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The first time someone hands me a new air quality monitor, they almost always ask the same thing within a minute of plugging it in. Not “how does it work,” but “is this number bad?” They are staring at a screen showing PM2.5, a VOC index, CO2, and humidity, and none of it means anything without a yardstick.
That yardstick is what this piece is about. A monitor only reports; it does not interpret. So let me give you the target numbers I use, metric by metric, plus what counts as good, fair, and poor.
PM2.5: the number worth watching first
PM2.5 refers to fine particles smaller than 2.5 micrometers, small enough to slip deep into your lungs. Cooking, candles, wood smoke, and outdoor wildfire haze all drive it up. It is reported in micrograms per cubic meter (ug/m3).
This is the reading I check first, because it maps most directly to health. The EPA’s particle pollution standards set a 24-hour outdoor limit around 35 ug/m3, and indoors you want to sit well below that.
| PM2.5 (ug/m3) | Rating | What it means |
|---|---|---|
| 0-12 | Good | Clean air, no action needed |
| 12-35 | Fair | Acceptable, but find the source |
| 35-55 | Poor | Ventilate or run a purifier now |
| 55+ | Very poor | Sensitive people should leave the room |
A mistake I see all the time: people panic when searing a steak sends PM2.5 to 150 for ten minutes. That short spike is normal. What matters is whether it drifts back under 12 within an hour or two once you turn on the range hood.
If cooking is your main source, a good exhaust fan does more than any purifier. Reducing PM2.5 from cooking usually starts at the stove, not the living room.
CO2: your ventilation report card
Carbon dioxide indoors is not really a poison at household levels. It is a proxy. Since people exhale CO2 constantly, a rising number tells you fresh air is not replacing stale air fast enough.
Outdoor air sits around 400-420 ppm. That is your floor. Everything above it is the CO2 your home is trapping.
| CO2 (ppm) | Rating | What it means |
|---|---|---|
| 400-800 | Good | Ventilation is keeping up |
| 800-1,000 | Fair | Getting stuffy, crack a window |
| 1,000-1,400 | Poor | Stale air, focus and sleep suffer |
| 1,400+ | Very poor | Ventilate immediately |
A closed bedroom with two sleepers can climb past 1,500 ppm by morning. That heavy-headed feeling when you wake is often this, not bad sleep. Cracking the door an inch usually knocks it back down.
CO2 is the one reading that tells you almost nothing about pollutants and almost everything about airflow.
VOC index: read the trend, not the digit
Volatile organic compounds come off paint, cleaners, air fresheners, new furniture, and dozens of everyday products. Here is the catch that trips people up: most home monitors do not report VOCs in a true concentration. They report an index, usually scaled 0-500, that measures change against your home’s own recent baseline.
So a reading of 100 is meant to be “typical for your space.” When it jumps to 250 after you spray glass cleaner, the number is telling you something changed, not that the air is a fixed level of toxic.
| VOC index | Rating | What it means |
|---|---|---|
| 0-150 | Good | Normal for your baseline |
| 150-250 | Fair | Something released VOCs recently |
| 250-400 | Poor | Strong source active, ventilate |
| 400+ | Very poor | Find and remove the source |
Because it is relative, chasing one perfect VOC number is a waste of time. Watch for the spike and how fast it clears. If you want the deeper background, our piece on what VOCs in the home are and why they matter covers the chemistry side.
Humidity: the quiet metric that controls the rest
Relative humidity gets ignored on these monitors, which is a shame, because it quietly drives comfort, mold risk, and even how bad other pollutants feel. It is reported as a percentage.
The EPA recommends keeping indoor humidity below 50 percent to discourage mold and dust mites, and I like a floor of 30 percent so noses and wood floors do not dry out.
Aim for 30-50 percent relative humidity year round. Below 30 you get static, cracked skin, and irritated sinuses. Above 60 you invite mold and dust mites.
In a humid climate basement, I have watched RH sit at 68 percent for weeks while the homeowner blamed a “musty smell” on old carpet. It was the moisture. A dehumidifier pulling it down to 45 percent fixed the smell in three days.
