Understanding AQI: The Air Quality Index Explained

Last updated April 29, 2026 · 9 min read

When you open an air-quality app and see a number like 87 or 152, that's the AQI — the Air Quality Index. It's the most important number in air quality and arguably the most misunderstood. This guide takes it apart: what it measures, how it's calculated, what each level means, and the fine print most apps don't bother to explain.

What the AQI actually is

The Air Quality Index is a 0-to-500 scale, defined by the U.S. Environmental Protection Agency, that translates measured concentrations of common air pollutants into a single unitless number you can act on. The scale is intentionally aligned with health outcomes, not concentrations: the same AQI value across different pollutants is meant to represent roughly equivalent risk to a typical person. AQI 100 from ozone and AQI 100 from fine particulate matter are designed to deliver about the same health hit even though the underlying concentrations are nothing alike.

The EPA covers six "criteria" pollutants in the AQI: ground-level ozone (O₃), fine particulate matter (PM2.5), coarse particulate matter (PM10), carbon monoxide (CO), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂). Each pollutant has its own concentration-to-AQI lookup table (called a "breakpoint table") published in 40 CFR Part 58 Appendix G. The reported AQI for a location at any moment is whichever pollutant produces the highest sub-index — that pollutant becomes the "responsible" or "dominant" pollutant. In a typical U.S. summer it's ozone. In a wildfire week it's PM2.5.

The six AQI categories, decoded

The EPA divides the 0-to-500 range into six categories, each with its own color and its own health guidance.

"Sensitive groups" is a specific term of art. It includes people with asthma, COPD, lung or heart disease, diabetes, or active respiratory infections; pregnant people; children; and adults aged 65 and older. If you fall into any of those buckets, the orange (101–150) threshold is your action threshold, not the red one. We unpack what each level means in practice in AQI and Your Health.

How the index is actually calculated

The math is simpler than it looks. For each pollutant, the EPA defines a piecewise-linear function that maps concentration to AQI. The breakpoints look like this for PM2.5 (revised in 2024 to align with the tightened annual standard):

Within each row, the relationship is linear: a PM2.5 of 22 µg/m³ falls about halfway between 9.1 and 35.4, so its AQI is about halfway between 51 and 100, or AQI 75. The EPA's full equation:

AQI = ((I_high − I_low) / (C_high − C_low)) × (C − C_low) + I_low

where C is the measured concentration and the high/low values come from the breakpoint row that contains C.

The same approach is applied to ozone (using an 8-hour average), to CO, NO₂, and SO₂ (with their own breakpoints and averaging windows), and to PM10 (24-hour average). The dominant pollutant — the one with the highest AQI — is reported.

NowCast: why the displayed AQI doesn't whipsaw

For PM2.5 and ozone, raw hourly concentrations are extremely noisy. A single gust of wind can spike or drop the reading. If apps reported AQI from raw hourly data, the number would jump 30 points back and forth all day and become useless for decision-making. To fix this, the EPA invented NowCast: a weighted average that emphasizes recent hours but smooths out single-hour noise. During stable conditions the smoothing is heavier; during rapidly changing conditions (a smoke plume rolling in) the algorithm gives recent hours more weight so the AQI catches up faster.

What NowCast actually does, in plain English: take the last 12 hours of measurements, compute a weight based on how much the readings have varied, and produce a single number that's somewhere between "the most recent reading" and "the 12-hour average". This is the AQI you see in essentially every consumer air-quality app, including Smog Report. We unpack the data path in How AirNow Data Works.

What the AQI is not

It's worth being clear about what the index doesn't capture, so you can spot the gaps:

• It's not a measurement of pollution. It's a translated index. If you want raw concentrations (µg/m³, ppb), most regulatory dashboards still show those underneath.
• It's location-dependent and only as good as the nearest monitor. A monitor 20 miles away on the other side of a ridge can be measuring an entirely different airshed than what you're breathing in your backyard.
• It does not include every harmful pollutant. Some toxic substances — many volatile organic compounds, formaldehyde, hydrogen sulfide near industrial sources — aren't in the AQI at all because they don't have a National Ambient Air Quality Standard. A "Good" AQI doesn't guarantee the air is free of all hazards.
• It's a public-health-average index. It's calibrated for typical responses across the population. Your personal sensitivity may be much higher or much lower; over time, paying attention to how you feel at different AQI levels gives you a personal threshold that may differ from the EPA's general guidance.

Putting it to work

The practical use of AQI is small and concrete. Three habits cover most of it:

1. Glance at it before outdoor exertion. Running, cycling, anything where you'll be breathing hard for 20+ minutes — that's where AQI matters most. Resting indoors, the AQI matters much less.
2. Know your category threshold. If you're in a sensitive group, your action level is AQI 100; if you're not, it's AQI 150.
3. Check the dominant pollutant, not just the number. The same AQI 130 from PM2.5 (smoke) and from ozone calls for different responses. PM2.5 — wear an N95 if you must be outside, run an air purifier indoors. Ozone — masks don't help, exercise indoors instead.

Primary sources: EPA AirNow — AQI Basics · EPA AQI Calculator · EPA NAAQS Table