How do I test the air quality in my home?

The three things a monitor answers well

Fine particles, carbon dioxide, and relative humidity are the three an indoor air quality monitor really does measure, and they are worth measuring. What makes them work is not the precision of the sensor. It is how long you leave it running. Start with fine particles, because the standards themselves prove the point. The health-based reference from the World Health Organization is an annual average of 5 micrograms per cubic meter.

The federal standard is an annual average of 9.0, alongside a 24-hour standard of 35 set as a 98th percentile across three years. None of them is a number you can read off a screen in one glance. All of them are built from many days of data. The average is the exposure. A single reading is a moment.

That is why a spot check misleads people in both directions. Frying dinner drives the particle count up fast, and the cloud can hang in the room for hours after the pan is off. The spike itself is not a malfunction. What matters is how high it goes and how long it takes to clear, which is the part a range hood vented outside changes and a single glance at the screen cannot show you.

A week of data reads the pattern instead. Sharp peaks at dinner time point at cooking, and a multi-day rise during regional smoke points outside. A flat elevated baseline means outdoor air is seeping in through the envelope. Put the sensor in the bedroom and in the main living area, and take a short reading in the kitchen during a normal meal if you want the cooking contribution on its own.

A consumer particle sensor reports total mass in micrograms per cubic meter, and nothing about what the particles are made of. It cannot separate wildfire smoke from cooking grease from road dust. The timing identifies the source, not the number.

Carbon dioxide needs the same patience, narrowed to one room and one time of day. The reading that matters is the overnight average in the bedroom with the door closed, because that is where and when it builds up. A daytime spot check in the living room tells you close to nothing. And the number is a ventilation signal rather than a safety score. ASHRAE's own 2022 position is that indoor carbon dioxide concentrations are not overall indicators of indoor air quality, though they can be a useful tool if you understand the limits of what they mean.

Two cautions if you do buy one. The reliable sensor type measures the gas directly, by infrared absorption. Some inexpensive devices print a carbon dioxide number that is actually estimated from a volatile organic compound sensor instead. That is not a carbon dioxide measurement, and it moves when you spray cleaner rather than when a room runs out of fresh air.

The second caution matters on a long run. Most consumer infrared sensors self-calibrate by assuming they periodically see fresh outdoor air, using the lowest recent reading as the anchor. In a tight house with a closed bedroom, a basement, or a place nobody airs out, the sensor never sees that low reading, the calibration drifts downward, and it under-reports over weeks. A fifteen-minute check catches the drift, though it does not correct it. Carry the sensor outside when you first set it up. The clean global background is around 430 parts per million, and a real yard reads somewhat above that, because traffic and nearby combustion add to it. If your sensor reads well under 430 outdoors, its baseline has drifted and every indoor number it has shown you is low.

Humidity is where cheap equipment wins outright. The sensing parts in a decent unit are specified to within a couple of percentage points, which is more than enough for the decision it informs. EPA's guidance is to keep indoor humidity below 60 percent, ideally between 30 and 50. The research consensus tightens the good end of that to 40 to 50 percent, which is where breathing, sleep, eyes, and skin all do best at once. EPA also notes that a moisture or humidity meter is a small, inexpensive instrument, ten to fifty dollars at many hardware stores. Nothing else on this page buys you that much certainty for that little money.

One thing a monitor is not. An air quality monitor is not a carbon monoxide alarm. Carbon monoxide is a medical emergency rather than a trend line, and it needs a dedicated alarm on every level of the home.

Why a mold air test is the weakest thing you can buy

Mold is where the testing industry is at its worst, and the correction comes from the agencies rather than from us. CDC does not recommend mold testing. EPA gives the structural reason: no EPA or other federal limits have been set for mold or mold spores, so a sample cannot be used to check a building against a federal standard. EPA adds that in most cases, if visible mold growth is present, sampling is unnecessary.

EPA says surface sampling may be useful for determining whether an area has been adequately cleaned or remediated. It has one other narrow use, which is confirming what is growing at a spot someone has already found. What it cannot do is scan a clean-looking home and return a number.

The method itself is why two houses with the same problem get opposite reports. An air cassette pumps room air through a spore trap for a short window. Active growth inside a wall often produces a false negative, because settled spores never enter the air stream while the pump is running.

The indoor-versus-outdoor ratio used to interpret the result has the mirror-image problem. It depends on the outdoor spore baseline, which swings by region and by season. The same house can produce a false alarm in one month and a false comfort in another.

The dust-based index sold direct to consumers has a narrower problem. It was built as a research index for comparing across populations, not for interpreting one home. A number designed to rank a thousand houses does not tell you what to do about yours.

What does work costs almost nothing. Look, smell, and check for water. Visible growth plus a moisture source in an otherwise well-kept home is actionable on its own, whatever a spore count says. A moisture reading on the wall behind a suspicious spot tells you more than an air sample from the middle of the room. It points at the water, and the water is the thing you can turn off.

Ask who profits from the result

One question separates a useful test from an expensive sales call. Does the company reading the result also sell the fix? That is not an accusation. It is a structural question with a real answer, and it is fair to ask out loud.

Florida wrote it into law. Under the state's licensing statute a mold assessor may not perform or offer mold remediation on a structure it assessed within the previous twelve months, with a narrow exception for one class of licensed contractor. The same assessor may not accept compensation or a reward from a remediator for referring business. Nor may that assessor take an engagement in which the assessment, or the fee for it, is contingent on what the assessment concludes. A remediator carries the mirror-image restriction. That is state law rather than a national rule, and Florida is not the only state that draws the line.

California draws no such line. No state license is required to call yourself a mold assessor here, and nothing stops the company that tests you from selling you the remediation. Check your own state rather than assuming the separation exists.

The pattern is bigger than mold. Free in-home HVAC inspections, free roof inspections, free mold inspections from remediation companies, and utility-sponsored energy audits all share one shape. Keep the inspection cheap enough to give away and the finding rate high, and keep the checklist to things the inspector can sell. You do not pay at the test stage. You pay later.

The counterweight matters, because a blanket accusation would be wrong. When we scanned mold specialists in our own market, most of the ones we sampled were inspection-only, and several market that separation as the reason to hire them. So the instruction is to ask, not to assume. The answer varies by company, and asking costs you a sentence.

Apply it to us as well. Stasis sells some fixes and refers others, so we are not standing outside this. What we can say is that our fee covers the visit rather than the finding, which means a report that says nothing here needs fixing is a survivable outcome for us. That is the question worth putting to any company you are about to pay: what happens to your revenue when the answer is nothing.

Where do I start?

1. Look up your county's EPA radon zone and find your state radon program. A low zone is not a clearance, and EPA recommends testing every home.
2. Check whether your library, county, or air agency lends out an air sensor. EPA runs loan programs, so a free week of data may be available.
3. Measure humidity first. It is the cheapest useful answer in indoor air, and 40 to 50 percent is a target you can act on today.
4. If you buy a monitor, read the bedroom overnight average, not just the peaks. One spike is a moment. A week shows which spikes clear.
5. Take a new carbon dioxide sensor outdoors for fifteen minutes at setup. A reading well under 430 parts per million there means the calibration drifted low.
6. Test for radon on its own. Most air quality monitors do not measure it, and a test runs two to 90 days short, more than 90 long.
7. If you suspect mold, look, smell, and take a moisture reading instead of ordering an air sample. No federal limit exists to grade a spore count.
8. Before you pay anyone, ask whether the company that tests also sells the fix, and what happens to them when the answer is nothing.

What does the science say?

What actually works?

Sources cited on this page

Every number on this guide page traces to one of the 17 sources below: 15 government or regulatory.