Air Quality Has Seasons. Here Is the Calendar.

Q: Why is air quality bad in winter in mountain valleys?

Temperature inversions. Utah DEQ's description, verbatim: "In a temperature inversion, the situation 'inverts,' and cold air at the surface gets trapped under a layer of warmer air." A valley under that lid keeps everything it emits, so wood smoke and traffic exhaust accumulate for days. Salt Lake City, Boise, Missoula, Logan, Reno and above all Fairbanks run this season every winter.

Q: When is ozone season?

Wherever there is sun, heat and traffic: roughly April into September in the sun belt (Phoenix's season is among the longest), June through August in the northern and coastal metros. EPA's summary is that ozone "is most likely to reach unhealthy levels on hot sunny days in urban environments." Within any ozone day, afternoons are the peak, because the chemistry needs hours of sunlight.

Q: Where should I check the current AQI?

AirNow.gov, EPA's official service, publishes the live reading and forecast; its Fire and Smoke Map adds monitor-level detail during smoke events. These pages carry the calendar (which months, which mechanism, from five years of EPA regulatory data) and the live NWS alert products, as issued.

Q: Are these pages a health guide?

No. Every category meaning, sensitive-group list and cautionary statement on these pages is EPA's language, quoted verbatim and attributed, and every live alert is reproduced as the issuing office wrote it. The pages add the seasonal analysis of that public record. For decisions on a given day, use the official product for your location and your own clinician.

Which months the air goes bad where you live, and why: smoke season, inversion season, ozone season and dust, state by state and metro by metro, from five years of EPA's own record. Today's AQI is published by AirNow.

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State calendars

Metro calendars

EPA record

2020 to 2024

The four mechanisms behind every bad-air month

EPA record, 2020 to 2024

Jul to Oct

smoke season

the West, and anywhere downwind of Canada

Nov to Feb

inversion season

mountain valleys, Fairbanks to Salt Lake

Apr to Sep

ozone season

the sun belt and the urban corridors

Spring, monsoon

dust

the desert Southwest and the dry plains

Spans above are the national envelope, not any one place's answer. Every state and metro page carries its own twelve-month chart computed from EPA's regulatory record.

The mechanisms

Smoke, inversions, ozone and dust: what actually happens

Wildfire smoke season

Smoke is fine particulate (PM2.5) that travels. In the West it floods the valleys nearest the fires; on a strong continental flow it crosses the country, as the East learned in June 2023. Washington's Department of Ecology states it flatly: “Smoke from wildfires is the largest source of particle pollution in Washington.” The season peaks in late summer, but it is a wind-and-fire question, not a calendar one.

Satellite view of wildfire smoke filling the mountain valleys of central Idaho, with active fire detections marked in red — Smoke filling the valleys of the Sawtooth Range and Salmon River Mountains, NASA MODIS, September 12, 2022. Smoke behaves like a fluid: it pours into terrain and pools where people live. “Smoke Shrouds the Sawtooth Range and Salmon River Mountains (MODIS 2022-09-12)” — MODIS Land Rapid Response Team, NASA GSFC — Public domain

Winter inversion season

Utah's Department of Environmental Quality explains its famous January haze: “In a temperature inversion, the situation ‘inverts,’ and cold air at the surface gets trapped under a layer of warmer air. A typical Utah winter sees about five to six multi-day inversion episodes and on average, 18 days with high PM2.5 levels exceeding the National Ambient Air Quality Standard (NAAQS).” The same physics runs in every deep valley from Fairbanks to Missoula to the Cache Valley: the mountains that make the view also make the lid.

Historic photograph of smog blanketing Salt Lake City, downtown towers barely visible under a haze layer against the mountains — Salt Lake City under an inversion, EPA Documerica, early 1970s. Emissions have fallen enormously since; the meteorology is permanent. “Smog Blankets Salt Lake City (Documerica)” — Bruce McAllister, US EPA, NARA 544791 — Public domain

Summer ozone season

Ozone is not emitted; it is cooked. EPA's description: “Ozone is most likely to reach unhealthy levels on hot sunny days in urban environments, but can still reach high levels during colder months.” That is why ozone season tracks the sun: April into September in Phoenix, June through August in the Northeast corridor, and afternoon by afternoon everywhere, since the chemistry needs hours of light to run.

Aerial photograph of downtown Los Angeles in the 1970s under thick brown photochemical smog — Downtown Los Angeles under photochemical smog, EPA Documerica, 1970s. The machine that made this photo, sun plus heat plus emissions plus a lid of marine air, still runs; controls have turned it down, not off. “Sunlight and Smog (Documerica)” — Gene Daniels, US EPA, NARA 552395 — Public domain

Dust

Coarse particles (PM10) on the wind: monsoon haboobs in Arizona, spring windstorms in New Mexico and West Texas, drought-year dust on the plains, and a growing lakebed dust question at the shrinking Great Salt Lake. Dust events are short, violent and local, which makes the live alert feed below the right tool for them.

