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A "good" indoor air quality score is one where each underlying reading sits in its healthy band: fine particulates (PM2.5) roughly under 9–12 µg/m³, carbon dioxide under about 800 ppm, relative humidity between 30% and 50%, and VOCs low and stable. A single combined "score" is only as trustworthy as the sensors feeding it, so the individual numbers matter more than the headline figure.
What does an air quality "score" actually represent? Particulate matter and AQI
Most monitors collapse several readings — particulate matter, CO2, VOCs, humidity — into one 0–100 index or a color band (green/yellow/red). It's a convenience summary, not an official standard, and different brands weight the inputs differently, which is why two monitors in the same room can disagree on the overall score while agreeing on the raw numbers.
The most comparable public benchmark is the US EPA's Air Quality Index (AQI), whose "Good" band runs 0–50. That index is built mainly around outdoor PM2.5 and ozone, but the particulate matter thresholds translate directly to indoor air, which is why PM2.5 is the reading worth anchoring to when a proprietary score feels vague.
What's a good PM2.5 level indoors?
Broadly, PM2.5 under roughly 9–12 µg/m³ is considered good — that range covers the EPA's "Good" AQI band and its tightened 2024 annual particulate standard of 9 µg/m³. Below about 12 µg/m³ most guidance treats the air as clean; sustained readings above 35 µg/m³ move into the range health agencies flag.
Indoors, PM2.5 spikes are usually events rather than a constant: frying, toasting, candles, a fireplace or wildfire smoke pushed in from outside. A good score isn't a flat line at zero — it's air that returns to that low single-digit range once the cooking is done and the room has had a chance to clear.
What's a good CO2 level indoors?
Under about 800 ppm is good and indicates fresh, well-ventilated air; 800–1,000 ppm is acceptable; sustained readings above 1,000 ppm signal the room needs more outdoor air. Outdoor air sits near 420 ppm, so an indoor number climbing well above that is really a ventilation reading, not a toxicity one.
CO2 is the reading that most directly reflects how many people are in a room versus how much fresh air it's getting. A bedroom that starts the night at 700 ppm and reads 1,600 ppm by morning with the door closed isn't dangerous, but it is telling you the space is short on ventilation — which is exactly the kind of thing you can't sense but a monitor makes obvious.
What about VOCs, humidity and formaldehyde?
For humidity, 30–50% relative humidity is the target — comfortable, and low enough to discourage mold and dust mites. VOC readings are best treated as relative: a good result is a low, stable baseline, with spikes that fade after painting, cleaning or unboxing something new, rather than a specific certified number.
Consumer VOC sensors report a total (TVOC) rather than identifying specific compounds, and they drift, so the useful signal is the pattern, not the digit. Formaldehyde is a specific VOC that off-gasses from new furniture, flooring and cabinetry, and some higher-end monitors report it separately because it carries its own health guidance rather than folding into the general VOC number. If VOCs climb every time you clean and settle afterward, the monitor is working exactly as intended. Humidity, by contrast, is one of the more accurate consumer readings and one of the easiest to act on.
Why do two monitors show different scores in the same room? Sensor accuracy and calibration
Different sensor accuracy, different placement, and different scoring formulas. One monitor may use a true NDIR CO2 sensor while another estimates CO2 from its VOC sensor; one may sit near a window while the other is by the kitchen. Small differences in each input, weighted differently, produce noticeably different overall scores.
This is why the consensus favors reading the component numbers over trusting the badge. If both monitors say PM2.5 is 6 µg/m³ and CO2 is 750 ppm, the air is good regardless of whether one calls it "92" and the other "green." Treat the summary score as a glanceable shortcut and the underlying readings as the truth. Calibration also matters: a monitor that's never been calibrated against a known reference can drift over months, especially on VOC and CO2 sensors, so a periodic check against a second device is worth doing if the numbers ever look implausible.
Real-time readings and temperature
Most monitors update in real-time, refreshing every few seconds to a minute, and pair the air quality reading with temperature so you can see whether a spike lines up with cooking, heating or an open window. A monitor with a laggy or averaged display is harder to use for spotting the actual cause of a spike.
What features matter beyond accuracy: app control, alerts and data logging?
An app extends a monitor from a single glanceable number into a history you can review, with alerts that notify you when a reading crosses a threshold you set. Data logging over weeks is what actually reveals a room's pattern, rather than a single reading which only tells you about this exact moment.
Brands like Airthings, Temtop, IQAir and Awair all pair their monitors with an app for data logging and alerts, though the depth of history and the alert customization varies between them. If you want to catch a slow-building problem — creeping CO2 in a bedroom, humidity drifting up over a season — data logging is the feature that actually shows it, not the live number alone.
Is it realistic to keep every reading in the "good" zone all the time?
No, and chasing a flat, perfect score is the wrong goal. Cooking will spike particulates, a closed occupied room will lift CO2, and winter heating will drop humidity — these are normal daily swings. Good air quality is defined by where the readings sit at rest and how quickly they recover, not by never moving.
A monitor that shows a temporary red band while you fry dinner is working correctly, and reacting to every brief spike leads to fiddling rather than fixing. The pattern that matters is the baseline and the recovery: particulates that fall back to single digits within a while of cooking, CO2 that drops when you open a door, humidity that returns to the 30–50% band. Persistent out-of-range readings signal a real problem worth acting on; transient ones are just the room living.
What should you actually do with the number for your health?
Use it to decide which fix a room needs, not to chase a perfect figure. High particulates point to filtration or source control; high CO2 points to ventilation; high humidity points to a dehumidifier or better airflow; low humidity in winter points to a humidifier. The score's job is to route you to the right lever.
A monitor is a diagnostic tool, so the win is turning a reading into an action. When particulates are the problem, our guide to improving indoor air quality covers the fixes; when humidity is off, the ideal indoor humidity level page explains the target range. Start at the air quality monitors hub for how to choose a device you can trust the numbers from.