Understanding the AQI: A Complete 2026 Guide to Air Quality Index Categories and Health Actions

The Air Quality Index (AQI) standardizes raw pollution concentrations into a 0 to 500 scale established by the EPA. Higher numbers reflect greater acute respiratory and cardiovascular risk. The index translates multi-pollutant sensor data into actionable health tiers.

The AQI Scale at a Glance

Regulated Pollutants in the Index

The United States AQI evaluates five criteria pollutants regulated under the Clean Air Act:

• Ground-level ozone forms in sunlight from vehicle and industrial emissions, peaking during warm afternoons.
• Fine particulate matter (PM2.5) dominates during wildfire events and winter inversions, penetrating deep into pulmonary alveoli and the bloodstream.
• Coarse particulate matter (PM10) comprises crustal dust, biological pollen, and mechanical debris.
• Carbon monoxide originates primarily from incomplete combustion in vehicle exhaust, concentrating near highway corridors.
• Sulfur dioxide and nitrogen dioxide originate from industrial power generation and diesel transport.

The reported AQI value represents the highest single pollutant concentration at that monitoring station at that hour, designated as the driving or critical pollutant.

Interpreting Regional Index Fluctuations

Evaluating personal exposure requires understanding seasonal and diurnal drivers:

• Monitor afternoon forecasts rather than morning snapshots during ozone season, because photochemical conversion accelerates under peak ultraviolet exposure.
• Compare current readings against local annual baselines. An AQI of 60 represents clean conditions in an industrial corridor but reflects elevated pollution in well-ventilated coastal regions.
• Identify whether the driving pollutant is gaseous ozone or fine particles. Ozone exposure requires timing outdoor exertion around early morning hours, while PM2.5 requires physical filtration.

Activity Guidance by AQI Category

1. 0 to 50 (Good): Satisfactory air quality with no activity restrictions.
2. 51 to 100 (Moderate): Acceptable conditions, though individuals with severe respiratory sensitivities should monitor symptoms during prolonged exertion.
3. 101 to 150 (Unhealthy for Sensitive Groups): Individuals with asthma, cardiovascular disease, older adults, and children should shorten prolonged outdoor exertion.
4. 151 to 200 (Unhealthy): The general population should reduce strenuous outdoor exertion; sensitive groups should avoid outdoor exertion.
5. 201 to 300 (Very Unhealthy): All individuals should remain indoors with windows closed and mechanical HEPA filtration active.
6. 301 or higher (Hazardous): Serious emergency conditions requiring complete avoidance of outdoor air.

The NowCast Calculation Algorithm

Real-time AQI reported on public weather dashboards does not represent an instantaneous raw reading. Instead, the EPA calculates particle indices using the NowCast algorithm, an hourly weighted averaging method designed to respond rapidly to changing air quality while dampening erratic sensor spikes.

The NowCast computes a weighted average of the preceding 12 hours of PM2.5 concentrations. When air quality changes rapidly, such as during a sudden wildfire smoke plume arrival, the algorithm assigns up to 90 percent of the total weight to the most recent three hours. Conversely, during periods of steady, stagnant air quality, the algorithm distributes weights evenly across all 12 hours. This mathematical dampening prevents public health alerts from fluctuating erratically while ensuring rapid detection of dangerous smoke events.

Regulatory Federal Monitors vs Consumer Laser Sensors

Understanding AQI reports requires distinguishing between Federal Reference Method (FRM) regulatory monitors and consumer optical particle counters. Official government stations operated by state environmental agencies utilize gravimetric beta-attenuation monitors that measure particulate mass under controlled relative humidity.

Consumer sensors use light-scattering optical counters calibrated to standard spherical aerosols. During periods of high ambient humidity or dense fog, water droplets swell airborne particles, causing low-cost optical monitors to overestimate PM2.5 concentrations by 25 to 50 percent. When interpreting hyper-local neighborhood sensor data, always verify whether national calibration factors have been applied to harmonize consumer readings with regional regulatory benchmarks.

Physiological Thresholds and Systemic Inflammation

Fine particulate matter below 2.5 microns behaves differently in human physiology than larger coarse dust. Because of their microscopic diameter, PM2.5 particles bypass upper respiratory cilia and penetrate directly into pulmonary alveoli. From the alveolar membrane, combustion byproducts and heavy metal fractions diffuse across the epithelial barrier directly into systemic circulation.

This trans-membrane migration triggers systemic oxidative stress, elevated fibrinogen levels, and acute vasoconstriction. Clinical epidemiological studies link multi-day exposures above 35 µg/m³ (AQI 100+) to elevated risks of ischemic events, arrhythmic episodes, and acute asthma exacerbations, highlighting the medical necessity of mechanical filtration during high-index days.

Where to Check Your AQI

Use our city air quality pages for real-time AQI plus historical and seasonal trends, or check our rankings to see how your area compares nationally.

Related Reading

• Summer 2026 Ozone Season: Cities at Highest Risk
• 2026 Wildfire Season Outlook
• Best Air Purifiers for Wildfire Smoke (2026)
• Our data methodology

Note: AQI categories and health guidance summarized here follow EPA conventions. Always consult official sources and your healthcare provider for personalized health decisions.