Indoor AQI Explained: What Your Monitor's Air Quality Score Really Means

How EPA's AQI works

EPA sets AQI values for ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide and nitrogen dioxide. For each pollutant, AirNow explains, an index of 100 generally corresponds to the short-term national air quality standard, and values at or below 100 are generally considered satisfactory. The federal rule says the reported AQI corresponds to the pollutant with the highest calculated index.

AQI categories, from AirNow, with PM2.5 breakpoints from the 2024 rule (24-hour averages).

Category	Index	Color	PM2.5 (µg/m³)
Good	0 to 50	Green	0.0 to 9.0
Moderate	51 to 100	Yellow	9.1 to 35.4
Unhealthy for sensitive groups	101 to 150	Orange	35.5 to 55.4
Unhealthy	151 to 200	Red	55.5 to 125.4
Very unhealthy	201 to 300	Purple	125.5 to 225.4
Hazardous	301 and up	Maroon	225.5 and up

EPA changed the PM2.5 breakpoints in 2024, effective May 6, 2024. The Good band used to run to 12.0 µg/m³; it now ends at 9.0, the level of the revised annual standard.

Averages vs live readings

The daily AQI for particles is based on 24-hour averages. For a near-real-time number, EPA uses the NowCast AQI, which AirNow builds from hourly monitoring data and shows on its website and app; for PM2.5 it weights the last 12 hours, leaning on recent hours when air quality is changing fast. Your home monitor, by contrast, may refresh every few seconds. A 30-second spike to 150 µg/m³ while you sear a steak is real, but it is not the same thing as a 24-hour average of 150 µg/m³, which would be in EPA's Very Unhealthy band.

The practical rule: react to short spikes by ventilating and running a purifier; judge your home's air by the daily average in the app.

What a monitor's AQI or score usually means

• PM2.5 converted to an AQI. Many monitors put the live PM2.5 reading through EPA's breakpoints. Check whether the app uses the 2024 breakpoints; with the old ones, a reading of 11 µg/m³ would still show Good.
• A blended score. Some makers combine PM2.5, CO2, VOCs, humidity and temperature into one score. Awair's Element, for example, turns its five readings into an overall score. These scores are the maker's own design, not an EPA scale.
• Color lights. Screenless monitors such as the Airthings Wave Enhance show a color and leave the numbers to the app.

A single score can hide what is wrong. A poor score might come from CO2 in a closed bedroom, which calls for ventilation, or from cooking smoke, which calls for a range hood and a purifier. Always open the detail.

The numbers worth acting on

Reference points, each from the agency or standard named.

Reading	Reference point	Source
PM2.5	9.0 µg/m³ annual standard; AQI Good up to 9.0 (24-hour)	EPA
PM2.5	5 µg/m³ annual and 15 µg/m³ 24-hour guidelines	WHO 2021
CO2	A ventilation indicator, not an overall IAQ measure; ASHRAE 62.1 sets no indoor CO2 limit	ASHRAE position document
Humidity	30 to 50%, keep below 60%	EPA
Radon	Fix at 4 pCi/L or more; consider fixing from 2 to 4	EPA
VOCs	No federally enforceable standard for homes; use the trend	EPA

For more on each, see PM2.5 explained, why CO2 matters at home and the ideal indoor humidity level.

Indoor vs outdoor AQI

EPA estimates that Americans spend about 90% of their time indoors and that some pollutants are often 2 to 5 times higher indoors than typical outdoor levels. For particles, the picture is different: in homes without smoking or other strong sources, EPA expects indoor PM levels to be about the same as or lower than outdoors. Comparing the two tells you what to do:

• Indoor PM2.5 higher than outdoor: the source is inside (cooking, candles, a fireplace). Ventilate and filter.
• Indoor close to a bad outdoor AQI: outdoor air is leaking in. Close up, run the HVAC fan with a good filter and use a purifier. See the wildfire smoke picks.
• Indoor PM2.5 low but CO2 high: the room needs fresh air, not filtering.

Monitors that show indoor and outdoor side by side, such as the IQAir AirVisual Pro, make this comparison easy.

Check your app's settings

1. Find whether the display shows a converted AQI, a raw µg/m³ figure or the maker's own score, and switch to raw figures if you can.
2. Check which AQI breakpoints it uses; the 2024 EPA scale ends Good at 9.0 µg/m³.
3. Look for a daily or 24-hour average view; that is the figure comparable with EPA's bands.
4. Set alerts on the individual readings you care about rather than on the overall score.

Choosing a monitor that reports honestly

Look for one that shows raw PM2.5 in µg/m³, uses an NDIR sensor for CO2, keeps history you can export and has an independent accuracy result from South Coast AQMD's AQ-SPEC program where one exists. Our best air quality monitors page and accuracy guide cover the options.