"Indoor Air Quality" is shorthand for a single, blended number meant to summarize how the air feels and what risks it carries: the way the U.S. Air Quality Index does for outdoor air. The Terrestream IAQ value is computed from the gases and particles the sensor reads (CO2, VOCs, NOx, PM2.5, and others) and rendered on the same 0–500 scale as the EPA AQI, so the numbers feel familiar.
The reason for one number is decision-making. If you have to glance at six different sensors and remember thresholds for each, you don't. You ignore them. A composite score with a color band tells you whether the air right now is fine, borderline, or worth doing something about, and the per-parameter pages tell you which parameter is driving it.
The thresholds come from EPA 40 CFR Part 58, Appendix G: the same federal rule that defines outdoor AQI categories. The World Health Organization's 2021 guideline values are tighter than EPA's on PM2.5; the device exposes both interpretations so you can choose how conservative to be.
A few things IAQ does not capture: radon, carbon monoxide, mold spores (the device sees their precursors via VOC bursts and humidity, but not the spores themselves), and outdoor allergens drifting indoors. For those, the dashboard cross-references outdoor AQI, pollen, and ozone feeds. See the related articles below.
References
- EPA - AirNow: AQI Basics www.airnow.gov
- 40 CFR Part 58, Appendix G - Uniform Air Quality Index www.ecfr.gov
- WHO Global Air Quality Guidelines (2021) www.who.int
- Sensirion - SEN66 datasheet and VOC index info sensirion.com
