How to Test Indoor Air Quality at Home (2026 Guide)

Quick answer: To test your indoor air quality at home, measure four things: fine particles (PM2.5), carbon dioxide (CO2), relative humidity, and volatile organic compounds (VOCs). One consumer air quality monitor in the $80 to $200 range covers all four, and it is the most useful purchase you can make. Test for radon separately with a radon kit or a dedicated radon monitor. Then keep one limit in mind: air tests measure what is floating in the air, and much of what collects indoors sits on surfaces.

Start with the free tests

Two tests cost nothing and tell you more than most people expect. Do them before you buy anything.

The re-entry smell test. Leave your home for at least an hour. When you walk back in, pay attention to the first few seconds. Your nose gets used to your own home within minutes, which is why the people who live there are the last to notice a lingering odor. What you smell on re-entry points you somewhere:

A musty or earthy smell points to moisture and possible mold.
A stale, heavy feeling points to poor ventilation and CO2 buildup.
A chemical or "new product" smell points to off-gassing from paint, furniture, flooring, or cleaning products.

The pattern log. For one week, write down how you feel and where you are, with the time. The pattern tells you more than any single symptom:

If you feel better within hours of leaving the house and worse when you come back, the cause is more likely the building than the season.
If mornings are worst, look at the bedroom first: bedding, the mattress, and eight hours in a closed room.
If one room bothers you and the others don't, look for something specific in that room, such as a damp wall, new furniture, a carpet, or an HVAC vent.
If a closed room feels worse as the day goes on, suspect CO2 and ventilation.

The log tells you which room to measure first. That choice matters more than which monitor you buy. If anyone in the home has symptoms that are severe or don't improve, see a doctor; a log is useful to bring to that appointment too.

What is worth measuring

Four measurements cover most household air problems. A fifth, radon, needs its own test.

1. Fine particles (PM2.5)

PM2.5 means airborne particles 2.5 microns across or smaller. Indoors, they come from cooking (frying and searing especially), candles, fireplaces, smoking, vacuums without a sealed filter, and outdoor smoke that leaks in. It is the best-studied indoor pollutant and the number most air purifiers are built to lower. Expect sharp spikes while you cook. What matters is how fast the reading falls afterward and where it settles.

2. Carbon dioxide (CO2)

At normal household levels, CO2 is a stand-in for ventilation rather than a pollutant you need to fear. People breathe it out all the time, so a rising CO2 reading means stale air is not being replaced fast enough. When CO2 builds up, so do other things released indoors: odors, moisture, and VOCs. CO2 is the number that best explains a room that feels stuffy and makes it hard to concentrate.

Buy a monitor with an NDIR sensor (non-dispersive infrared, the type that measures CO2 directly). Some cheaper devices estimate "eCO2" from a VOC sensor, which is far less reliable.

3. Relative humidity

Humidity has the widest knock-on effects, because it decides whether mold and dust mites can thrive. Below about 30%, air feels dry on your skin, eyes, and throat, and static builds. Above 60%, damp surfaces invite mold and dust mites multiply. We aim for 40% to 60% indoors; the EPA recommends keeping humidity below 60% and ideally between 30% and 50% (EPA). A $10 hygrometer measures it, and most air quality monitors include one. If you only measure one thing, measure this.

4. VOCs (volatile organic compounds)

VOCs are gases released by paint, adhesives, pressed-wood furniture, new flooring, cleaning products, and fragranced products. Consumer monitors report a "TVOC" (total VOC) figure. It is a rough total, not a list of specific chemicals, so use it to spot sources and trends rather than to judge safety. Watch what makes the number jump; that is your source. For ways to bring it down, see reduce VOCs.

5. Radon: test it separately

Radon is a colorless, odorless radioactive gas that seeps in from the soil. According to EPA estimates, it is the number one cause of lung cancer among non-smokers (EPA). A standard air quality monitor does not measure it. You have two options:

A radon test kit. Short-term kits stay in place for 2 to 90 days and cost little. They are sold at hardware stores, by mail, and through the national radon hotline.
A consumer radon monitor. Battery-powered digital detectors such as the Airthings Corentium Home 2 give continuous readings and running averages, which helps if you want to watch radon over seasons.

