You've probably seen AQI numbers in weather apps or air quality alerts. "Today's AQI is 87 — Moderate." But what does that actually mean? Should you cancel your morning run? Is it safe for your child with asthma? And why do two cities report the same AQI value for conditions that feel very different?
The Air Quality Index is a standardized reporting tool created by the EPA to communicate air quality information to the public in an accessible way. Understanding how it works — and its limitations — is important for anyone making decisions based on air quality data.
How AQI is Calculated
The AQI is not a single measurement of "air quality." It is a composite index calculated from measurements of five major pollutants: ground-level ozone (O3), particulate matter 2.5 micrometers and smaller (PM2.5), particulate matter 10 micrometers and smaller (PM10), carbon monoxide (CO), and sulfur dioxide (SO2). A sixth pollutant, nitrogen dioxide (NO2), is included in some AQI calculations at the local level.
For each pollutant, a raw concentration measurement is converted to an index value using a piecewise linear function defined by the EPA. These breakpoints are based on the National Ambient Air Quality Standards (NAAQS) and the associated health evidence for each pollutant. The final reported AQI is the highest individual pollutant index among all the pollutants measured. This means a city's AQI might be driven by ozone on a hot summer afternoon and by PM2.5 on a smoky winter morning.
The AQI Scale
The AQI runs from 0 to 500, divided into six color-coded categories:
0–50 (Green): Good. Air quality poses little or no risk. Most people can be outdoors without concern.
51–100 (Yellow): Moderate. Air quality is acceptable. Some unusually sensitive people may experience mild symptoms during prolonged outdoor exertion.
101–150 (Orange): Unhealthy for Sensitive Groups. People with lung disease, older adults, children, and active people should reduce prolonged or heavy outdoor exertion.
151–200 (Red): Unhealthy. Everyone may begin to experience health effects. Sensitive groups should avoid prolonged outdoor exertion.
201–300 (Purple): Very Unhealthy. Health alert. Everyone should avoid prolonged outdoor exertion.
301–500 (Maroon): Hazardous. Health emergency conditions. Everyone should avoid all outdoor exertion.
What AQI Doesn't Tell You
The AQI is designed for the general population and communicates a single number. This simplification is useful but masks important nuance. First, the "sensitive groups" category is extremely broad. A 75-year-old with severe COPD and a healthy 35-year-old with mild seasonal allergies both fall into "sensitive groups," but their appropriate responses to an AQI of 115 are very different.
Second, the AQI doesn't distinguish between pollutants with different health profiles. An AQI of 120 driven by ozone primarily affects the respiratory system through oxidative stress. An AQI of 120 driven by PM2.5 carries cardiovascular risk in addition to respiratory effects. The health implications are meaningfully different, but the AQI number looks the same.
Third, the AQI is typically calculated from 8-hour or 24-hour average concentrations, depending on the pollutant. This averaging smooths out short-term spikes. If PM2.5 spikes to 200 µg/m³ for two hours and is otherwise at 10 µg/m³ for the rest of the day, the 24-hour average might report a moderate AQI. But the two-hour spike still represents a real acute exposure risk.
PM2.5: The Pollutant That Matters Most
If you only track one air quality metric for health purposes, it should be PM2.5. Fine particulate matter — particles smaller than 2.5 micrometers — is small enough to bypass the respiratory tract's natural defenses and penetrate deep into the alveoli of the lungs. From there, the smallest particles can enter the bloodstream directly.
The health evidence linking PM2.5 to adverse outcomes is among the strongest in environmental health science: associations with cardiovascular disease, stroke, lung cancer, premature birth, cognitive decline, and all-cause mortality. The 2024 NAAQS PM2.5 standard was tightened from 12 µg/m³ to 9 µg/m³ in annual average, reflecting updated health evidence showing effects at lower concentrations than previously understood.
PM2.5 is also the pollutant most dramatically affected by wildfire smoke, which is why Trace AQ focuses particular attention on it. During major wildfire smoke events, PM2.5 concentrations can spike to 200–500 µg/m³ — concentrations that are genuinely dangerous and require immediate protective action.
Why Forecasting Matters More Than Current Conditions
The AQI tells you what's happening right now. But for most protective health behaviors — staying indoors, canceling events, adjusting medication, pre-positioning resources — current conditions are too late. By the time AQI crosses the Unhealthy threshold, the exposure is already happening.
This is the core insight behind Trace AQ: the most valuable air quality information is accurate advance warning, not real-time reporting. Our forecasts combine the pollutant-level specificity of the standard AQI framework with the predictive horizon needed to actually act on the information. We deliver hourly PM2.5, ozone, and combined AQI forecasts up to 4 days ahead, giving healthcare providers, city managers, and informed individuals the time they need to prepare.
Understanding AQI is the foundation. Forecasting it is what changes outcomes.
