Understanding Air Quality: What the AQI Really Means

What Is the Air Quality Index?

The Air Quality Index is a standardized scale from 0 to 500 that the EPA uses to report how clean or polluted the air is and what associated health effects might be a concern. The index was designed to be intuitive: lower numbers mean cleaner air, and higher numbers mean more pollution. The AQI is divided into six color-coded categories, each tied to a specific level of health concern.

At the "Good" level (0-50), air quality is satisfactory and poses little or no health risk. At "Moderate" (51-100), conditions are acceptable for most people, though unusually sensitive individuals may experience minor symptoms. Above 100, the air becomes problematic — first for sensitive groups like children, the elderly, and people with asthma, and then for everyone as the index climbs toward "Unhealthy" (151-200), "Very Unhealthy" (201-300), and "Hazardous" (301-500).

How the AQI Is Calculated

The AQI is not a single measurement. It is calculated separately for five criteria pollutants: ground-level ozone, fine particulate matter (PM2.5), coarse particulate matter (PM10), carbon monoxide, nitrogen dioxide, and sulfur dioxide. Each pollutant has its own breakpoint table — a set of concentration thresholds that map measured values to the 0-500 AQI scale. The highest individual pollutant AQI becomes the reported AQI for that day and location.

This means a city with an AQI of 85 could be experiencing moderate ozone levels on a hot summer day, or moderate PM2.5 from nearby wildfire smoke, or elevated NO2 from heavy traffic. The number alone does not tell you which pollutant is driving it — which matters because different pollutants affect your body in different ways.

The Six AQI Categories and What They Mean for Your Body

Good (0-50): Air quality is satisfactory. No precautions needed. You can exercise outdoors, open windows, and go about your day without concern. Most cities in northern New England, the northern Great Plains, and parts of Hawaii regularly fall in this range.

Moderate (51-100): Air quality is acceptable. However, some pollutants may pose a moderate health concern for a very small number of people who are unusually sensitive. If you have severe asthma or a serious heart condition, you might notice mild symptoms during extended outdoor activity.

Unhealthy for Sensitive Groups (101-150): Members of sensitive groups — children, older adults, and people with heart or lung disease — may experience health effects. The general public is not likely to be affected. This is the level where schools in some jurisdictions bring outdoor recess indoors.

Unhealthy (151-200): Everyone may begin to experience health effects. Sensitive groups may experience more serious effects. Limit prolonged outdoor exertion. This level is common during severe wildfire events in western cities.

Very Unhealthy (201-300): Health alert — the entire population is more likely to be affected. Avoid all outdoor exertion. During the 2020 and 2023 wildfire seasons, cities like Portland, San Francisco, and even New York temporarily reached this level.

Hazardous (301-500): Health emergency. Everyone should avoid all outdoor activity. This level is rare in the United States but has occurred during catastrophic wildfire events.

The Two Pollutants That Drive Most AQI Readings

While the AQI accounts for five pollutants, two dominate the picture in modern America: ozone and PM2.5. Across the 1,020 cities tracked by AirHistory, ozone is the dominant pollutant in roughly 55% of cities and PM2.5 dominates in about 40%. The remaining pollutants — PM10, NO2, CO, and SO2 — are the primary driver in fewer than 5% of cities combined, a testament to decades of emission control progress.

Ozone tends to dominate in warmer climates and during summer months because sunlight drives its formation. PM2.5 dominates in areas affected by wildfire smoke, heavy industry, or winter inversions that trap emissions near the ground. Understanding which pollutant drives your city's AQI is important because the health effects differ: ozone primarily irritates the respiratory system, while PM2.5 penetrates deep into the lungs and enters the bloodstream, affecting the cardiovascular system as well.

Why 10-Year Trends Matter More Than Today's Reading

Daily AQI readings are useful for deciding whether to jog outside this afternoon. But they are poor indicators of a city's overall air quality health profile. A single wildfire event can push a normally clean city into the "Hazardous" range for days. A cold front can temporarily clear pollution from a chronically poor-air city. The real question — "Is the air where I live getting better or worse?" — can only be answered by looking at trends over years, not days.

AirHistory analyzes 10 years of EPA monitoring data (2014-2023) for every city. This timeframe is long enough to separate signal from noise. A city whose median AQI has dropped from 65 to 48 over a decade is genuinely improving. A city that has bounced between 40 and 80 each year depending on wildfire season needs a different assessment than a city that has climbed steadily from 45 to 62.

Currently, 346 of the 1,020 tracked cities show an improving trend, 430 are stable, and 244 are worsening. The national picture is mixed — regulations continue to drive down industrial and vehicle emissions, but wildfire smoke and climate-driven ozone are pushing some cities in the wrong direction.

How AirHistory's Air Quality Grade Works

The Air Quality Grade condenses all of this into a single A-F letter grade. The grade is calculated from four factors: 5-year average AQI (40% weight), 10-year trend direction (30% weight), annual unhealthy day count (20% weight), and dominant pollutant type (10% weight). This weighting prioritizes cities that combine clean air with an improving trajectory over cities that happen to have a low average but are getting worse. Grades are recalculated annually when new EPA data becomes available.