Every state, one line each

The worst month for air quality, state by state

Computed from EPA's daily AQI record, 2020 to 2024: each state's worst month by share of days at Unhealthy for Sensitive Groups or worse, and the pollutant that most often set the index that month. Click through for the full twelve-month chart.

State	Worst month	Usually set by	Season type
Maine	June	ozone	Summer ozone, plus imported smoke
New Hampshire	July	ozone	Short summer ozone season
Vermont	July	fine particles (PM2.5)	Summer transport, winter valley smoke
Massachusetts	July	ozone	Summer ozone season
Connecticut	June	ozone	The Northeast's toughest ozone corridor
Rhode Island	June	ozone	Summer ozone season
New York	June	ozone	Summer ozone, and a new smoke wildcard
New Jersey	June	ozone	Summer ozone season
Pennsylvania	June	ozone	Summer ozone, valley inversions
Delaware	June	ozone	Summer ozone season
Maryland	June	ozone	Summer ozone season
Virginia	June	ozone	Mild summer ozone season
West Virginia	June	ozone	Valley inversions, summer ozone
North Carolina	June	ozone	Summer ozone, mountain smoke
South Carolina	July	ozone	Summer ozone, burn season smoke
Georgia	June	fine particles (PM2.5)	Summer ozone, winter particulate
Florida	June	fine particles (PM2.5)	Spring: the surprising peak
Kentucky	June	ozone	Summer ozone season
Tennessee	June	fine particles (PM2.5)	Summer ozone season
Alabama	June	ozone	Summer ozone, year-round burn smoke
Mississippi	June	ozone	Fall burning, summer ozone
Arkansas	June	ozone	Early summer ozone season
Louisiana	June	ozone	Long warm-season ozone
Ohio	June	ozone	Summer ozone season
Michigan	June	ozone	Lake breeze ozone, imported smoke
Indiana	June	ozone	Summer ozone season
Illinois	June	ozone	Lake breeze ozone season
Wisconsin	June	ozone	Lakeshore ozone, winter stagnation
Minnesota	June	ozone	Canadian smoke season
Iowa	June	ozone	Winter stagnation, June ozone
Missouri	June	ozone	Summer ozone season
Kansas	June	ozone	April burn smoke, summer ozone
Nebraska	June	fine particles (PM2.5)	Smoke and dust on the wind
South Dakota	July	ozone	Imported summer smoke
North Dakota	August	fine particles (PM2.5)	Canadian smoke season
Colorado	July	ozone	Front Range ozone, mountain smoke
Wyoming	July	ozone	Summer smoke, winter basin ozone
Montana	September	fine particles (PM2.5)	Smoke season
Idaho	September	fine particles (PM2.5)	Smoke season, winter inversions
Utah	August	ozone	Winter inversions, summer ozone
Nevada	August	ozone	Ozone, smoke, and dust
Arizona	July	ozone	One of the longest ozone seasons
New Mexico	July	ozone	Spring dust and fire, summer ozone
Texas	August	ozone	Long ozone season, dust and smoke
Oklahoma	August	ozone	Summer ozone, spring smoke
California	August	ozone	Every season, somewhere
Oregon	September	fine particles (PM2.5)	Smoke season, winter wood smoke
Washington	September	fine particles (PM2.5)	Smoke season
Alaska	January	fine particles (PM2.5)	Winter inversions, summer boreal fire
Hawaii	January	fine particles (PM2.5)	Vog, when the volcano is active

Washington, DC is covered by its metro page. Puerto Rico and the US Virgin Islands are not covered: our state framework and alert filtering are built on the 50 states, and we would rather name that gap than publish a half-built page.

Live, as issued

Every NWS air quality product active right now

One request to api.weather.gov, reproduced verbatim: Air Quality Alerts (written by or for state air agencies and relayed by NWS) plus the smoke, dust, ashfall and stagnation advisories NWS issues itself. Each is reproduced in full. Today's AQI reading is published by EPA's AirNow.gov and its Fire and Smoke Map.

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Reading the index

The AQI scale, in EPA's words

AQI	Category	What it means
0 to 50	Good	Air quality is satisfactory, and air pollution poses little or no risk.
51 to 100	Moderate	Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution.
101 to 150	Unhealthy for Sensitive Groups	Members of sensitive groups may experience health effects. The general public is less likely to be affected.
151 to 200	Unhealthy	Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects.
201 to 300	Very Unhealthy	Health alert: The risk of health effects is increased for everyone.
301 and higher	Hazardous	Health warning of emergency conditions: everyone is more likely to be affected.

