What an indoor air quality monitor actually measures

An indoor air quality monitor measures fine particles (PM2.5), carbon dioxide (CO₂), relative humidity, VOCs, and sometimes radon and temperature, in the room where you put it. It does not measure one score. It is five or six separate readings, and each one goes wrong for a different reason.

That distinction matters the first time a number looks bad. A bedroom that reads fine for PM2.5 but climbs on CO₂ overnight has a ventilation problem, not a particle problem, and the fix is a cracked door, not an air purifier. Buying a monitor and not knowing which reading does what is how people end up running the wrong fix for months.

This post goes through what an indoor air quality monitor reports, reading by reading, what each one is actually standing in for, and what even a good monitor cannot tell you.

Key Takeaways

• An indoor air quality monitor reports PM2.5 in µg/m³, CO₂ in ppm, relative humidity in %, and VOCs, with radon in pCi/L on monitors that include it.
• CO₂ is a stand-in for ventilation, not air quality on its own. The CDC names under 800 ppm as a target for a room getting enough outdoor air.
• PM2.5 only means something next to the air outside your own address. The EPA's outdoor standard is 35 µg/m³ over 24 hours.
• Radon needs months of data, not a glance at today's number, because the EPA's action level of 4 pCi/L is a long-term average.
• A monitor does not measure carbon monoxide, mold spores or allergens, and it does not diagnose anything. It reports a number; you decide what to do with it.

The readings an indoor air quality monitor actually reports

Most consumer monitors, including Measured Air's kit, report some version of the same core set:

• PM2.5, in µg/m³, is fine particles from cooking, smoke, candles and dust, small enough to stay in the air for hours.
• CO₂, in ppm, is how stale a room is getting. It rises when a space is not getting enough outdoor air, whether or not anyone is burning or spraying anything. The CDC names under 800 ppm as a target for a room getting enough outdoor air.
• Relative humidity, in %, decides whether the air is dry enough to crack skin and wood, or wet enough to grow mold. The EPA's guidance is to keep it below 60%, ideally between 30% and 50%.
• VOCs are the off-gassing from paint, new furniture, flooring and cleaning products.
• Temperature, in °F, gives context for the other four. A room that reads humid at 65°F can read dry at 78°F with the same amount of water in the air.

None of these numbers is a verdict on its own. A monitor's job is to report each one with enough history that you can tell a bad hour from a bad month.

Radon: the one reading that can't answer today

Radon, in pCi/L, is a gas that rises out of the ground into the lowest level of a house. A monitor that includes it reports a number from day one, but that number is close to meaningless on its own, because radon swings by the hour with wind, pressure and whether a door was open.

The EPA's action level is 4 pCi/L, and it says to consider fixing a home that reads between 2 and 4 pCi/L. Both figures describe a long-run average, so a monitor that reports radon is worth more after three months of data than after three days.

Why the outdoor reading matters as much as the indoor one

An indoor air quality monitor that only reports indoor numbers is missing half the comparison. Indoor PM2.5 of 18 µg/m³ is a bad afternoon when the outdoor air is at 4, and a good result when the outdoor air is at 60. The EPA's outdoor PM2.5 standards, 35 µg/m³ over 24 hours and 9.0 µg/m³ as an annual average, exist because particle exposure is a regional and seasonal story, not a fixed line.

Measured Air's kit puts the outdoor reading at your own address next to every indoor number it reports, in plain language, so a spike has a reason attached rather than just a number. More on what the particle reading means specifically in PM2.5 indoor levels and on the gas that climbs overnight in CO₂ levels in the bedroom.

What an indoor air quality monitor does not measure

A monitor reports what its sensors are built to report, and that list has real gaps worth knowing before you rely on one.

• Carbon monoxide. CO is a different gas from CO₂, has no smell, and needs its own dedicated alarm. A monitor that reports CO₂ is not a CO detector, and treating it as one is a real risk.
• Mold spores and allergens. A monitor reports the humidity that lets mold grow, not the mold itself. High humidity with no visible growth is a warning, not a diagnosis.
• A health outcome. A reading does not diagnose, treat or prevent anything. The EPA, CDC and WHO publish population-level thresholds. What a given number means for a given person is a question for a doctor, not a sensor.
• A fix. The monitor tells you the bedroom is running past 1,000 ppm of CO₂ by 3am. It does not open the door, run the exhaust fan, or change the filter. That part is still yours.

Where to put the monitor so the reading means something

Placement changes a reading more than most people expect. A monitor on a sunny windowsill, directly above the stove, or right in a supply vent's air stream will report that spot, not the room.

Put it at breathing height, about four feet off the floor, in the rooms you actually spend hours in: the bedroom you sleep in, wherever you cook, wherever you work, and the basement if you have one. One monitor in a hallway cannot tell you whether the bedroom or the kitchen is the actual problem. Most useful answers come from two rooms disagreeing with each other.

What changes once you have the readings

The value of an indoor air quality monitor is not the number itself. It is the decision the number settles.

• CO₂ climbing past 1,000 ppm overnight: crack the bedroom door first, since that costs nothing. If the line still climbs, the next step is ventilation, not a purifier, because CO₂ is not something filtration removes.
• PM2.5 spiking at dinner: range hood on high, cooking on the back burner the hood actually covers.
• Humidity sitting over 60%: exhaust fans running longer during showers and cooking, and a look for the water source feeding it, covered in indoor humidity levels.
• Radon over 4 pCi/L after a few months of data: get quotes from radon mitigation contractors in your state. Measured Air ships kits and software and nothing else, so nobody comes to the house, and mitigation is a separate trade. More on the threshold in radon levels in the home.

FAQ

What does an indoor air quality monitor actually measure? Most report PM2.5 in µg/m³, CO₂ in ppm, relative humidity in %, VOCs, and temperature, with some including radon in pCi/L. Each reading answers a different question: particles, ventilation, moisture, off-gassing and long-term gas exposure are five separate problems with five separate fixes.

Can an indoor air quality monitor detect carbon monoxide? Not unless it specifically lists CO as a sensor, and most do not. CO₂ and CO are different gases, and a CO₂ reading tells you nothing about carbon monoxide. Keep a dedicated CO alarm regardless of what your monitor reports.

Do I need more than one monitor in my house? One monitor tells you about one room. A house with a stuffy bedroom and a smoky kitchen needs at least two monitors to tell those problems apart, since a single hallway reading blends both into one number that fits neither room.

How accurate do indoor air quality monitors need to be to be useful? Useful enough to show direction and relative change: whether the line went up or down after you did something, not a lab-grade absolute value. The EPA and CDC thresholds this post links are the fixed points to compare against; the monitor's job is showing you where your room sits relative to them over time.

Is a higher number always worse? For PM2.5, CO₂ and radon, yes, lower is generally better. Humidity is the exception: too low and too high both cause problems, so the target is a range (roughly 30% to 50%) rather than a floor.

Where this leaves you

An indoor air quality monitor measures PM2.5, CO₂, humidity, VOCs, and often radon and temperature, room by room, with the outdoor air as the reference that makes any of it mean something. It does not measure carbon monoxide, mold itself, or a health outcome, and it does not fix anything on its own. What it gives you is the number that tells you which fix to try first.

If you want your own rooms reporting instead of guessed at, order a kit sized to your rooms and start with the room you spend the most hours in.