How to test indoor air quality in your home

To test indoor air quality at home, measure five things in the rooms you actually use: PM2.5 in µg/m³, CO₂ in ppm, relative humidity in %, VOCs, and radon in pCi/L. Four of those give you an answer the first day. Radon takes months, because the number that matters there is a long average rather than a moment.

People usually go looking for a test after something specific. A smoke day when the sky goes orange and you want to know whether shutting the windows did anything, or a bedroom that is fine at 11pm and closed in by 3am.

This post covers what to measure, the thresholds the EPA and the CDC publish, where to put a monitor, and how to run a test that settles something. If you would rather watch your own rooms than read about them, that is what a kit for homeowners does.

Key Takeaways

• Five readings cover a house: PM2.5 in µg/m³, CO₂ in ppm, relative humidity in %, VOCs, and radon in pCi/L. Each one needs a room attached to be worth anything.
• The EPA's outdoor limits for PM2.5 are 35 µg/m³ over 24 hours and 9.0 µg/m³ as an annual average. There is no federal indoor standard, so compare your indoor line to the air outside your own address.
• The CDC names under 800 ppm of CO₂ as a target for a room getting enough outdoor air. Past roughly 1,000 ppm, most people start calling a room stuffy.
• The EPA's radon action level is 4 pCi/L, and it says to consider fixing a home that reads between 2 and 4 pCi/L. Radon is the one reading an afternoon cannot answer.
• A monitor measures. It does not ventilate, filter or seal anything. The value is in what you change after the reading.

What you are testing for, reading by reading

An indoor air quality test is not one score. It is a handful of readings that go wrong for different reasons and get fixed in different ways.

• PM2.5, in µg/m³, is fine particles: cooking smoke, wildfire smoke, candles, dust. Small enough to hang in a room for hours after whatever made them stopped.
• CO₂, in ppm, is how stale the air is. It works as a stand-in for ventilation, because when it climbs the room is not getting much outdoor air and nothing else in the room is being flushed out either.
• Relative humidity, in %, decides whether the house is drying out or growing mold.
• VOCs are the off-gassing from cleaning products, paint, new furniture and new flooring.
• Radon, in pCi/L, is a gas that comes up out of the ground into the lowest level of a house. No smell, and no bad day you can point at, only an average over time.
• The air outside at your own address is what makes an indoor number mean anything. Indoor PM2.5 of 18 µg/m³ is a bad afternoon when it is 4 outside and a good result when it is 60.

The air outside is measured. The air you breathe isn't. A Measured Air kit reports each of these room by room next to the outdoor reading at your address, and keeps the history.

Three ways to test indoor air quality at home

Your senses are free, immediate, and blind to most of it. A room that smells stuffy is usually a ventilation problem you could confirm in one night of CO₂ readings, but particles and radon have no smell at all. Carbon monoxide is a different gas from CO₂, also has no smell, and needs its own alarm: a monitor is not a substitute for one.

A single-use test kit is the next step up. A charcoal radon kit sits out for two to seven days and goes to a lab, and a lab canister for VOCs works the same way. You get one number, for one period, in one spot: enough to screen for radon, thin for anything that changes by the hour.

Continuous monitoring, room by room, is the only version that shows you the shape of a day. Monitors stay in the rooms and keep reporting: when the kitchen spikes, when the bedroom climbs, whether the purifier moved the line or just made noise.

The tradeoff is worth saying plainly. A week of readings with no change made at the end of it has told you something and fixed nothing.

What a good reading looks like

Every threshold below has a published source, because most of the numbers quoted in air quality marketing do not.

• PM2.5: the EPA's outdoor standards are 35 µg/m³ over 24 hours and 9.0 µg/m³ as an annual average (the NAAQS table). There is no federal indoor limit, so treat those as a yardstick. Indoors, single digits is a quiet house, and a climb to 80 or 150 µg/m³ for twenty minutes is usually dinner.
• CO₂: under 800 ppm is the figure the CDC names for a space getting enough outdoor air. Past roughly 1,000 ppm most people start describing a room as stuffy, and a closed bedroom often runs well past 2,000 ppm by morning. There is more in what bedroom CO₂ levels mean.
• Relative humidity: the EPA's guidance is to keep indoor humidity below 60%, and ideally between 30% and 50% (its brief guide to mold and moisture).
• Radon: the EPA's action level is 4 pCi/L, and it says to consider fixing a home between 2 and 4 pCi/L, while noting that getting below 2 pCi/L is difficult.
• VOCs: there is no consumer threshold worth quoting. Read the shape instead. A spike the day you paint that fades over a week is the paint, and a line that never comes back down is something still sitting in the room.

