate deep into the lungs and enter the bloodstream.
Ozone (O3)
A reactive gas formed at ground level when sunlight triggers chemical reactions between nitrogen oxides and volatile organic compounds.
PM10 (Coarse Particulate Matter)
Inhalable particles with diameters between 2.5 and 10 micrometers, typically from dust, construction, and agricultural activities.
Nitrogen Dioxide (NO2)
A reddish-brown gas produced primarily by vehicle engines and power plants that contributes to smog and respiratory problems.
Sulfur Dioxide (SO2)
A pungent gas released by burning sulfur-containing fossil fuels, primarily from power plants and industrial facilities.
Carbon Monoxide (CO)
A colorless, odorless gas produced by incomplete combustion of carbon-containing fuels, primarily from vehicles and industrial processes.
Particulate Matter (PM)
A mixture of solid particles and liquid droplets suspended in the air, ranging in size from visible dust to microscopic particles.
Volatile Organic Compounds (VOCs)
Carbon-containing chemicals that easily evaporate at room temperature and contribute to ozone formation and direct health effects.
Smog
Visible air pollution, primarily composed of ground-level ozone and fine particles, that reduces visibility and harms health.
Measurement & Monitoring
Air Quality Index (AQI)
A standardized EPA scale from 0 to 500 that communicates daily air quality and associated health risks.
AQI Categories
The six color-coded health concern levels, Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, and Hazardous, used by the EPA to describe air quality.
Good Air Quality
Air quality conditions with an AQI of 0-50, posing little or no health risk for any population group.
Air Quality Monitor
A device that measures pollutant concentrations in ambient air, operated by government agencies as part of the national monitoring network.
Air Quality Forecast
Predictions of future AQI levels issued by state and local agencies, typically covering 1-2 days ahead.
AQI Breakpoints
The specific pollutant concentration values that correspond to each AQI level, used to convert raw measurements to the 0-500 scale.
Health & Exposure
Sensitive Groups
Population groups more vulnerable to air pollution, including children, older adults, pregnant women, and people with heart or lung conditions.
Pollen vs. Pollution
Distinguishing between biological allergens (pollen, mold) and chemical air pollutants, which are measured and regulated differently.
Indoor Air Quality
The quality of air inside buildings, which can be 2-5 times more polluted than outdoor air due to poor ventilation and indoor sources.
Air Toxics (Hazardous Air Pollutants)
Pollutants known or suspected to cause cancer, birth defects, or other serious health effects, regulated separately from criteria pollutants.
Regulation & Standards
National Ambient Air Quality Standards (NAAQS)
EPA-established limits on concentrations of six criteria pollutants that must be met nationwide to protect public health and the environment.
Clean Air Act
The primary federal law governing air quality in the United States, first enacted in 1963 and significantly amended in 1970 and 1990.
EPA Air Quality System (AQS)
The EPA's national database of ambient air quality data collected from monitoring stations across the United States.
Nonattainment Area
A geographic area that fails to meet one or more National Ambient Air Quality Standards for a criteria pollutant.
Criteria Pollutants
The six common air pollutants regulated by the EPA under the Clean Air Act: ozone, PM2.5, PM10, CO, NO2, and SO2.
Sources & Causes
Wildfire Smoke
Airborne particulate matter and gases produced by wildfires, now the leading source of hazardous air quality events in the western United States.
Temperature Inversion
A weather condition where a layer of warm air traps cooler air near the ground, preventing pollutants from dispersing.
Acid Rain
Precipitation made acidic by sulfur dioxide and nitrogen oxide emissions, which damages ecosystems, buildings, and water quality.
Emissions Inventory
A comprehensive accounting of air pollutant emissions from all sources in a geographic area, used for regulation and planning.
AirHistory Metrics
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