Indoor air quality is measured by tracking the concentration of specific pollutants, most commonly fine particulate matter (PM2.5), carbon dioxide (CO2), volatile organic compounds (VOCs), and formaldehyde. Monitoring can be as simple as a mail-in radon test kit or as sophisticated as a network of commercial-grade sensors logging data every minute. The approach you need depends on whether you’re checking a single concern like radon or trying to get a continuous picture of overall air health.
The Pollutants That Get Measured
Not all indoor air problems are the same, and different pollutants require different sensors. The EPA identifies several major categories of indoor contaminants: combustion byproducts like carbon monoxide and particulate matter, volatile organic compounds released by paints, cleaning supplies, and building materials, and biological pollutants like mold. Here’s what each measurement actually tells you.
PM2.5 (fine particulate matter) refers to tiny airborne particles smaller than 2.5 micrometers, roughly 30 times thinner than a human hair. These particles come from cooking, candles, tobacco smoke, and outdoor air that drifts inside. They’re measured in micrograms per cubic meter (µg/m³). In 2024, the EPA lowered its annual outdoor standard from 12.0 µg/m³ to 9.0 µg/m³, though that rule is currently under legal review. There’s no formal indoor standard, but the outdoor limit is a useful benchmark. Anything consistently above it indoors warrants attention.
CO2 (carbon dioxide) is measured in parts per million (ppm) and serves as a proxy for ventilation. Outdoor air sits around 400 ppm. Indoor levels between 600 and 1,000 ppm are typical in well-ventilated spaces. Once CO2 climbs above 1,000 ppm, it usually means fresh air isn’t circulating fast enough for the number of people in the room. You won’t feel “poisoned” at 1,500 ppm, but studies link elevated CO2 to reduced concentration and decision-making performance.
VOCs (volatile organic compounds) are gases released by everything from new furniture to cleaning products to air fresheners. Total VOC levels (often labeled TVOC) are reported in parts per billion (ppb) or µg/m³. Individual VOCs like formaldehyde can also be measured separately because formaldehyde is especially common indoors and has well-established health effects at low concentrations.
Carbon monoxide is a colorless, odorless gas produced by gas stoves, furnaces, and fireplaces. Dedicated CO detectors are required by law in many states and measure in ppm. Even low chronic exposure can cause headaches and fatigue.
Radon is a radioactive gas that seeps up from soil through foundation cracks. It’s the second leading cause of lung cancer. The CDC recommends testing every home, with short-term kits measuring levels over 2 to 90 days and long-term kits running over 90 days for a more accurate yearly average. The longer you test, the better the result reflects your actual exposure. Radon is measured in picocuries per liter (pCi/L), and the EPA recommends taking action if levels reach 4 pCi/L or higher.
Consumer Monitors vs. Professional Equipment
Consumer-grade air quality monitors have become widely available, typically costing anywhere from $80 to $400. Most combine several sensors in one device and display readings for PM2.5, CO2, VOCs, temperature, and humidity on a screen or smartphone app. They’re useful for spotting trends: you can see that PM2.5 spikes when you cook, or that CO2 climbs overnight in a closed bedroom.
However, the EPA cautions that there’s limited information on the accuracy of these low-cost monitors, and no widely accepted indoor performance criteria exist for them. Accuracy varies not only across manufacturers but even within product lines from the same company. Factors like temperature, humidity, sensor age, and the presence of multiple pollutants can all skew readings. A higher price tag doesn’t necessarily mean better performance either. Cost differences often reflect features like display quality, internet connectivity, and housing design rather than sensor precision.
Professional and regulatory-grade monitors are a different class of instrument. The EPA uses a formal evaluation process for Federal Reference Method (FRM) and Federal Equivalent Method (FEM) devices, considered the gold standard for air quality monitoring. These instruments cost thousands of dollars and require trained operators. Currently, there’s no equivalent reference monitoring network for indoor air, which is part of why indoor air quality remains less standardized than outdoor measurements.
For radon specifically, professional testing through a certified technician uses continuous radon monitors that log hourly readings over several days. These are more precise than charcoal test kits, though the inexpensive mail-order kits are a perfectly reasonable first step.
Where to Place a Monitor
Sensor placement matters more than most people realize. A monitor tucked behind a bookshelf or sitting on a windowsill can give readings that don’t represent what you’re actually breathing. The EPA recommends placing sensors at breathing zone height, between 3 and 6 feet above the floor. Below 3 feet, ground-level effects can distort readings.
Keep monitors away from windows, doors, and HVAC vents. These locations introduce rapidly changing temperature and humidity that can throw off sensors, and the air near them doesn’t represent average indoor conditions. The RESET Air Standard for commercial buildings specifies placing monitors at least 16 feet from operable windows and 16 feet from air filters and fresh-air diffusers. In smaller residential spaces, that distance isn’t always possible, but the principle holds: aim for a central location that reflects the room’s typical air, not the air right next to a draft or a supply vent.
Free airflow around the sensor is essential. Don’t place it in a corner, behind furniture, or inside a cabinet. The sensor needs to sample the same air you’re breathing in the middle of the room.
Continuous Monitoring vs. One-Time Testing
Some pollutants are best caught with continuous monitoring, while others only need a single test. Radon levels are relatively stable and tied to geology, so a well-executed test lasting 90 days or more gives you a reliable number. Mold assessment typically involves a one-time inspection, sometimes with air sampling or surface swabs sent to a lab.
PM2.5, CO2, and VOCs, on the other hand, fluctuate constantly. Cooking a stir-fry can send PM2.5 levels above 200 µg/m³ for 20 minutes. CO2 rises through the night in a bedroom with the door closed and drops the moment you open a window. VOC levels spike after painting or cleaning, then gradually fall. A single snapshot reading of these pollutants tells you very little. Continuous monitoring over days or weeks reveals patterns you can act on, like running a range hood during cooking or cracking a window during certain hours.
Commercial building standards reflect this reality. The RESET Air Standard requires projects to provide occupants with hourly air quality data, and its proof-of-uniformity testing protocol involves recording data at one-minute intervals. For home use, most consumer monitors log data continuously and let you review trends through an app.
Making Sense of the Numbers
Without formal indoor air quality standards in most countries, interpreting your readings requires some context. A few practical benchmarks can help.
- PM2.5: Below 9 µg/m³ as a daily average is a reasonable target based on the most recent EPA outdoor standard. Brief spikes during cooking are normal, but sustained levels above 25 µg/m³ suggest a ventilation or filtration problem.
- CO2: 400 to 800 ppm is well-ventilated. 800 to 1,200 ppm is acceptable but worth improving. Above 1,200 ppm consistently means the space needs more fresh air.
- TVOC: Below 250 ppb is generally considered good. Above 500 ppb, you may notice odors and irritation.
- Radon: Below 2 pCi/L is low risk. Between 2 and 4 pCi/L, consider mitigation. At 4 pCi/L or above, the EPA recommends taking corrective action.
- Carbon monoxide: Detectable levels above 0 ppm indoors should be investigated. Most residential CO alarms trigger at 70 ppm after one to four hours of sustained exposure.
If your monitor shows a pollutant that’s consistently elevated, the fix usually falls into three categories: source control (removing or reducing the thing producing the pollutant), ventilation (bringing in more fresh air), and filtration (using HEPA filters or activated carbon to clean recirculated air). Knowing which pollutant is the problem tells you which strategy will actually help.