Nothing sold for air purification, including our products, affects radon. If your result is high, the fix is a radon mitigation system installed by a qualified contractor.

What the numbers should look like

Use these ranges for orientation, not diagnosis. Different agencies set different figures for different time periods, so treat one high reading as a reason to look closer rather than as an emergency. The PM2.5 figures below come from outdoor air standards; there is no official indoor PM2.5 limit, so many people use the outdoor numbers as a yardstick.

Measurement	Generally good	Worth watching	Investigate	Source
PM2.5 (long-term average)	9 µg/m³ or lower	9 to 35 µg/m³	Sustained above 35 µg/m³	EPA annual standard is 9 µg/m³ and 24-hour standard is 35 µg/m³ (EPA); WHO guidelines are stricter: 5 µg/m³ annual and 15 µg/m³ over 24 hours (WHO)
CO2	Under about 800 ppm	800 to 1,000 ppm	Often above 1,000 to 1,400 ppm	Common rule of thumb; ASHRAE standards set ventilation rates, not a CO2 limit (ASHRAE)
Relative humidity	40% to 60%	30% to 40% (common in winter)	Below 30% or above 60%	EPA: below 60%, ideally 30% to 50% (EPA)
TVOC	Near your device's normal baseline	Climbs after a new product or activity	Stays high for days	No single standard; readings vary by device
Radon	Under 2 pCi/L	2 to 4 pCi/L: EPA says consider fixing	4 pCi/L or higher: EPA says fix	EPA action level (EPA)

Two tips for reading a monitor:

Trends beat single numbers. Consumer sensors vary from unit to unit and drift over time. The same sensor comparing 9 a.m. with 9 p.m. in the same room is still reliable and useful.
Place it with care. Put it about chest height, away from windows, vents, and the stove, in the room where problems seem worst.

Got your numbers? See what to do about them

How to run a full home air quality check

A complete check takes about a week of passive monitoring plus an afternoon of hands-on inspection.

Log symptoms and smells for seven days, with the room and the time of day. This tells you where to point everything else.
Put a monitor in the room of most concern for 48 hours. Keep to your normal routine, including cooking and sleeping, so you get a true baseline.
Read the daily curve, not the average. Match each spike to what you were doing. A PM2.5 spike at 7 p.m. is usually dinner. A CO2 climb overnight in a closed bedroom points to ventilation.
Move the monitor to the bedroom for another 48 hours. You spend about a third of your life there, so overnight readings matter. If CO2 climbs past 1,000 ppm by morning, try leaving the door or a window cracked and measure again.
Run a radon test following the kit's instructions. The EPA says to test on the lowest level of the home you use regularly, in a room you use often (not a kitchen or bathroom), at least 20 inches off the floor. If a short-term result is 4 pCi/L or higher, run a follow-up test before deciding on repairs (EPA).
Inspect for moisture, which sits behind most mold and musty-smell problems. Check under sinks, around windows, behind toilets, along basement walls, and anywhere you see staining or feel damp. Fix what you find before you buy any device.
Decide whether you need a professional. Call one if you find mold covering more than about 10 square feet (the EPA's cutoff for do-it-yourself cleanup), mold in your HVAC system, a moldy smell with no visible source, a radon result at or above 4 pCi/L on follow-up, or ongoing symptoms despite normal readings.

Do home mold test kits help?

Mostly not. "Settle plate" kits ask you to leave a petri dish open and see what grows. Mold spores are present in nearly all indoor air, so something will grow on almost any plate. The kit can't tell you whether your level is unusual. A moisture inspection, a look at hidden spots, and professional sampling when you suspect hidden mold tell you far more. If you can see mold, fix the moisture and clean it up; knowing the exact species rarely changes what you do.

What air tests can't measure

Every tool above measures what is suspended in the air at that moment. That is a useful thing to know. It is also only part of the picture.

Up to 80% of indoor allergens and microbes sit on surfaces rather than in the air at any given time (research). They settle into carpet, bedding, upholstery, along baseboards, and inside ductwork, where they build up over time. Dust mite allergen is a clear example. It collects in mattresses, pillows, and upholstered furniture and only becomes airborne for a few minutes when someone sits down, rolls over, or makes the bed.

