What Does an "Unhealthy" Air Quality Level Actually Mean?
The short answer
"Unhealthy" is a label, not a measurement. On any air quality index, the category words are names given to ranges of index values by the agency that publishes the scale, so the word only means something once you know which index you are reading. On the U.S. Air Quality Index, AirNow defines six categories — Good (0–50), Moderate (51–100), Unhealthy for Sensitive Groups (101–150), Unhealthy (151–200), Very Unhealthy (201–300), and Hazardous (301 and above).
The number under the label
Category names sit on top of a calculation, not the other way around. EPA's AQI technical assistance document sets the concentration breakpoints that anchor each band, and the boundary at 100 is the important one: for each pollutant, an index value of 100 corresponds to the level of that pollutant's short-term national ambient standard. Everything above 100 is, by construction, air above the standard. The actual levels and averaging times are published in EPA's NAAQS table, and our guide to reading the NAAQS table explains how to interpret the columns.
This is why a single category word carries so little information on its own:
- The same label can come from different pollutants. An Unhealthy reading driven by ozone and one driven by fine particles are the same word describing different air.
- A label covers a wide band. The distance between the bottom and top of a category can be large in concentration terms.
- Labels are boundaries, not thresholds in nature. Nothing changes physically when a value crosses from 100 to 101; the cut point is a reporting convention tied to a standard.
What the categories are for
Each U.S. AQI category is paired with published cautionary language aimed at the general public and at groups the agency identifies as more sensitive — that pairing is the whole point of grouping a continuous number into named bands. Plain-language descriptions of what each category is meant to convey are published by AirNow and by the American Lung Association. Those are the authorities to read for the wording itself; this page is about how the labeling system is built, not about what any individual should do with a reading.
Why a PSI "unhealthy" band is not a U.S. "Unhealthy" band
If the reading in front of you is on a Pollutant Standards Index rather than the U.S. AQI, the U.S. category table does not apply to it. Each index defines its own pollutant coverage, averaging times, and band boundaries, and those definitions come from the agency that publishes the index — so the only correct source for what a PSI category means is that agency's own documentation. Two scales can attach identical English words to genuinely different concentrations, which is exactly why translating a foreign category into an American one produces a wrong answer. Our page on AQI vs. PSI covers why national scales diverge.
How to find out what a category means on any index
Working from the label back to something concrete takes three steps:
1. Identify the publisher. Every index reading belongs to an agency. Find which one, because its scale definition governs.
2. Find the concentration behind the value. Most reporting systems publish the pollutant concentration alongside the index number, in µg/m³ for particles or ppm/ppb for gases.
3. Compare that concentration to a standard you understand. For U.S. readers, that means the NAAQS levels for the six criteria pollutants.
Done this way, the category word becomes a convenience rather than the whole message. It is also why two apps can display different category labels for the same address at the same moment — different data sources and averaging choices, described further in our page on why apps show different AQI values.