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, PM2.5, 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 thresholds, but reports the air in the room around you. Indoor sensors include parameters the outdoor index does not measure (VOCs, indoor CO2). The shared scale is deliberate: glancing at a "92" on either should feel like the same kind of "moderate," even though the inputs differ.
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, a closed bedroom can run an indoor IAQ of 150 from CO2 and VOC accumulation. The dashboard shows both side-by-side so the right action (open a window, or do not open a window) is obvious.
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 has tighter PM cutoffs. Terrestream defaults to the U.S. EPA scale; international users can switch.
References
- EPA - AirNow: AQI Basics www.airnow.gov
- 40 CFR Part 58, Appendix G - Uniform Air Quality Index www.ecfr.gov
- AirNow API documentation docs.airnowapi.org
- European Environment Agency - Common Air Quality Index www.eea.europa.eu
