Montana Air Quality: The Seasonal Shape

In EPA's monitoring record for 2020 to 2024, the worst stretch in Montana is July through September. September leads: 8.2% of monitored days reached Unhealthy for Sensitive Groups or worse and 36% were at least Moderate, with the index most often set by fine particles (PM2.5). July runs second. The month-by-month chart on this page shows the full shape.

The data puts it in late summer: the months ranked worst in Montana are July through September, driven by fine particles, which in that part of the calendar means smoke. The season tracks the fire map upwind rather than a fixed date, so treat those months as the watch window rather than a guarantee either way.

Wildfire smoke from Montana's own forests, the Northwest, and Canada, filling the western valleys from July into October. In the EPA record the September index is most often set by fine particles (PM2.5), which is the fingerprint of that mechanism.

April, in the 2020 to 2024 record: only 8% of monitored days even reached Moderate, and 0.0% went beyond it. If you are choosing a month for a visit or an outdoor project on air quality grounds, that is the statistical answer.

AirNow.gov, EPA's official public AQI service, publishes the current reading and forecast at its Montana state page. This page adds the calendar (which months are like this every year, and why) plus the live NWS alert products above, reproduced as issued.

EPA's definition, verbatim: "Members of sensitive groups may experience health effects. The general public is less likely to be affected." It covers AQI 101 to 150, shown in orange. Who counts as a sensitive group depends on the pollutant; for ozone EPA lists "People with lung disease, such as asthma; children, including teenagers; people who are active outdoors; and older adults." The full pollutant-by-pollutant table is on this page, quoted from EPA's Technical Assistance Document.

Check the current category on AirNow, then apply EPA's cautionary statement for that category. For orange days and ozone, EPA's guidance reads: "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." The equivalent statements for other pollutants are quoted in the scale section above.

From EPA's Air Quality System pre-generated daily AQI files, the official regulatory record: every daily AQI value for every monitored Montana county, 2020 to 2024, grouped by calendar month into EPA's six categories. The snapshot is rebuilt annually when EPA finalizes a new year. It is climatology, not a forecast: it says what the months are usually like, not what next week holds.