What does AQI stand for?

Air Quality Index — the EPA's 0-to-500 scale that reports current outdoor air pollution as one number, sorted into six color-coded categories from Good (green) to Hazardous (maroon).

EPA labels 0–50 as Good (green). The next band, Moderate (51–100, yellow), is still below the level where any category with "unhealthy" in its EPA name begins.

Is an AQI of 150 bad?

An AQI of 150 sits at the very top of the orange Unhealthy for Sensitive Groups band (101–150); one point higher crosses into the red Unhealthy band (151–200). EPA's category names are the scale's own severity labels.

How is the AQI calculated?

Each of the six pollutants is converted to a sub-index by linear interpolation within EPA's breakpoint table, and the highest sub-index becomes the reported AQI. Hourly values use AirNow's NowCast, a weighted average of the last 12 hours.

What pollutant drives the AQI during wildfire smoke?

Fine particles — PM2.5. Smoke episodes push the PM2.5 sub-index far above the other five pollutants, so it becomes the reported number under the highest-sub-index rule.

Why do AirNow and PurpleAir show different numbers?

AirNow reports regulatory monitors plus corrected sensor data, while a raw PurpleAir reading has no EPA correction applied — and the low-cost sensors tend to read high in dense smoke. The Fire and Smoke Map applies EPA's correction equation before display.

Can the AQI go above 500?

Concentrations can exceed the top of the breakpoint table, but the reported index is capped at 500. Some agencies label such conditions "beyond the AQI" — the June 2023 smoke episode produced readings in the 400s in the Northeast, approaching that cap.

The AQI Scale, Explained

The Air Quality Index is the EPA's 0-to-500 scale for reporting how polluted the outdoor air is right now. One number summarizes six different pollutants, and six color-coded categories turn that number into a word — Good, Moderate, and on up to Hazardous — so a reading makes sense without a chemistry background.

The scale is piecewise, not linear: each category covers a fixed span of index points that maps back to a measured pollutant concentration. This guide walks through the 6 bands and their exact boundaries, the arithmetic that converts a raw measurement into the index, the 2024 revision that tightened the fine-particle numbers, and where the readings on a map actually come from.

Where each AQI category begins

The EPA index runs 0–500 across six color-coded categories. The ladder shows the first five; the sixth — Hazardous, maroon — begins at 301.

US AQISensitive Groups 101 AQIUnhealthy 151 AQIVery Unhealthy 201 AQI

EPA AQI categories: Good 0–50 (green), Moderate 51–100 (yellow), Unhealthy for Sensitive Groups 101–150 (orange), Unhealthy 151–200 (red), Very Unhealthy 201–300 (purple), Hazardous 301+ (maroon); the reported index caps at 500. Source: US EPA / AirNow.

The six AQI categories and their colors

EPA divides the index into six bands: Good (0–50, green), Moderate (51–100, yellow), Unhealthy for Sensitive Groups (101–150, orange), Unhealthy (151–200, red), Very Unhealthy (201–300, purple), and Hazardous (301 and above, maroon). The colors are part of the standard itself — a maroon map pixel means the same thing on every app and agency site that follows the EPA convention.

The band edges matter more than small moves within a band, because the category label — EPA's own wording, like "Unhealthy for Sensitive Groups" — changes at 101, 151, 201, and 301, not at every point in between. A 100 and a 101 sit two points apart on the number line but in different categories.

Six pollutants, one number: the highest sub-index wins

The US AQI is computed from 6 pollutants: ground-level ozone, fine particles (PM2.5), coarse particles (PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. Each gets its own sub-index on the same 0-to-500 scale, and the reported AQI is simply the highest of the six — the index never averages pollutants together.

That maximum rule is why the "primary pollutant" line on a report matters. During wildfire smoke, PM2.5 is almost always the driver; on hot, stagnant, sunny afternoons, ozone usually takes the top slot. Two cities can post the same AQI of 121 for entirely different physical reasons.

How the number is actually computed

Each pollutant has an EPA breakpoint table pairing concentration ranges with index ranges. The conversion is linear interpolation within the matching row: the index equals the fraction of the way the measured concentration sits through its concentration range, scaled onto the corresponding index range and added to that range's floor. The same formula covers every pollutant and every band; only the table values differ.

For hour-by-hour reporting, AirNow does not wait for a full 24-hour average. Its NowCast method weights the most recent 12 hours of particle measurements, and the weighting shifts toward the newest hours when conditions change quickly — the weight factor never drops below 0.5. That is how the map can jump from yellow to red within a couple of hours as a smoke plume arrives.

