An air-quality app may show “current AQI,” “today,” “daily AQI,” and “forecast” on the same screen. Those labels do not describe the same number.
The distinction matters most when conditions are changing quickly. A smoke plume can arrive after a morning forecast, ozone can build later in the day, and a final daily AQI cannot be known until the required daily measurements exist.
This guide uses the U.S. EPA system. Other countries may use different indexes, pollutants, averaging periods, and category boundaries.
The four numbers at a glance
| Label | What it represents | Best use | | --- | --- | --- | | Current AQI | A near-real-time public report, generally based on recent observations | Deciding what to do now | | NowCast | EPA's method for relating recent ozone or particle readings to the AQI scale | Producing a responsive current AQI | | Daily AQI | The AQI calculated from the complete required daily data | Reviewing what happened on a completed day | | Forecast AQI | A prediction of future daily or sub-daily air quality | Planning and preparing a backup option |
Do not compare two numbers until you know which row each one belongs to.
What “current AQI” means on AirNow
AirNow receives hourly observations from monitoring agencies. For ozone and particle pollution, it reports current conditions using the NowCast. The EPA's May 2026 AQI technical document says the NowCast relates recent hourly readings to the AQI and can change during the day as conditions change.
Current does not mean an instantaneous reading from this second. AirNow explains that most observations arrive after the measured hour and are normally released by the end of the following hour. Its data page also notes that current AirNow data receive preliminary quality checks but not the full validation used for EPA's regulatory Air Quality System.
Before using a current number, check:
- its timestamp;
- the reporting location;
- the dominant pollutant;
- whether the trend is improving, stable, or worsening; and
- whether a local sensor or source could make your exact location different.
What the NowCast does
A conventional daily PM2.5 AQI depends on a 24-hour average. Waiting until the day ends would make that result poor guidance for an activity happening now. The NowCast addresses this by using recent hourly measurements and giving more weight to recent hours when air quality is changing rapidly.
EPA uses a NowCast for ozone, PM2.5, and PM10. The exact algorithms are documented in the AQI technical document. Sulfur dioxide and nitrogen dioxide sub-indexes already use one-hour concentrations, so EPA says a NowCast is not needed for them.
The NowCast is not:
- a one-minute sensor value;
- a prediction of the next few hours;
- the final daily AQI; or
- proof that the same AQI applies at every address.
It is a current communication value calculated from observations.
What a final daily AQI means
EPA states that obtaining the daily AQI requires the complete daily data: 24 hourly values for particle pollution, or the applicable maximum one-hour or eight-hour value for other pollutants. A completed daily AQI is therefore best suited to historical review and reporting.
If a page presents a final daily AQI for a day that is still in progress, inspect the label. It may actually be a current value, a forecast, or a provisional daily estimate.
This also explains why yesterday's certified or historical value may differ from what you remember seeing live. AirNow data are preliminary public-reporting data; regulatory data go through fuller validation in EPA's Air Quality System.
What an AQI forecast means
An AQI forecast is a prediction. EPA says state, Tribal, and local forecasters commonly issue next-day forecasts in the afternoon. Some providers publish additional days or hourly model output, but farther-out detail should not be mistaken for a measurement.
Forecasts use information such as expected weather, emissions, transport, smoke, and recent observations. They can be wrong when winds shift, cloud cover changes, an inversion breaks sooner than expected, or a wildfire changes behavior.
A forecast is still useful. It tells you when to prepare, not what is guaranteed to happen.
Always check the pollutant
The same overall AQI number can imply different causes and timing:
- Ground-level ozone often increases on sunny days and may be worse later in the day.
- PM2.5 can rise during wildfire smoke, stagnant winter conditions, or combustion events.
- PM10 may dominate during dust events.
The overall AQI is the highest pollutant-specific sub-index. Follow the advice for that pollutant and your health group. AirNow's activity guides provide pollutant-specific recommendations instead of a one-size-fits-all outdoor time limit.
This distinction is also essential for our calculator: only a confirmed U.S. PM2.5 AQI should be converted to a PM2.5 concentration and cigarette-equivalent estimate.
How to plan without false precision
The day before
- Check the official local forecast and any forecast discussion.
- Note the expected AQI category and dominant pollutant.
- Prepare a lower-exposure time, route, or indoor alternative.
- For smoke, prepare filtration before conditions deteriorate.
The goal is a flexible plan, not a schedule built around one exact predicted number.
Before the activity
- Check current AQI or the NowCast.
- Confirm the timestamp and location.
- Look at the trend and nearby monitors during variable smoke.
- Apply the relevant AirNow activity guidance.
- Recheck if the activity lasts several hours.
Our AQI Activity Decision Tool can organize the category, group, and activity context, but official alerts and medical advice take priority.
After the day ends
Use historical or regulatory data if you need to analyze what happened. Do not reconstruct a completed day's exposure from one screenshot of the current AQI. Personal exposure would also require time-location and indoor-air information that an outdoor AQI does not provide.
Why current and forecast values disagree
The forecast and current reading answer different questions. A forecast may have predicted afternoon ozone while the morning NowCast is still Good. Smoke may arrive earlier or later than expected. The forecast may cover a reporting area while the current value comes from one monitor.
When the difference is large:
- verify that both use the same AQI system;
- compare the same pollutant;
- check the timestamps and locations;
- read any local advisory or forecast discussion; and
- prioritize current official guidance for an immediate decision.
For a broader diagnostic checklist, read Why Air-Quality Apps Show Different AQI.
Wildfire smoke needs more frequent checks
Smoke can vary sharply with wind and terrain. The AirNow Fire and Smoke Map combines permanent monitors, temporary monitors, corrected sensor data, fire information, smoke plumes, and Smoke Forecast Outlooks where available.
Use the map to answer three separate questions:
- What do nearby particle measurements show now?
- Is the trend improving or worsening?
- What does the available Smoke Outlook say about the next period?
The map is not an evacuation service. During an active emergency, follow local fire and emergency-management instructions. Our map-reading guide explains each layer.
Common mistakes
Treating a seven-day number as measured air quality
It is a forecast and uncertainty generally increases with lead time. Use farther-out days to prepare, then recheck closer to the activity.
Comparing a ten-minute sensor average with the NowCast
They use different locations, instruments, processing, and time windows. A disagreement can be expected during a fast change.
Assuming “today” always means the same thing
It may refer to a current observation, today's forecast category, or a provisional daily value. Open the provider's explanation or legend.
Ignoring heat while avoiding smoke
EPA warns that staying inside with windows closed can be dangerous during extreme heat if adequate cooling is unavailable. Seek an air-conditioned cleaner-air space when needed and follow local heat guidance.
Bottom line
Use a forecast to prepare, a current NowCast-based AQI to adjust today's activity, and a completed daily AQI to review the past. Check the pollutant, timestamp, location, and trend every time.
If you need a source that matches your region and use case, compare verified options with the Air Quality App Finder. If you plan to use a reading in the cigarette-equivalent calculator, first confirm that it is a U.S. PM2.5 AQI.