Methodology

Collection

A scheduled job checks the official AirNow and National Weather Service files every few minutes, normalizes them and stores the latest good copy. Pages read that copy; visitors' browsers never contact AirNow or the National Weather Service. If a check fails, the last good data are kept and their age is shown. Values are never filled in or estimated by this site.

Each figure

Current AQI for a place

The reporting area's current AQI from AirNow's reporting-area file, with the pollutant AirNow marks as primary. AirNow's time marks the end of the measurement hour, in the agency's local time zone.

Monitor readings

Each monitor's hourly NowCast AQI for PM2.5, PM10 and ozone (hourly AQI for nitrogen dioxide) from AirNow's hourly observation file. The hour shown is the hour the measurement began.

Map shading

AirNow's current AQI contours, as published: an interpolated estimate across the lower 48 states. The combined map is AirNow's own combination of PM2.5 and ozone. The site redraws the contours in the official AQI colors.

Forecasts

The agency forecast for each reporting area and local date, with its issue date. A category without a number is shown as a category only. If no forecast exists for today, the page says so.

Alerts

Active National Weather Service air quality messages. An alert is matched to a county through the county codes in the message, and to a state through its counties and zones. A message whose expiry time has passed is shown as expired.

History

Days with AQI above 100 per county and year from EPA AirData, with the days that had an AQI. A year is complete with an AQI on at least 75% of its days (274 of 365). Data as of June 25, 2026.

Freshness limits

Each source has a limit for current data and a longer limit after which data are withheld. Between the two, data are shown and marked delayed. Past the second, the page says the data are out of date and shows no value or color. Age is measured with the server's clock from the time the data were observed; for the alert and wildfire lists, from the time the list was last checked.

The status page marks a source late after 3 missed checks (at least 20 minutes).

State and local layers

• The publisher's own words. Burn restrictions, action days, smoke outlooks, forecast discussions, emission events, releases, burn authorizations and enforcement notices are shown with the agency's own title, status words and dates, a link to the official record, the source, the time it was retrieved and the feed's caveat. Emission events, releases and enforcement notices cover the last 14 days, newest first, 15 at a time. A feed's copy older than four expected checks, and at least six hours, is withheld.
• Kept apart from the AQI. None of these items is turned into an AQI or given an AQI color, and the site never states that a fire, burn, event or facility caused a reading.
• Private individuals. Items name businesses and public bodies. When a record concerns a private person, such as a burn permit issued to a resident, the person's name, phone number and address are left out and the location is rounded to about 1 km.
• State sensors. Sensor networks some states run for local information are shown as the agency publishes them. They are separate from the regulatory monitors behind the AQI, and their readings can differ.
• Wildfires. Fires come from the National Interagency Fire Center's incident records, with size and containment as fire managers report them; an unreported size or containment is shown as not reported. The national list keeps an incident 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 collapsed into one line on the pages and drawn on the map only when "Older fire records" is switched on. Prescribed fires are listed separately when discovered or updated in the last 14 days. The list is withheld when the copy is more than 6 hours old.
• Pilot-state records. Facility lists, enforcement records, permits and air-toxics results for Connecticut, Massachusetts and Rhode Island are copies made when the site is built, each dated on the page. They describe permits and past compliance and do not measure the air today.

Missing data

Missing or unknown data are never shown as zero, as green or as Good. No-data and out-of-date states have their own grey, hatched look, which matches none of the AQI colors.

ZIP codes and places

AirNow assigns every ZIP code to one reporting area. A ZIP code search shows that reporting area's reading, which describes the whole area around the agency's reference point. Place search uses U.S. Census Bureau names and points and runs in your browser.

Each city page uses the county that contains its reporting area's reference point for its history section and names that county on the page.

Times and time zones

Every time on the site names its time zone. A place's readings and forecasts are shown in its local time zone, using the zone label the agency publishes (for example EST or PDT). National tables use Eastern Time. Dates are written as month, day and year.

Corrections

Values from AirNow, the National Weather Service and EPA are shown as the agencies publish them. When an agency revises its data, the site shows the revision at its next check or data refresh. Errors in this site’s own text and calculations are corrected when they are found. Found a wrong reading or a broken link? Send a correction.