So a monitor can read "good" while the house still smells musty or a bedroom still collects dust. The monitor is working. It is answering a narrower question than the one you are asking. The same gap explains why a filter can run for months and leave a musty smell in place: the source is microbial growth on a damp surface, and very little of it passes through the filter.

Read a clean air test as good news with a footnote. If your numbers look fine and your home still doesn't, shift your attention from the air to the surfaces:

Bedding and mattress: wash bedding hot every week and use zippered allergen covers on the mattress and pillows.
Carpet and rugs: vacuum with a sealed HEPA vacuum and consider hard flooring in bedrooms.
Upholstery: vacuum it regularly and wash removable covers.
Damp rooms: bring humidity below 60% and fix leaks.
HVAC: change filters on schedule and check that the drain pan under the cooling coil drains.

Surfaces are also where environmental probiotics work. Beneficial Bacillus bacteria settle on surfaces and consume the organic residue that odor-causing bacteria and mold feed on. That is a different job from filtration, and it supports a balanced indoor microbiome. A room unit such as the Biotica 800 ($299, for rooms up to 800 sq ft) treats those surfaces continuously. It will not change the PM2.5 number on your monitor, because airborne particles are not what it treats. If PM2.5 is your problem, a HEPA purifier is the right tool.

The bottom line

Testing your indoor air is worth doing, and it costs less than most people expect. Start with the free tests: log how you feel by room and time, and trust your nose in the first few seconds after you walk back in. Then measure PM2.5, CO2, humidity, and VOCs with one consumer monitor, and test radon separately because nothing else will catch it. Read trends rather than single numbers, and fix moisture before you buy a device. If the readings come back clean and your home still doesn't feel right, look at the surfaces your monitor can't see.

Frequently asked questions

How do I test the air quality in my home? Use a consumer air quality monitor that measures PM2.5, carbon dioxide, relative humidity, and VOCs. Place it at chest height in the room of most concern, leave it for 48 hours on your normal routine, and read the daily pattern rather than one number. Test for radon separately with a radon kit or a dedicated radon monitor, because standard air quality monitors don't measure it.

Are home air quality monitors accurate? They are accurate enough to track trends, but readings vary between units and can drift over time. That makes them better at showing when and where your air changes than at giving a lab-grade number. Comparing the same room morning and evening on the same device reliably shows sources such as cooking, candles, or a stuffy bedroom overnight.

What is a good PM2.5 level indoors? There is no official indoor limit, so most people use outdoor standards as a yardstick. A long-term average at or below 9 µg/m³ meets the EPA's annual outdoor standard, and the stricter WHO guideline is 5 µg/m³. Readings that stay above 35 µg/m³, the EPA's 24-hour standard, are worth investigating. Short spikes while cooking or vacuuming are normal; what matters is how fast the level falls afterward.

How can I test my air quality without a monitor? Leave the house for an hour and notice what you smell in the first few seconds after you return, since your nose adapts to your own home. Track how you feel by room and time of day for a week to see whether problems follow the building. Then inspect for moisture, staining, and condensation, which often reveal the source faster than a device.

Do at-home mold test kits work? Petri-dish "settle plate" kits are a poor way to judge a mold problem. Mold spores are in nearly all indoor air, so something will grow on almost any plate, and the kit can't tell you whether your level is unusual. A moisture inspection, plus professional sampling when you suspect hidden mold, is far more informative.

Why does my air quality monitor read good but my home still doesn't feel right? Air monitors measure only what is floating in the air at the moment. Up to 80% of indoor allergens and microbes sit on surfaces such as bedding, carpet, and upholstery, and they become airborne only briefly when disturbed. Clean readings alongside lingering dust or odor usually point to a surface source, and anyone with ongoing symptoms should also see a doctor.

How often should I test my indoor air quality? Do a full check once, then keep a monitor running if you own one, because the long-term trend is where the value is. Check again after anything that changes the building, such as renovation, new flooring or furniture, water damage, or HVAC work. Retest radon if you start using a lower level of the home, after major renovation or foundation work, and when you sell, since buyers often expect a test from the past two years.

How to Test Your Indoor Air Quality at Home (And What Every Test Misses)