Air quality guide
Understanding the Air Quality Index (AQI)
What the numbers mean, how each level affects your health, and when to limit outdoor activity.
Key Takeaway
According to the U.S. EPA, the AQI runs from 0 to 500. Below 50 is "Good." Above 100, air quality becomes unhealthy, first for sensitive groups like children, the elderly, and people with asthma, then for everyone. The most important pollutants driving AQI in most US locations are ground-level ozone (summer) and PM2.5 fine particles (winter and wildfire season).
What Is the AQI?
The Air Quality Index is a standardized scale the EPA uses to communicate how clean or polluted the air is and what health effects people might experience. It translates complex pollutant concentration measurements into a single number anyone can understand. An AQI of 50 means the air is clean. An AQI of 150 means everyone should consider reducing prolonged outdoor activity.
The EPA calculates the AQI for five major pollutants: ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. The highest individual pollutant AQI becomes the overall AQI reported for that area. This "worst pollutant wins" approach ensures the number reflects the most significant current health risk.
The Six AQI Levels
| AQI Range | Level | Health Guidance |
|---|---|---|
| 0–50 | Good | Air quality is satisfactory. No health risk. |
| 51–100 | Moderate | Acceptable. Unusually sensitive people may experience minor effects. |
| 101–150 | Unhealthy for Sensitive Groups | Sensitive groups should reduce prolonged outdoor exertion. |
| 151–200 | Unhealthy | Everyone may experience health effects. Limit outdoor activity. |
| 201–300 | Very Unhealthy | Health alert. Everyone should avoid prolonged outdoor exertion. |
| 301–500 | Hazardous | Emergency conditions. Stay indoors. Entire population affected. |
What Drives AQI in Most Areas
In most US locations, two pollutants dominate AQI readings: ground-level ozone in summer and PM2.5 (fine particle pollution) in winter and during wildfire events. Ozone forms when sunlight reacts with vehicle exhaust and industrial emissions, it peaks on hot, sunny afternoons. PM2.5 comes from combustion sources, vehicle emissions, and wildfires. During major wildfire seasons, PM2.5 levels can push the AQI into the Hazardous range even hundreds of miles from the fire.
PlainAir tracks both ozone days and PM2.5 days for every county and metro area. Browse county-level data to see which pollutant drives your area's AQI.
Annual AQI: Good Days vs. Unhealthy Days
Real-time AQI tells you about today. PlainAir focuses on the annual picture, how many days each county or metro experienced Good, Moderate, or Unhealthy air quality over the course of a year. This is arguably more useful for long-term decisions like where to live, because a single "Good" day doesn't tell you whether the area has 300 Good days or 200.
- Good · 157,710 d · 74.6%
- Moderate · 51,698 d · 24.4%
- USG · 1,640 d · 0.8%
- Unhealthy · 319 d · 0.2%
- Very Unhealthy · 40 d · 0.0%
- Hazardous · 43 d · 0.0%
Our best air quality rankings and worst air quality rankings use the annual "good day percentage", the proportion of monitored days with AQI under 50, to rank areas from cleanest to most polluted.
How to Use AQI Data
For day-to-day decisions, check real-time AQI at AirNow.gov before planning outdoor activities. For long-term decisions, choosing where to live, evaluating a neighborhood, or understanding regional air quality trends, use PlainAir's annual data. Compare state-level summaries or drill into specific metro areas to see multi-year trends.
Key metrics to compare:
- Good day percentage: Higher is better. A county with 85% Good days has much cleaner air than one with 55%.
- Unhealthy days: Even a few Unhealthy (AQI 151+) days per year are significant, especially for sensitive groups.
- Median AQI: The typical daily AQI, lower is better.
- Max AQI: Shows worst-case conditions. Areas prone to wildfire smoke spikes will have high max AQI.
Frequently Asked Questions
What does an AQI of 100 mean?
An AQI of 100 corresponds to the national air quality standard for the pollutant. Below 100 is generally considered satisfactory. Above 100, air quality is unhealthy, first for sensitive groups (101-150), then for everyone (151+). The standard is set by the EPA to protect public health with an adequate margin of safety.
Who are "sensitive groups" in air quality warnings?
Sensitive groups include people with asthma, chronic lung disease, or heart disease; older adults (65+); children and teenagers; people who work or exercise outdoors; and pregnant women. These groups experience health effects at lower pollution levels than the general population.
Can the AQI exceed 500?
The AQI scale officially runs from 0 to 500. Values above 500 are considered "Beyond the AQI" and represent emergency conditions. This has occurred during extreme wildfire smoke events in states like California, Oregon, and Washington, where PM2.5 levels vastly exceeded the Hazardous threshold.
How is the AQI calculated?
The EPA measures concentrations of five major pollutants (ground-level ozone, PM2.5, PM10, carbon monoxide, and sulfur dioxide) and converts each to a sub-index using pollutant-specific breakpoints. The highest sub-index becomes the reported AQI. This means the AQI reflects the single worst pollutant at any given time.
Does the AQI measure all types of air pollution?
No. The AQI covers five criteria pollutants regulated by the Clean Air Act. It does not measure many other harmful substances like volatile organic compounds (VOCs), toxic metals, or specific chemicals from industrial emissions. Areas can have "Good" AQI while still having localized pollution issues not captured by the index.
How often is AQI data updated?
Real-time AQI from monitoring stations is updated hourly at AirNow.gov. PlainAir uses EPA AQS annual summary data, which provides a comprehensive yearly picture, how many Good, Moderate, and Unhealthy days each area experienced. Annual data is published by the EPA each spring for the previous calendar year.
Sources
- U.S. Environmental Protection Agency, Air Quality System (AQS), 2020-2025
- EPA AirNow, AQI basics and health guidance
- EPA, National Ambient Air Quality Standards (NAAQS)
This content is for informational purposes only. Always follow official guidance from local authorities. For current air quality conditions, visit AirNow.gov.