What is IAQ?

On the Terrestream device, IAQ is a particle index. The firmware converts PM_2.5 and PM₁₀ into sub-indices using the U.S. EPA's 2024 AQI breakpoints and shows the worse of the two, on the same 0–500 scale and six color bands as the outdoor AQI. It is calculated, not measured, and it leaves out CO₂, the VOC and NO_x indexes, humidity and temperature: each of those has its own reading and its own bands.

The reason for one particle number is decision-making: a single color tells you whether particles are fine, borderline, or worth doing something about. It is not a whole-room score. A crowded bedroom can show a green IAQ while CO₂ sits in orange, and a freshly painted room can show a green IAQ with a high VOC index. Read IAQ for particles, then check the other readings; the per-parameter pages explain each one.

The breakpoints come from the EPA's AQI rules in 40 CFR Part 58, Appendix G, with the PM_2.5 values updated in 2024: up to 9.0 µg/m³ is Good, up to 35.4 is Moderate, up to 55.4 is Unhealthy for Sensitive Groups. The World Health Organization's 2021 guidelines are stricter (5 µg/m³ annual, 15 µg/m³ over 24 hours), but the device has no WHO mode. The one alternative is AQHI+, the PM_2.5-only form of Canada's Air Quality Health Index, which you can choose in the app's device settings; see Canadian AQHI.

A few things IAQ does not capture: gases of any kind (CO₂, VOCs, NO_x), radon, carbon monoxide, and mold spores. Humidity and VOC readings can point to conditions that favor mold, but the device does not see spores. For outdoor context, Terrestream separately checks outdoor air quality, pollen, and ozone. See the related articles below.