You can measure indoor air quality using a combination of consumer-grade monitors for everyday pollutants and specialized test kits for things like radon and mold. No single device captures the full picture, so the approach depends on what you’re trying to detect. The core pollutants worth tracking in a home are particulate matter (PM2.5), carbon dioxide (CO2), carbon monoxide (CO), volatile organic compounds (VOCs), formaldehyde, and radon.
What to Measure and Why It Matters
Each indoor pollutant tells you something different about your air. CO2 levels are a direct proxy for ventilation: the higher the reading, the less fresh air is circulating. In occupied rooms, CO2 should stay below about 700 to 800 ppm. Once it climbs past 1,000 ppm, the space is clearly under-ventilated, and you may notice drowsiness or difficulty concentrating.
PM2.5 refers to tiny particles from cooking, candles, wildfires, or dust that are small enough to reach deep into your lungs. VOCs are gases released by paint, cleaning products, new furniture, and building materials. Formaldehyde, a specific VOC, off-gases from pressed-wood products, certain flooring, and adhesives. Carbon monoxide is an odorless, potentially lethal gas produced by gas stoves, furnaces, and attached garages. Radon is a naturally occurring radioactive gas that seeps up from the ground into basements and lower floors.
If you or your household members are experiencing recurring headaches, eye or throat irritation, dry cough, dizziness, fatigue, or difficulty concentrating that improves when you leave the building, those are classic signs of poor indoor air quality. Symptoms like chest tightness, fever, chills, or muscle aches point to possible biological contamination from mold or bacteria. Either pattern is a strong reason to start measuring.
Consumer Air Quality Monitors
Most consumer monitors sold today detect some combination of PM2.5, CO2, VOCs, temperature, and humidity. They range from around $50 for a basic single-pollutant sensor to $200 or more for multi-sensor devices with app connectivity and logging. These are useful for spotting trends, identifying problem times of day, and confirming whether ventilation changes are working. They are not laboratory instruments, so treat the readings as reliable indicators rather than precise measurements.
The accuracy of a monitor depends heavily on the sensor technology inside. For CO2, look for devices that use an infrared sensor (sometimes listed as NDIR on the spec sheet). These are highly selective and accurate for CO2 specifically. For VOCs, sensors based on metal-oxide semiconductor technology are common and affordable but have low selectivity, meaning they respond to a broad range of gases without distinguishing between them. More advanced monitors use photoionization detectors, which can identify thousands of individual VOCs at very low concentrations but are sensitive to humidity. For carbon monoxide, electrochemical sensors offer the best accuracy at low concentrations.
A dedicated CO alarm is non-negotiable in any home with gas appliances, an attached garage, or fuel-burning heating. This is a safety device, not an air quality monitor, and should be on every floor regardless of whether you buy anything else.
Where to Place Your Monitor
Position an indoor air quality monitor at breathing zone height, roughly 3 to 6 feet above the floor. The EPA recommends prioritizing the rooms where you spend the most time: bedrooms, living rooms, and home offices.
Keep the sensor away from localized pollution sources like toasters, gas stoves, or space heaters, and also away from pollution sinks like air purifiers or HVAC vents. These will skew your readings and won’t reflect what you’re actually breathing throughout the room. Similarly, avoid placing monitors right next to doors, windows, or duct openings, since those spots are heavily influenced by outdoor air or the HVAC system and won’t represent the room’s average conditions. The sensor needs free airflow around it, so don’t tuck it behind furniture or into a corner.
Testing for Radon
Radon requires its own testing method. Consumer air quality monitors don’t typically detect it, and it demands a different approach because concentrations fluctuate significantly over hours and days.
Short-term radon test kits are the simplest starting point. These use activated charcoal or electret detectors placed in the lowest livable level of your home for 2 to 10 days under closed-building conditions (windows and exterior doors shut). You mail the kit to a lab, and results come back within a week or two. Research indicates a minimum of four days of exposure yields the most accurate short-term results. A one-week test achieves over 95% confidence in predicting annual averages when radon levels are low (below about 75 Bq/m³, or roughly 2 pCi/L).