Putting the four numbers together
No single reading tells the whole story. The skill is reading them as a set. A stuffy room with high CO2 but clean PM2.5 needs a window, not a purifier. Clean CO2 but climbing PM2.5 during dinner means your range hood is losing.
Here is the mental checklist I run when I glance at a monitor.
- Is PM2.5 under 12 and staying there between cooking sessions?
- Is CO2 under 1,000 in occupied rooms, especially bedrooms overnight?
- Did the VOC index return to baseline after the last spray or spill?
- Is humidity parked between 30 and 50 percent?
Cheap monitors can drift over time, especially CO2 and VOC sensors. If your CO2 never drops near 420 outdoors on a breezy day, the sensor likely needs recalibration or fresh-air auto-calibration.
What a good reading actually looks like
On a normal afternoon in a healthy home, you should see something close to this: PM2.5 around 3-8, CO2 near 550-700, VOC index under 120, and humidity around 40-45 percent. That is a genuinely good indoor air quality reading, and it means your ventilation, filtration, and moisture control are all doing their jobs.
You will not hit those numbers every hour, and you should not expect to. Cooking, cleaning, and a full house all move the needle. What you want is a home that returns to those baselines on its own within an hour or two.
If yours does not, pick the one metric that stays worst and start there. A stuck PM2.5 points to filtration or a source. Stuck CO2 points to ventilation. Stuck humidity points to a dehumidifier or a leak. Fix one lever, watch the screen for a day, and let the numbers tell you whether you guessed right.
Frequently asked questions
What is the single most important reading to watch?
PM2.5 is the one I check first because it links most directly to health. Fine particles reach deep into the lungs, and the target is clear: stay under 12 micrograms per cubic meter for a good daily average. CO2 and humidity matter too, but a stuck PM2.5 number usually points to an active source you can fix fast, like cooking without a range hood.
Is high CO2 in my house dangerous?
At normal household levels, no. Readings of 1,000 to 1,500 ppm will not harm you, but they signal poor ventilation and can leave you groggy or unfocused. True danger from CO2 needs concentrations far higher than a home reaches. Treat a high number as a nudge to open a window, not an emergency. It is a stale-air warning, not a toxic-gas alarm.
Why does my VOC reading jump around so much?
Because most monitors report a relative index, not a fixed concentration. The sensor learns your home's baseline and flags changes against it. Spraying cleaner, opening a new package, or painting sends it up, then it settles as the air clears. Do not chase one perfect value. Watch for spikes and how quickly they return to your normal range.
What humidity level should I aim for?
Keep relative humidity between 30 and 50 percent. Below 30 percent you get dry sinuses, static, and cracking wood. Above 60 percent you invite mold and dust mites. Around 40 to 45 percent is the comfortable middle. In damp basements a dehumidifier does the work; in dry winter homes a humidifier fills the gap. Check it seasonally since both ends drift.
Should I worry about a single high spike?
Usually not. Searing food or lighting candles can send PM2.5 or VOCs soaring for a few minutes, and that is normal. What matters is whether the reading returns to baseline within an hour or two. Sustained high numbers over many hours deserve attention. Brief spikes that clear on their own are just your monitor doing its job of showing you real events.
My readings never look perfect. Is something wrong?
Probably not. Real homes with cooking, cleaning, and people rarely hold perfect numbers all day. The goal is a home that returns to good baselines on its own after activity settles. If one metric stays poor for hours no matter what, start there. Otherwise, occasional movement across all four readings is exactly what a working monitor should show you.
Dharm Solanki
Dharm Solanki spends a lot of time thinking about the air inside people's homes, usually when no one else is. He writes about humidity, ventilation, and dehumidifiers, along with the small fixes that turn a stuffy or damp room into one that feels comfortable again. He would rather explain why a recommendation works than just point at a product, so his guides tend to answer the question you were about to ask next. And if something looks great on paper but struggles in a normal house, he says so.
