What is IAQ?
One number for particles: the worse of the PM₂.₅ and PM₁₀ sub-indices, on the same 0–500 scale as the U.S. EPA AQI. It does not include CO₂, VOCs or NOₓ.
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Outdoor & weather
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AQI vs IAQ: the same scale, two contexts
Both numbers use the same 0–500 color band, but the math is different and so is the answer to "is it bad?"
WHO vs EPA: two number systems, two philosophies
EPA NAAQS is regulatory. WHO 2021 is aspirational. Terrestream uses the EPA scale; this is how to compare your readings with both.
Co-movement patterns: the fingerprint catalog
A single metric tells you a number. Several metrics moving together tell you what happened.
