Why Do Two Air Quality Apps Show Different AQI Numbers for the Same Place?
Two apps can show different AQI values for the same address at the same moment, and usually neither is broken. The most common causes are that they are using different indexes (the U.S. AQI is not the only national scale), different data sources (a regulatory monitor miles away versus a nearby low-cost sensor versus a model), or different averaging windows (a NowCast estimate versus a completed hourly or daily average). Sorting out which of those is in play usually explains the whole gap.
1. Different index, different number
An index value is only meaningful alongside the scale that produced it. National systems set their category boundaries at different concentrations and sometimes use different ranges entirely, so the same air can legitimately produce two different index numbers. An app built for an international audience may default to a scale that is not the U.S. AQI. AQI vs. PSI walks through why those scales are not convertible into one another.
Check the app's settings first — many let you pick which national index to display, and the setting is easy to miss.
2. Different monitor, or no monitor at all
The U.S. AQI reported by official channels comes from a network of regulatory monitors run by state, local, and tribal agencies, with data flowing through EPA systems. Those instruments are sited and operated under specified methods, and the data they produce is archived in EPA's outdoor air quality data portal. The agencies responsible for the daily report are described in where daily air quality reports come from.
Many consumer apps supplement or replace that with something else:
- Low-cost sensor networks, which are dense but use different measurement principles and often need correction factors, especially in smoke or high humidity.
- Modeled or interpolated values, estimating conditions between monitors.
- Satellite-derived estimates, which infer surface conditions from column measurements.
None of those is automatically wrong, but they are answering a slightly different question than a regulatory monitor is.
3. Different averaging window
This is the one that trips up careful readers. The AQI is defined over specific averaging times — an 8-hour window for ozone, 24 hours for fine particles, and so on, as listed in the NAAQS table. For real-time display, EPA's technical assistance document defines the NowCast, a weighted average that leans on recent hours so the number can move quickly when conditions change.
An app showing a NowCast, an app showing the last completed hourly value, and an app showing a forecast for the day will disagree by construction. The same is true when one is showing a forecast issued that morning and another is showing current conditions — see what AQI means in a weather forecast.
4. Different pollutant driving the number
The reported AQI is the highest sub-index among the pollutants measured at a site. If one source tracks fine particles only and another also tracks ozone, the second can report a higher number on a sunny afternoon simply because it is watching a pollutant the first one ignores. The National Weather Service's air quality page explains why ozone and particle pollution follow different daily and seasonal patterns.
5. Timing and refresh lag
Monitor data goes through automated quality screening before publication, and apps cache. A few tens of index points of difference can simply be one source running an hour behind another.
How to compare fairly
Before concluding that an app is wrong, line up four things: which index it uses, which station or data source it is drawing from, what averaging period it is showing, and which pollutant it names as the driver. Good apps disclose all four somewhere in the interface. When they match and the numbers still disagree, the difference is in the underlying data, not in the arithmetic — and the official agency report is the reference point.