Category names, index ranges and colors are reproduced exactly from the US EPA, Technical Assistance Document for the Reporting of Daily Air Quality (EPA-454/B-24-002, May 2024), Tables 1 and 2. Meanings quoted from AirNow.gov, AQI Basics.

Who EPA means by sensitive groups

When this pollutant has an AQI above 100	EPA lists these sensitive groups
Ozone	People with lung disease, such as asthma; children, including teenagers; people who are active outdoors; and older adults.
Fine particle pollution (PM2.5) and particle pollution (PM10)	People with heart or lung disease; older adults; and children, including teenagers. In addition, some minority groups, people with lower incomes, and outdoor workers may experience higher exposure that can worsen underlying health conditions.
Carbon monoxide (CO)	People with heart disease.
Nitrogen dioxide (NO2)	People with lung disease, such as asthma; children, including teenagers; and older adults.
Sulfur dioxide (SO2)	People with asthma; children, including teenagers; and older adults.

What EPA says sensitive groups should do on an orange day

Sensitive groups: Make outdoor activities shorter and less intense. Take more breaks. Watch for symptoms such as coughing or shortness of breath. Plan outdoor activities in the morning when ozone is lower. People with asthma: Follow your asthma action plan and keep quick-relief medicine handy.
EPA cautionary statement for Ozone (8-hour), Unhealthy for Sensitive Groups, quoted from the US EPA, Technical Assistance Document for the Reporting of Daily Air Quality (EPA-454/B-24-002, May 2024), Table 5.

Sensitive groups: Make outdoor activities shorter and less intense. It's OK to be active outdoors but take more breaks. Watch for symptoms such as coughing or shortness of breath. People with asthma: Follow your asthma action plan and keep quick relief medicine handy. People with heart disease: Symptoms such as palpitations, shortness of breath, or unusual fatigue may indicate a serious problem. If you have any of these, contact your health care provider.
EPA cautionary statement for PM2.5 (24-hour), Unhealthy for Sensitive Groups, quoted from the US EPA, Technical Assistance Document for the Reporting of Daily Air Quality (EPA-454/B-24-002, May 2024), Table 5.

Everything above is EPA’s language, quoted. On any day that matters, the decision belongs with the official product for your location and your own clinician.

Where the question is asked

Metro air quality calendars

The 49 metros where the seasonal question is sharpest: every inversion-prone valley city, the western smoke corridor, and the ozone belt. Each page cuts the same EPA record to the metro area.

State by state

Air quality seasons by state

Smoke season states

Late summer, when wildfire smoke floods the valleys and rides the continental wind. The season follows the fire map, not a date.

Inversion season states

Winter, when a lid of warm air seals cold valleys and everything emitted underneath stays until the weather breaks.

Ozone season states

Summer, when heat and sun cook traffic and industry emissions into ozone on exactly the afternoons people are outside.

States with several seasons

Two or more of the above sharing one calendar, usually a summer problem and a winter problem back to back.

Common questions

Air quality seasons FAQ

When is wildfire smoke season?

For the West, the high-risk months are July through September, extending into October in a bad year; the northern tier can see Canadian smoke as early as May. But smoke season is set by fires and wind, not by dates: the June 2023 East Coast episode came in what used to be considered the clean early summer. Each state and metro page on this site ranks its own months from the EPA record, and the live NWS alert feed carries the day-to-day answer.

Why is air quality bad in winter in mountain valleys?

When is ozone season?

What is a good AQI? What do the colors mean?

EPA's scale has six categories: Good (0 to 50, green), Moderate (51 to 100, yellow), Unhealthy for Sensitive Groups (101 to 150, orange), Unhealthy (151 to 200, red), Very Unhealthy (201 to 300, purple), and Hazardous (301 and higher, maroon). The definitions on this page are quoted verbatim from EPA. The planning threshold most people care about is 101: the first category EPA ties to health effects.

Where should I check the current AQI?

Are these pages a health guide?

Sources: EPA Air Quality System daily AQI files (2020 to 2024, public domain regulatory data, refreshed annually); api.weather.gov active alerts, read live in your browser and reproduced as issued; AQI definitions from EPA's Technical Assistance Document (EPA-454/B-24-002, May 2024) and AirNow.gov, quoted verbatim. Agency quotes are reproduced exactly with links; today's AQI reading is at AirNow, and health guidance on these pages is EPA's, quoted.