The New 2024 AQI Scale for PM2.5, Explained
Last updated 2026-07-15
On May 6, 2024, the EPA changed how fine-particle pollution (PM2.5) converts into an Air Quality Index number. The categories kept their old names and colors, so at a glance nothing looks different — but the same amount of smoke now produces a higher AQI than it did before. If a monitor reads 56 today where a 2023 app would have said 50, this is why.
In short: EPA moved the top of the "Good" range down from 12.0 to 9.0 micrograms per cubic meter (ug/m3) and rescaled "Moderate" to match, following the tightened national health standard for annual PM2.5. Here is what shifted and how to read the current scale.
What actually changed on May 6, 2024
EPA revised the PM2.5 concentration breakpoints — the ug/m3 cut points that map raw readings onto the 0-500 AQI ladder. Only PM2.5 changed; ozone, CO, and the rest were untouched. The two visible edits:
- "Good" now ends at 9.0 ug/m3 instead of 12.0. That top edge is still AQI 50.
- "Moderate" now runs 9.1 to 35.4 ug/m3 and still maps to AQI 51-100, so the band is squeezed into a narrower slice of concentrations.
The higher categories kept their cut points, and the category names, colors, and health messages are all unchanged. That is why the update slipped past most people — "Moderate (yellow)" looks identical whether you enter it at 12.1 or 9.1. We list the exact breakpoints we use on our methodology page.
Why 12.0 ug/m3 is now AQI 56, not 50
AQI is a straight-line interpolation inside each band. Under the old table, 12.0 ug/m3 sat exactly at the top of "Good," so it computed to AQI 50. Under the 2024 table, "Good" stops at 9.0, so 12.0 now falls inside "Moderate."
Moderate spans 9.1-35.4 ug/m3 mapped onto AQI 51-100. Interpolating 12.0 within that range lands at roughly AQI 56. Nothing got dirtier — the same reading just sits higher on the redrawn ladder. A few points on the current scale: 9.0 ug/m3 is AQI 50 (top of Good), 12.0 is about 56, and 35.4 is AQI 100 (top of Moderate).
Want the number for a specific reading? Our can-I-go-outside tool uses the 2024 breakpoints, and the cigarette-equivalent estimator turns a PM2.5 concentration into a rough exposure comparison.
The six AQI categories and what they mean
The AQI runs 0-500 in six colored bands. For wildfire smoke, PM2.5 is almost always the pollutant driving the number. EPA's guidance in plain terms:
- Good — AQI 0-50 (0.0-9.0 ug/m3): Air quality poses little or no risk.
- Moderate — AQI 51-100 (9.1-35.4): Acceptable, but unusually sensitive people may want to limit prolonged heavy exertion outdoors.
- Unhealthy for Sensitive Groups — AQI 101-150 (35.5-55.4): Sensitive groups may feel effects; the general public usually will not.
- Unhealthy — AQI 151-200 (55.5-125.4): Everyone may begin to feel effects; sensitive groups feel them more seriously.
- Very Unhealthy — AQI 201-300 (125.5-225.4): Health alert; risk is increased for everyone.
- Hazardous — AQI 301+ (225.5+): Emergency conditions; the entire population is likely affected.
Per EPA, "sensitive groups" means people with heart or lung disease, older adults, children and teens, and pregnant people. Once smoke pushes you into orange or higher, it is worth running a purifier or a DIY Corsi-Rosenthal box — size it with our purifier sizer.
NowCast: why the current number is not a flat 24-hour average
The AQI formula was built for a full 24-hour PM2.5 average, but during a fast-moving smoke event you need a number now. AirNow solves this with NowCast — a weighted average of the most recent hours of monitor data (up to about 12) that leans harder on the latest readings when conditions change quickly.
Practical effects to keep in mind:
- When smoke rolls in fast, NowCast climbs quickly instead of being dragged down by clean morning hours.
- When smoke clears, the current AQI can lag reality by an hour or two as older, dirtier hours age out.
- The "current" AQI on most maps is a NowCast value; a forecast AQI is a prediction for a full day.
To act minute-to-minute, watch the raw PM2.5 trend rather than the smoothed AQI, and sign up for free smoke alerts.
Why many apps and sites still show the old scale
More than two years on, plenty of weather widgets and third-party air apps still report pre-2024 numbers. Common reasons:
- Hardcoded breakpoint tables. Many services baked the old ug/m3 cut points into their code and never updated the conversion.
- Cached or upstream data. Some pull AQI from an API that itself hasn't switched, so the error is inherited.
- The change is invisible. With names and colors unchanged, there is no visual cue a site is stale — the only tell is comparing a raw concentration to the AQI it produces.
Quick self-test: find a source's raw PM2.5 in ug/m3 and check whether 12.0 reads as AQI 50 (old) or about 56 (2024). Everything here uses the current EPA breakpoints, documented on our methodology page.
FAQ
Did the AQI numbers actually change, or just the health standard?
Both. EPA tightened the national annual PM2.5 standard and, as part of the same May 6, 2024 update, redrew the AQI breakpoints so a given amount of smoke now maps to a higher AQI value. The air did not change; the scale did.
So is a PM2.5 of 12 now Good or Moderate?
Moderate. Under the 2024 scale, Good ends at 9.0 ug/m3, so 12.0 falls into the Moderate band and computes to about AQI 56. Under the old scale it was exactly AQI 50, the top of Good.
What is NowCast and why does the current AQI change so fast?
NowCast is a weighted average of recent hourly PM2.5 readings that emphasizes the latest hours when conditions shift quickly. It lets a map show a meaningful current AQI instead of waiting for a full 24-hour average, so the number can climb or fall within an hour during a smoke event.
How do I know if a weather app is using the new or old scale?
Compare a raw PM2.5 reading in ug/m3 to the AQI it shows. If 12.0 ug/m3 displays as AQI 50, the app is on the old pre-2024 table; if it shows about 56, it is using the current 2024 breakpoints.
Check the air where you are
Live AQI on the 2024 EPA standard, with the reporting monitor and observation time on every reading.
Where this guidance comes from
We paraphrase published guidance rather than inventing our own. The primary sources for everything on this page:
- AirNow — Air Quality and Health (U.S. EPA): the AQI scale and its activity guidance by level.
- EPA — Particulate Matter (PM) Basics: what PM2.5 is and why it reaches deep into the lungs.
- CDC — Wildfire Smoke Safety: who is at higher risk, and when to seek care.
- EPA — Wildfires and Indoor Air Quality: clean rooms, filtration and what actually works indoors.
- AHAM: the CADR standard and the two-thirds room-sizing rule.
Our own AQI arithmetic is documented on the methodology page.
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Some links are affiliate links — see disclosure. This guide is general health information and is not a substitute for medical advice — see disclaimer.