The catch is that short-term tests predict the true annual average with only about 50% accuracy at moderate radon levels. If your initial test comes back elevated or borderline, a long-term follow-up of at least 90 days using an alpha track detector gives a much more reliable picture. The EPA’s action level is 4 pCi/L (about 148 Bq/m³), while the World Health Organization recommends keeping levels below 2.7 pCi/L (100 Bq/m³). Short-term kits cost $15 to $30 including lab analysis. Continuous electronic radon monitors that log hourly readings run $150 to $250 and can be used indefinitely.
Testing for Mold
Mold testing is trickier than gas or particle monitoring because what you find depends entirely on how you sample. Air sampling and dust sampling detect different species in different proportions. Smaller spores like those from Cladosporium stay airborne longer and dominate air samples, while larger spores like Epicoccum settle quickly and show up primarily in dust. A study comparing five sampling methods found no correlation between fungal concentrations in air versus dust samples, largely because of these differences in spore behavior.
For homeowners, the two practical options are DIY mold test kits and professional mold inspections. DIY kits typically use a petri dish or air cassette to capture spores over a set period, then send them to a lab for identification. These can confirm that mold spores are present, but interpreting the results without context is difficult since some level of mold is always present indoors. Professional inspectors use calibrated air pumps that pull a known volume of air through a collection medium, allowing them to calculate spore concentrations per cubic meter. They can also collect dust samples and use DNA-based analysis (the ERMI index) to identify 36 mold species and score overall mold burden. If you can see or smell mold, testing to confirm it is generally unnecessary. Testing is most useful when you suspect hidden mold behind walls or under flooring.
Using CO2 as a Ventilation Check
A CO2 monitor is one of the most useful single purchases you can make because it doubles as a real-time ventilation gauge. Every person exhales CO2, so in an occupied room, concentrations rise steadily when fresh air isn’t coming in fast enough. Outdoor air typically sits around 420 ppm. Indoors, the gap between your reading and outdoor levels tells you how well the space is ventilated.
In a classroom with 20 to 25 people over a 6 to 8 hour day, research suggests keeping CO2 below 700 ppm to meaningfully reduce airborne infection risk. In a restaurant with 50 to 100 people and a 2 to 3 hour visit, the threshold is about 900 ppm. For a home office or bedroom where you spend several hours, aim to keep levels below 800 ppm. If you consistently see readings above 1,000 ppm, opening a window, running an exhaust fan, or adjusting your HVAC system’s fresh air intake will bring numbers down quickly, and you can watch the monitor to confirm.
Key Thresholds to Know
- CO2: Below 800 ppm in occupied rooms for good ventilation. Above 1,000 ppm signals inadequate fresh air.
- PM2.5: The WHO guideline for annual average exposure is 5 µg/m³. Indoor levels from cooking or candles can spike well above that temporarily, but sustained readings above 12 µg/m³ warrant action.
- Formaldehyde: International guidelines recommend a maximum of 0.1 ppm (about 120 µg/m³) for continuous exposure in homes.
- Carbon monoxide: Any sustained reading above 0 ppm indoors deserves investigation. Alarms typically trigger at 70 ppm.
- Radon: The EPA recommends mitigation at or above 4 pCi/L. The WHO sets a lower reference of 2.7 pCi/L.
Professional Air Quality Assessments
A professional indoor air quality audit makes sense when consumer monitors flag persistent problems you can’t explain, when you’re investigating health symptoms, or when buying or renovating a home. Certified inspectors bring laboratory-grade instruments that measure pollutant concentrations with far higher precision than consumer sensors. They can also test for pollutants that consumer devices miss, including specific VOC compounds, asbestos fibers, and biological contaminants.
A typical residential assessment involves a walkthrough to identify potential sources, placement of sampling equipment for a set period, and lab analysis of collected samples. Costs vary widely by region and scope, generally running $200 to $500 for targeted testing (radon, mold, or a single pollutant) and $400 to $1,000 or more for a comprehensive multi-pollutant audit. The value of professional testing is in the interpretation: not just what’s in your air, but where it’s coming from and what changes will fix it.