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How to Interpret AQI on an Indoor Air Monitor

The AQI number on a monitor looks universal and is not. It is calculated from one pollutant at a time (almost always PM2.5 indoors), on a scale that differs between the US, Europe, China and India, and it assumes a 24-hour average that an instantaneous reading does not represent. Here is how to read it properly.

By the Akermin Editorial Team · 3 min read

AQI on an indoor monitor is a translation of the PM2.5 reading (occasionally PM10 or VOC) into a 0 to 500 scale with color bands, usually following the US EPA breakpoints: 0 to 50 Good (up to 12 µg/m³), 51 to 100 Moderate (up to 35.4), 101 to 150 Unhealthy for Sensitive Groups (up to 55.4), 151 to 200 Unhealthy (up to 150.4), 201 to 300 Very Unhealthy, 301 and above Hazardous. Those breakpoints assume a 24-hour average; a monitor applying them to a live reading exaggerates short spikes. Read the µg/m³ value for decisions and treat AQI as a color code.

Which pollutant

An official AQI is the highest of several sub-indices (PM2.5, PM10, ozone, NO₂, SO₂, CO). Indoor monitors have one or two of those sensors, so their AQI is almost always the PM2.5 sub-index alone. Some monitors add a VOC-based index, which has no official basis. Check the manual for what drives the number; if it does not say, assume PM2.5.

Which scale

Scale Range PM2.5 for “Good” upper limit Notes
US EPA AQI 0 to 500 12 µg/m³ (AQI 50) Most common on consumer monitors
China AQI 0 to 500 35 µg/m³ (AQI 50) Looser at the low end; same numbers mean more pollution
India AQI 0 to 500 30 µg/m³ (AQI 50) Different breakpoints again
European Air Quality Index 1 to 6 bands 10 µg/m³ (Good) Bands, not a 0 to 500 number
UK Daily Air Quality Index 1 to 10 11 µg/m³ (index 1) Ten bands

A monitor made for one market may ship with that market’s scale. The same air can read AQI 45 on one scale and 75 on another.

The averaging-time problem

Breakpoints are defined for 24-hour mean PM2.5. A 20-minute cooking spike to 200 µg/m³ pushes an instantaneous AQI to “Very Unhealthy” while the day’s average barely moves. The dramatic number is not wrong about the moment but wrong about exposure. Some monitors compute a running 1-hour or 24-hour AQI, which is more meaningful; check the settings.

The US bands in µg/m³

AQI Category PM2.5 (24 h) Indoor action
0 to 50 Good 0 to 12.0 None
51 to 100 Moderate 12.1 to 35.4 Find the source if persistent; run purifier
101 to 150 Unhealthy for sensitive groups 35.5 to 55.4 Purifier on; stop combustion sources; close windows if outdoor
151 to 200 Unhealthy 55.5 to 150.4 Purifier on high; sensitive people to a clean room
201 to 300 Very unhealthy 150.5 to 250.4 Seal the clean room; N95 if going out
301+ Hazardous 250.5+ Relocate vulnerable people if it persists

Reading it well

  1. Find the µg/m³ value; it is the measurement.
  2. Note whether the AQI is instantaneous or averaged.
  3. Use the color as a prompt, the µg/m³ trend as the decision.
  4. Compare devices in µg/m³, never in AQI, unless both use the same scale.

For where the particles come from, see how to compare PM1, PM2.5 and PM10 readings; for the sensor behind the number, how to choose a PM2.5 monitor. Monitors are compared in best air quality monitors.

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Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.

Frequently Asked Questions

What does AQI 50 mean on my monitor?

On the US scale, the top of the Good band, corresponding to about 12 µg/m³ PM2.5 over 24 hours. On other scales it may mean something different; check which one the monitor uses.

Why does the AQI jump so fast when I cook?

The monitor converts an instantaneous PM2.5 reading using breakpoints designed for 24-hour averages. Short spikes produce dramatic AQI values that do not represent daily exposure.

Should I use AQI or µg/m³?

µg/m³ for decisions; it is the measured quantity and comparable across devices. AQI is a convenience for color coding.

Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.