Local air quality reporting has become an important health resource for people around the world. Several terms are used to describe air quality reporting, including the U.S. Air Quality Index, country-specific air quality indices, AirNow’s NowCast AQI, and IQAir’s AQI⁺.

What is an air quality index?

An air quality index is a tool that relates air pollution concentrations to the health risk posed by those pollution levels.

The U.S. Air Quality Index was developed by the United States Environmental Protection Agency to help communicate air pollution levels and related health risk. It uses color-coded categories for five major pollutants regulated under the Clean Air Act: ground-level ozone, particle pollution, carbon monoxide, nitrogen dioxide, and sulfur dioxide.

AirNow, which is operated by the EPA and partner agencies, also provides the NowCast AQI, a current air quality estimate that uses recent monitoring data to show changing conditions.

IQAir’s AirVisual Platform reports air quality using AQI⁺, which applies the U.S. AQI health categories but calculates readings from hourly pollutant averages for a more real-time view of local conditions (1) (2).

Because these systems do not always use the same averaging period, their readings can differ even when they are describing the same general air quality conditions.

The AirVisual Platform is IQAir’s air quality monitoring platform. AirVisual data comes from government-operated reference monitors as well as low-cost sensor stations, including community-based monitors and IQAir AirVisual monitors. Together, the platform includes data from more than 80,000 locations.

Additional information on the IQAir platform includes 24-hour air quality history, air quality forecasts, and health recommendations.

IQAir's AQI+ vs. AirNow's NowCast data

IQAir AQI⁺ updates hourly based on the average of the previous hour’s available pollutant measurements. AirNow’s NowCast AQI is also updated hourly, but it is calculated using an extended time frame. For particulate matter, NowCast uses a weighted average of recent hourly measurements, with up to 12 hours of data contributing to the reported value (3).

In general, NowCast is designed to smooth short-term fluctuations, while IQAir AQI⁺ is designed to provide a more responsive view of recent changes in air quality.

Because the two methods use different averaging periods, they may report different AQI levels at the same time.

Why differences in reports occur

Differences between AirVisual data and other air pollution information providers can happen for several reasons:

  • Different averaging methods: IQAir AQI⁺ is based on hourly pollutant averages, which can make it more responsive to recent changes in air quality. AirNow’s NowCast AQI also updates hourly, but for particulate matter it uses a weighted average of recent hourly measurements, with earlier hours still contributing to the reported value. Because of this, NowCast may change more gradually during rapidly changing events.
  • Different time windows: During events such as wildfire smoke, traffic-related pollution, or other localized pollution episodes, PM2.5 concentrations can rise or fall quickly. IQAir AQI⁺ may reflect those changes more quickly, while NowCast may show a smoother value because it incorporates more than the most recent hour of data.
  • Air pollution index differences: Many countries have their own air pollution indices, and some platforms use their own systems for representing and communicating health risk based on pollutant concentrations. IQAir’s AirVisual Platform uses the U.S. AQI health categories, color scale, and breakpoint values, but AQI⁺ is calculated using hourly pollutant averages rather than the same time-averaging periods used by some other AQI systems.
  • Different data sources: AirNow relies on government-operated regulatory monitors, while IQAir’s AirVisual Platform includes government stations, low-cost sensors, and community-based monitors. Differences in station location, sensor placement, calibration, and local pollution conditions can lead to different readings, especially when air quality varies from neighborhood to neighborhood.
  • Category thresholds: AQI categories are based on concentration breakpoints. When pollution levels are near the boundary between two categories, even a small difference in measured or averaged pollutant concentration can result in a different AQI category.

     

Why differences can appear even with hourly updates

The key difference is not whether AirNow NowCast and IQAir AQI⁺ update hourly. Both can update hourly. The difference is what each hourly value represents.

IQAir AQI⁺ reflects hourly pollutant averages, while NowCast incorporates a weighted average of recent hours. This means AQI⁺ may respond more quickly to sudden changes, while NowCast may change more gradually.

During stable air quality conditions, the two readings may be similar. During rapidly changing events, such as wildfire smoke, traffic-related pollution, or localized pollution episodes, the differences may be more noticeable.

Conclusion

AirNow NowCast and IQAir AQI⁺ can both describe current air quality conditions, but they do not always calculate AQI the same way. IQAir AQI⁺ is based on hourly pollutant averages, while NowCast uses a weighted average of recent hourly measurements for particulate matter. This can make IQAir’s AQI⁺ more responsive to rapid changes, while the NowCast AQI may provide a smoother value over a longer recent time window.

Differences can also occur because platforms may use different monitoring stations, sensor networks, data quality processes, and AQI category thresholds. When air quality is stable, readings may closely align. When air quality is changing quickly, two valid systems can report different AQI values at the same time.

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