Indoor Air Quality Testing: What to Test, How, and What the Results Mean (2026)

"Testing indoor air quality" covers several very different tests. Some, like radon and carbon monoxide, are about specific dangers with clear action levels; others, like particles and VOCs, tell you how well your home is ventilated and filtered.

Start with the two the EPA and CPSC treat as essential: a radon test, with action recommended at 4 picocuries per liter or higher, and carbon monoxide alarms on every level. Then a monitor for particles and humidity, kept between 30% and 50%, shows how your ventilation, filtration and moisture control are working.

The six tests at a glance

Each test answers a different question, and only some need a laboratory. This table pairs each one with how you measure it and the number the EPA or CPSC attaches to it.

The radon, carbon monoxide and humidity numbers come with clear action points; the particle, VOC and mold rows are about trends and sources rather than a single pass-or-fail line.

Radon

Radon is a radioactive gas from the ground that you can't see or smell. The EPA recommends testing every home, and fixing it when the level is 4 picocuries per liter (pCi/L) or more.

Short-term test kits are inexpensive; a long-term test gives a better picture of the year-round average. If a test comes back high, a certified radon mitigation contractor can install a venting system.

The EPA draws two lines rather than one. At 4 pCi/L or more it recommends fixing the home; in the 2 to 4 band it suggests considering a fix, because no level is free of risk.

Carbon monoxide

Carbon monoxide comes from fuel-burning appliances, attached garages and generators. It needs an alarm, not a one-time test: install CO alarms on every level and outside sleeping areas. See our carbon monoxide detector guide.

Particles

Fine particles (PM2.5) come from cooking, smoke, candles and outdoor pollution. Home monitors with laser particle sensors show them in micrograms per cubic meter and react within minutes. For outdoor air, the EPA's annual standard for PM2.5 is 9.0 micrograms per cubic meter; indoors, a monitor is most useful for spotting spikes and checking whether filtration brings them down.

Humidity

Humidity is the easiest thing to measure and one of the most useful. The EPA recommends keeping indoor humidity between 30% and 50%, and below 60%, to limit mold and dust mites. An inexpensive hygrometer is enough; see the ideal indoor humidity level.

VOCs and formaldehyde

Consumer monitors report total VOCs as a relative reading, useful for seeing when cleaning or painting raises levels, but they don't identify compounds. Laboratory tests, with a sample collected on a sorbent tube or badge, measure specific compounds such as formaldehyde. See VOCs in your home.

Mold

If you can see or smell mold, the EPA says testing usually isn't necessary: clean it up and fix the moisture that caused it. Sampling can help when you suspect hidden mold, but results are hard to interpret without a professional. Moisture is the real target.

What a consumer monitor's sensors can and can't do

A typical home monitor bundles four sensors, and each has a different claim to accuracy. Knowing how each one works tells you which readings to trust and which to treat as a rough signal.

• Particle (PM2.5) sensor. A small laser shines through a stream of air and a photodetector counts the light scattered off passing particles, then estimates mass from that count. It reacts within minutes to cooking or smoke, but it infers mass rather than weighing it, and readings can drift upward in very humid air as water coats the particles.
• Carbon dioxide sensor. Better monitors use a nondispersive infrared (NDIR) sensor, which measures how much infrared light carbon dioxide absorbs in a sample chamber. CO2 itself is not the indoor hazard; it is a proxy for how much fresh air reaches the room, so a steady rise in a closed room points to weak ventilation.
• Humidity sensor. A capacitive element changes its stored charge as it absorbs water vapor, giving a reliable relative-humidity percentage, which is why an inexpensive hygrometer is enough to track the 30% to 50% target.
• Total-VOC sensor. A metal-oxide element reports a single combined index that rises and falls with total gases, so it flags that cleaning or painting raised levels without naming the compound; a laboratory sample is still needed to measure formaldehyde or any specific chemical.

Treat the humidity reading as the firm number, the particle reading as a good relative trend, and the total-VOC reading as a motion detector for fumes rather than a measurement.

Home monitors vs. professional testing

• Home monitors track particles, humidity, temperature, CO2 and total VOCs continuously, and are best for spotting trends and spikes.
• Professional tests measure specific pollutants to a laboratory standard, and are worth it for radon, formaldehyde or a health concern.

Reading a day of monitor data

Picture a monitor logging particles and humidity through one day. Two readings stand out, and each points to a specific fix.

Suppose fine particles sit at a low baseline all afternoon, then spike well above the EPA's 9.0 microgram reference while the stove burners run at dinner. The step is to run the range hood on high until the reading falls back toward the afternoon baseline.

Later the humidity sensor reads above the 50% top of the EPA's 30% to 50% target in the evening, though still under the 60% ceiling where mold risk climbs. Running a dehumidifier or a bath and kitchen exhaust fan pulls it back into range.

The pattern holds for any monitor: a short spike calls for ventilation at the source, while a reading that sits outside a target band calls for ongoing moisture or filtration control.

What to do with the results

The EPA's three strategies, in order: control the source, improve ventilation, then clean the air. Fix moisture, ventilate when cooking and cleaning, use exhaust fans, and add filtration; see how to improve indoor air quality.

Frequently asked questions

How do I test the air quality in my home?

Test for radon with a kit, install carbon monoxide alarms, and use a monitor for particles and humidity. Use laboratory tests for specific pollutants such as formaldehyde.

What radon level is dangerous?

The EPA recommends fixing your home if radon is 4 pCi/L or higher, and considering it between 2 and 4.

Should I test for mold?

Usually not, if you can see or smell it; the EPA advises cleaning it up and fixing the moisture source.

What indoor humidity is best?

Between 30% and 50%, and below 60%, according to the EPA.

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