Air quality history by county

Now. The shading is AirNow's current AQI contour map for the chosen product: Combined is AirNow's PM2.5-and-ozone map as supplied, and PM2.5 and Ozone are its single-pollutant maps. AirNow interpolates between monitors each hour, so the shading is an estimate for an area rather than a measurement at an address, and areas without enough nearby monitors are left unshaded as "no estimate." Monitor points show each site's latest hourly reading (NowCast for particles and ozone); in Combined each point shows the higher of its PM2.5 and ozone AQI. The map checks for new data every five minutes, and zooms no closer than the resolution of AirNow's estimate (about 5 km). When observations are more than six hours old they are withheld.

Forecast. Each marker is a forecast issued by a state, local or tribal air agency for an AirNow reporting area, for that area's own local date. Forecasts cover the areas agencies choose to forecast, so many areas have none for a given day; those are marked "not issued." A forecast is a prediction for the whole reporting area. Action days are declared by the agencies.

History. Each county is shaded by the number of days its monitors reported a daily AQI above 100 in the selected year, from the U.S. EPA AirData annual summary by county. A county is shaded only when it reported an AQI on at least 75% of the year's days; counties with fewer days are hatched, and counties without a monitor are grey. EPA publishes certified annual data months after the year ends.

Advisories. Official air quality alerts from NOAA's National Weather Service are drawn with a bold dashed outline and hatching over the zones or counties they name. They disappear when they expire.

Wildfires. Wildfires and fire complexes reported by fire managers to the National Interagency Fire Center are drawn as a flame in a dark badge, larger for larger reported sizes. The national fire list keeps a fire until it is declared out, often for months. Shown as active: wildfires discovered in the last 14 days, and older fires reported less than 90% contained with a record update in the last 14 days. Other fires still on the national list are older records. Older records are drawn only when "Older fire records" is switched on in Layers and filters, as a hollow grey badge one size smaller with a thin perimeter outline. At the national view the map shows active wildfires of 100 acres or more; smaller ones and fires without a reported size appear when you zoom in. Prescribed fires are shown when discovered or updated in the last 14 days. A perimeter shows the last mapped extent at the time stated; the fire may have moved since, and many fires have no perimeter. A fire on the map does not mean smoke is reaching the ground nearby: the air quality readings show that.

Burns and restrictions. Authorized agricultural and land-management burns published by state and local agencies, and prescribed fires reported nationally, are drawn as a small outlined flame. Burn restrictions, no-burn days and action days are drawn over the counties they name: a solid outline with dots when mandatory, a dashed outline when voluntary, each marked with a crossed-out flame. Notices that restrict nothing are listed in the details and the table rather than drawn.

Reported events, sensors and facilities. Emission events, air releases and enforcement notices published by state and local agencies are drawn as diamonds where the agency gives a location. Sensors are state- and city-run sensors outside the official AirNow network, drawn as small squares; their readings are shown as the operator publishes them and are not the official AQI. Facilities are air-permitted facilities listed by Connecticut, Massachusetts and Rhode Island. Listing a facility, a burn or an event does not indicate that it caused any reading.

Current observations are preliminary data from the U.S. EPA AirNow program and participating air quality agencies; for official readings visit AirNow.gov. Alerts: National Weather Service. Wildfires: National Interagency Fire Center. Burns, restrictions, events and sensors: the state, county and city agencies named in each item. History: EPA AirData. County and state outlines: U.S. Census Bureau. Basemap: © OpenStreetMap contributors, OpenMapTiles, served by OpenFreeMap.