The 2024 EPA update to PM2.5

In 2024, EPA tightened the annual PM2.5 standard from 12 to 9 µg/m³ and revised the AQI breakpoints for fine particles to match. Under the revised table, a 24-hour PM2.5 level of 9 µg/m³ is the top of the Good band, and 35.4 µg/m³ is the top of Moderate — so the same measured air now reports a somewhat higher index than it did before the change.

Practically, that means comparing this summer's readings against pre-2024 archives is an apples-to-oranges exercise near the Good/Moderate boundary: the air did not necessarily change, the ruler did.

Reading AQI vs raw µg/m³

Monitoring hardware measures concentration — micrograms of particulate per cubic meter — while the public number is the index. The two are related through the breakpoint table, not by a single multiplier: AQI 50 corresponds to 9 µg/m³ of 24-hour PM2.5, while AQI 100 corresponds to 35.4 µg/m³. Research papers and sensor dashboards usually speak in µg/m³; forecast pages and maps speak in AQI. Knowing which unit a chart uses is the first step to reading it correctly.

Where the numbers come from

The official readings behind AirNow come from regulatory-grade monitors operated by state, local, and tribal air agencies. The EPA and US Forest Service Fire and Smoke Map layers in a second source during fire season: low-cost PurpleAir sensors, shown only after an EPA correction equation adjusts their raw output, which tends to read high in dense smoke. During the June 2023 Canadian-smoke episode this network recorded readings in the 400s across parts of the Northeast — New York City among them — some of the highest index values in the archive for the region.

All six bands, drawn to scale

Each bar spans its band's index range — the categories widen toward the top of the scale.

Good0–50

Moderate51–100

Sensitive Groups101–150

Unhealthy151–200

Very Unhealthy201–300

Hazardous301–500

The scale, by the numbers

pollutants feed the index — the highest sub-index wins

US EPA

9µg/m³

annual PM2.5 standard after the 2024 EPA update (was 12)

US EPA

500

the reported index's hard cap

US EPA / AirNow

12hours

of data in AirNow's NowCast hourly weighting

AirNow

Check your local air quality

The worst modern AQI days east of the Rockies are wildfire-smoke days, and the transport corridor from the Canadian boreal fires runs across the Upper Midwest and Great Lakes. These metros sit in that path — check the current AQI for each, or look up any US city on the air quality hub:

Minneapolis, MN air quality — Sits directly under the main transport corridor for Canadian boreal wildfire smoke.
Milwaukee, WI air quality — Great Lakes geography puts it in the path of long-range smoke plumes riding the jet stream.
Chicago, IL air quality — Smoke from Canadian fires regularly settles over the lakefront during summer episodes.
Detroit, MI air quality — Downwind of both the boreal fire regions and Great Lakes smoke recirculation.
Duluth, MN air quality — One of the first US metros a southbound Canadian smoke plume reaches.
New York, NY air quality — Recorded AQI readings in the 400s when the June 2023 Canadian smoke plume arrived.

Frequently asked

What does AQI stand for?: Air Quality Index — the EPA's 0-to-500 scale that reports current outdoor air pollution as one number, sorted into six color-coded categories from Good (green) to Hazardous (maroon).
What is a good AQI?: EPA labels 0–50 as Good (green). The next band, Moderate (51–100, yellow), is still below the level where any category with "unhealthy" in its EPA name begins.
Is an AQI of 150 bad?: An AQI of 150 sits at the very top of the orange Unhealthy for Sensitive Groups band (101–150); one point higher crosses into the red Unhealthy band (151–200). EPA's category names are the scale's own severity labels.
How is the AQI calculated?: Each of the six pollutants is converted to a sub-index by linear interpolation within EPA's breakpoint table, and the highest sub-index becomes the reported AQI. Hourly values use AirNow's NowCast, a weighted average of the last 12 hours.
What pollutant drives the AQI during wildfire smoke?: Fine particles — PM2.5. Smoke episodes push the PM2.5 sub-index far above the other five pollutants, so it becomes the reported number under the highest-sub-index rule.
Why do AirNow and PurpleAir show different numbers?: AirNow reports regulatory monitors plus corrected sensor data, while a raw PurpleAir reading has no EPA correction applied — and the low-cost sensors tend to read high in dense smoke. The Fire and Smoke Map applies EPA's correction equation before display.
Can the AQI go above 500?: Concentrations can exceed the top of the breakpoint table, but the reported index is capped at 500. Some agencies label such conditions "beyond the AQI" — the June 2023 smoke episode produced readings in the 400s in the Northeast, approaching that cap.