An air-quality report becomes useful when you connect its number to a place, pollutant, reporting period and planned activity. Start with the official local report, then read the health guidance for the people who will be outdoors.
This guide uses the United States Air Quality Index, or AQI. Other countries use different indices and thresholds. Preserve the source’s index name when comparing apps or reports from different places.
Read five fields before acting
- Location: confirm the reporting area or monitor relative to your activity.
- Pollutant: identify particles, ozone or another reported pollutant.
- Time: check the timestamp, time zone and whether the result is current, daily or forecast.
- Category: read the category name as well as the number or color.
- Guidance: open the advice for the pollutant and relevant group.
AirNow’s AQI basics explains the US index and its six categories. AQI converts pollutant concentrations onto a communication scale. A value of 120 is an index number; it is not 120 micrograms per cubic meter, a percentage of polluted air or a personal dose.
On a narrow screen, scroll the table sideways. Keyboard users can focus it and use the arrow keys.
| AQI | Category | How to use the category |
|---|---|---|
| 0–50 | Good | Read the report together with current local conditions. |
| 51–100 | Moderate | Check pollutant-specific advice, especially for unusually sensitive people. |
| 101–150 | Unhealthy for Sensitive Groups | Use the guidance for groups sensitive to the reported pollutant. |
| 151–200 | Unhealthy | Check guidance for everyone as well as sensitive groups. |
| 201–300 | Very Unhealthy | Follow the stronger official activity guidance. |
| 301 and higher | Hazardous | Follow local emergency and public-health instructions. |
The table identifies categories; AirNow’s activity guides supply the practical advice. Sensitivity differs by pollutant, and an individual’s care plan may add specific precautions.
Separate current conditions from the daily story
AirNow explains daily, forecast and current AQI. The NowCast estimates current conditions from recent observations, weighting them to respond to changes. A daily value summarizes the relevant longer averaging period; a forecast describes expected conditions. Keep these labels when recording a result.
For a trip tomorrow, use the forecast to prepare an indoor alternative, then recheck current conditions near departure. For a rapidly changing smoke episode, an earlier screenshot may already be unrepresentative. Avoid comparing yesterday’s daily maximum with a nearby sensor’s latest short reading as though both described the same moment.
Apply a report to a real decision
Write down the activity’s duration and exertion as well as the AQI. A brief essential errand and prolonged vigorous exercise are different planning situations. Use official guidance to decide whether timing, location, intensity or an indoor alternative should change.
Understand why maps and sensors disagree
Check whether two services show the same pollutant, location, averaging period and conversion method. A particle-focused smoke map and a general AQI report can answer different questions. AirNow’s site guide explains its displays and directs readers to the Fire and Smoke Map during smoke events.
For low-cost equipment, use EPA’s Air Sensor Toolbox to understand siting, performance and evaluation. Nearby sensor disagreement can reflect genuine local variation as well as instrument or processing differences. Check the device’s condition and data flags before interpreting an isolated high point as a community-wide event.
Visibility adds context, especially during smoke or dust, but photographs cannot establish every pollutant concentration. Ozone and fine particles also require different interpretations. Keep visual observations alongside measured data with their respective limits.
Make the check easy to repeat
Save the official report and pollutant-specific activity guide for your area. Before an outdoor plan, check the five fields, record the intended adjustment and choose when to look again. Follow local advisories during an incident; if readings are unavailable, report the gap rather than inventing a favorable number.
For indoor planning, see sources, filtration and room assessment in an apartment. For a community measurement effort, the monitoring project guide helps define data quality and decisions before buying sensors.
Air-quality decision log
Air-quality decision log — plain-text download. Save a copy and fill it in with your own information. It includes instructions, example assumptions and source references.