Air Quality Alerts, Decoded
An Air Quality Alert is a regulatory message, not a weather forecast: a state air agency concludes that pollution is expected to reach a defined rung of the EPA's Air Quality Index, and the National Weather Service broadcasts that conclusion through the same feeds that carry storm warnings.
The trigger, the wording, and even the product name vary by state, which is why an identical forecast can produce an alert in one state and none across the border. This guide covers who issues alerts, the AQI 101 threshold most programs use, Ozone Action Day mechanics, the difference between smoke-driven and ozone-driven episodes, and the Code Orange color shorthand.
Alert color codes on the AQI scale
Most state alert programs trigger where the orange band begins — a forecast AQI of 101. The Code Orange / Red / Purple shorthand is the EPA scale's own colors.
Who actually issues an Air Quality Alert
The National Weather Service transmits the product headlined Air Quality Alert, but it does not decide when one is needed. That call belongs to state, local, and tribal air agencies — the same bodies that run the monitoring networks behind AirNow. When an agency's forecasters expect pollution to cross their threshold, the NWS relays the message so it lands in weather apps, NOAA Weather Radio, and the same alert feeds as any watch or warning.
That division of labor explains an oddity in the text itself: an alert usually names the pollutant, the affected counties, and the issuing agency rather than a weather office. The forecast behind it is an air-quality forecast, produced by the state agency and published through AirNow — not a temperature or precipitation product.
The trigger: AQI 101 in most states
There is no single federal trigger, but most programs draw the line where the orange band begins: an AQI of 101, the category EPA labels Unhealthy for Sensitive Groups. The Minnesota Pollution Control Agency issues an alert when the index is forecast to reach 101 or higher; the Wisconsin DNR's advisories key off a forecast above 100; Colorado's CDPHE issues an Action Day Alert when a pollutant is expected to reach that same category or beyond.
Because each of the 3 cited agencies — and every other state's — writes its own criteria, borders matter. A plume that pushes the forecast to 105 can produce an alert on one side of a state line and silence on the other, not because the air differs but because the paperwork does. AirNow's forecast map shows the underlying category either way.
Ozone Action Days: summer chemistry on a schedule
Ground-level ozone is not emitted directly. It forms when nitrogen oxides and volatile organic compounds react in sunlight, which is why ozone alerts cluster on hot, stagnant, sunny days — many agencies brand them Ozone Action Days. Concentrations typically build through the day and peak in the mid-to-late afternoon before easing after sunset.
The federal benchmark is the 2015 ozone standard: 0.070 ppm — 70 parts per billion — averaged over 8 hours. The AQI's breakpoint table is aligned to it: Moderate tops out at 70 ppb, and the orange band opens at 71 ppb. Most state ozone-forecast seasons run roughly May through September, matching the sun angle that drives the chemistry.
Smoke alerts vs ozone alerts
The same alert product covers two different atmospheres. Ozone-driven alerts are a summer-afternoon phenomenon tied to local sunshine acting on traffic and industrial precursors. PM2.5-driven alerts follow particle sources instead: wildfire smoke in the warm season and, in some mountain basins, wintertime inversions that trap emissions near the ground.
The "primary pollutant" line in a forecast says which regime is in charge. During the Canadian-smoke episodes that have crossed the Upper Midwest and Great Lakes since 2023, nearly every alert named PM2.5; on an August heat-ridge afternoon in Texas, the same headline usually carries ozone.
Code Orange, Code Red: the color shorthand
The Action Day programs EPA coordinates with state agencies use the AQI's own colors as shorthand. Code Orange announces a forecast in the 101–150 band; Code Red means 151–200; Code Purple, much rarer, marks 201 and above. The codes add urgency but no new information — a Code Red day is simply a day forecast to land in the red category of the standard EPA scale.
How long alerts last, and where to check
An Air Quality Alert carries explicit start and end times — commonly through the end of a day or an overnight period — and agencies extend, expand, or cancel it as forecasts update. Multi-day smoke episodes are typically handled as a chain of extensions rather than one long alert. Current status lives in two public places: the AirNow forecast for the underlying index, and the NWS active-alerts feed for the text product itself.
Alert taxonomy: alert vs Action Day vs advisory
One product family, three vocabularies — the forecast AQI band carries the meaning.
The NWS disseminates the Air Quality Alert product on behalf of state and local air agencies; Ozone Action Day programs follow the 2015 EPA 8-hour ozone standard of 70 ppb. “Advisory” tiers and names vary by state. Sources: NWS; US EPA / AirNow.
Alert machinery, by the numbers
forecast level where the cited Minnesota, Wisconsin, and Colorado programs begin
MN PCA / WI DNR / CO CDPHE
EPA 8-hour ozone standard since 2015 — Moderate tops out here
US EPA
where Code Red begins on the EPA scale
US EPA / AirNow
where Code Purple begins
US EPA / 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:
- Phoenix, AZ air quality — Relentless summer sun and stagnant valley air make ozone-season forecasts a daily ritual.
- Denver, CO air quality — The Front Range runs a formal summer Action Day Alert program for ozone.
- Houston, TX air quality — Gulf heat, sunshine, and precursor-rich air give it one of the longest ozone-watch seasons.
- Los Angeles, CA air quality — Mountain-ringed basin geography traps sunny-day ozone — the classic action-day setup.
- Minneapolis, MN air quality — The state agency's alerts begin at a forecast AQI of 101 — a threshold smoke episodes now test every summer.
- Milwaukee, WI air quality — Lake-breeze ozone along the Lake Michigan shore adds summer advisories on top of smoke-season ones.
Frequently asked
- What does an Air Quality Alert mean?
- A state or local air agency expects the AQI to reach its alert threshold — in most states the orange band that begins at 101 — and the National Weather Service has broadcast that forecast. The category itself comes from the standard EPA scale.
- Who issues air quality alerts?
- State, local, and tribal air agencies decide; the NWS disseminates. The Minnesota Pollution Control Agency, Wisconsin DNR, and Colorado CDPHE are typical issuing agencies, each with published criteria.
- What AQI triggers an air quality alert?
- Most state programs trigger on a forecast of 101 or higher — the start of the EPA category named Unhealthy for Sensitive Groups. Thresholds are state-defined, so the exact criteria and product wording shift at state lines.
- What is the difference between an alert and an advisory?
- Mostly vocabulary. Some agencies use "advisory" for a lower tier or a pollutant-specific notice, while the NWS text product is uniformly headlined Air Quality Alert. The forecast AQI band carries the meaning, not the noun.
- What is an Ozone Action Day?
- An agency's announcement that sunlight-driven ozone is forecast to reach its action level — territory defined by the 70 ppb federal 8-hour standard and the 101+ AQI band. They cluster on hot, stagnant afternoons, roughly May through September.
- What does Code Orange mean?
- A forecast in the AQI's orange band, 101–150 — Unhealthy for Sensitive Groups in EPA's naming. Code Red is 151–200 and Code Purple 201+; the codes are the scale's colors used as shorthand.
- Do air quality alerts expire?
- Yes — each alert states an end time, and agencies extend or cancel as conditions change. During long smoke episodes an alert is often re-issued daily, so the current AirNow forecast beats yesterday's headline.