AQI vs IAQ: the same scale, two contexts

The U.S. EPA Air Quality Index is a 0–500 number that summarizes outdoor air at a regulatory monitoring station. It is computed by taking the worst of five regulated pollutants (ozone, PM_2.5, PM₁₀, CO, NO₂, SO₂) and converting whichever is most elevated into its corresponding band. The math lives in 40 CFR Part 58 Appendix G.

Terrestream's IAQ uses the same color scale and the same PM breakpoints as the EPA AQI, but for the air in the room around you, and only for particles: it is the worse of the indoor PM_2.5 and PM₁₀ sub-indices. Indoor gases such as CO₂ and VOCs have their own readings and bands. The shared scale is deliberate: glancing at a "92" on either should feel like the same kind of "moderate."

A subtle point: indoor and outdoor can disagree sharply. An outdoor wildfire-smoke event can put outdoor AQI at 250 ("Very Unhealthy") while a well-sealed home with HEPA running stays under 50 indoors. Conversely, on a perfect outdoor day, cooking can push indoor IAQ past 100, and a closed bedroom can build up CO₂ and VOCs that IAQ does not count at all. Terrestream compares indoor readings with outdoor air quality so the right action (open a window, or do not open a window) is clear.

Outside the U.S., other indices exist on different scales: the European Common Air Quality Index runs 0–100 with different thresholds; China uses a 0–500 scale that looks like EPA's but uses different PM breakpoints. The Terrestream device uses the U.S. EPA scale by default; the only alternative is Canada's AQHI+ (see Canadian AQHI).