Where to put a monitor in each room

Placement decides a reading more often than the sensor does. Put the monitor at breathing height, four feet off the floor on a shelf or table, and keep it away from the spots that will lie to you: directly above the stove, on a sunny windowsill, in the path of a supply vent, or outside the bathroom door.

Cover the rooms where you spend hours, not the rooms that look tidy: for most houses, the bedroom you sleep in, the room where you cook, wherever you work, and the basement.

Radon has its own placement rules, and a test kit's instructions will state them: the lowest lived-in level of the house, at least 20 inches above the floor, away from exterior walls, drafts and high humidity, and left alone for the full test period.

How to test indoor air quality so the result settles something

1. Baseline for a week and change nothing. You cannot tell whether something helped without knowing what normal looked like.
2. Read it by hour, not by average. The average of a stuffy 3am and an open-window afternoon describes no actual moment in your house.
3. Change one thing: bedroom door open, or the range hood on high for the whole cook rather than the last two minutes.
4. Compare the same hours: last Tuesday 11pm to 6am against this Tuesday 11pm to 6am.
5. Keep what moved the line and drop what did not. A purifier that changes nothing in the room it is standing in has told you the problem is somewhere else.

Most single changes show up within a night or two. A monitoring run can also end in a written report covering what the air did and what to do about it, so running it again after a fix puts the before and after in one place.

What to do when a reading climbs

• CO₂ over 1,000 ppm overnight: open the bedroom door first, because it costs nothing. If the line still climbs, the next step is ventilation rather than filtration, since CO₂ is not something a purifier removes.
• PM2.5 spiking at dinner: range hood on high, and cook on the back burner where the hood catches the plume. How long the line takes to come down tells you more about the room than the peak does.
• PM2.5 up on a smoke day: windows shut, a HEPA room purifier in one room, a MERV 13 filter in the furnace with the fan set to circulate. The indoor line against the outdoor line tells you whether any of it worked.
• Humidity over 60%: exhaust fans during showers and cooking, a dehumidifier in the damp room, and a look for the water feeding it.
• Radon over 4 pCi/L: 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.

FAQ

How can I test the air quality in my house for free? You cannot measure it for free, but you can test ventilation for free. Sleep with the bedroom door open for a few nights and see whether the room feels different by morning. Particles, VOCs and radon do not announce themselves, so those need an instrument.

How long does an indoor air quality test take? PM2.5, CO₂ and humidity answer inside a day, often inside an hour. VOCs need a few days to separate a one-off event from a constant source. Radon needs two to seven days for a short-term kit and 90 days or more for a long-term test, which the EPA treats as the better picture of a year-round average.

Do I need a separate radon test if I already have an air quality monitor? Only if your monitor does not report radon, and many do not. Measured Air kits include radon as a long-term reading, which means letting it run for months rather than checking it on day two.

Can an air quality monitor tell me if the air is making me sick? No. A reading is a measurement, not a diagnosis, and no monitor can tell you why you feel the way you do. The thresholds published by the EPA, the CDC and the WHO are population guidance, and what a given number means for a given person is a question for a doctor.

Is one monitor enough for a whole house? One is enough to start and not enough to compare. A single reading in the hallway cannot tell you whether the bedroom is stuffy or the kitchen is the problem, and most useful conclusions come from two rooms disagreeing.

Where this leaves you

How to test indoor air quality at home comes down to four decisions: which rooms, which readings, where in the room, and what you change afterward. Cover PM2.5, CO₂, humidity, VOCs and radon in the rooms you use, compare every indoor number to the air outside your own address, then baseline for a week, change one thing, and compare the same hours.

A monitor tells you what the air is doing and when. Opening a door, running the hood, changing a filter or calling a mitigation contractor is what changes it.

If you want your own rooms measured instead of estimated, order a kit sized to your rooms and start with the bedroom.