Monitoring air quality starts with knowing what to measure, where to measure it, and which tools give you reliable data. Whether you’re tracking outdoor pollution in your neighborhood or checking the air inside your home, the process involves a mix of free online tools, affordable sensors, and some specific testing for pollutants like radon that electronics can’t easily detect.
What Pollutants to Track
Not all air pollutants behave the same way or come from the same sources, so monitoring air quality means paying attention to several different things at once. The most important ones break into two categories: outdoor pollutants and indoor-specific concerns.
PM2.5 (fine particulate matter) is the single most important pollutant to track. These tiny particles, smaller than 2.5 micrometers, penetrate deep into your lungs and enter your bloodstream. They come from vehicle exhaust, wildfires, cooking, and burning candles or incense. The World Health Organization’s 2021 guidelines recommend annual average exposure stay below 5 micrograms per cubic meter, a level that most populated areas worldwide fail to meet.
CO2 (carbon dioxide) works as a proxy for how well a room is ventilated. Outdoor air typically contains 380 to 500 ppm. The American Society of Heating, Refrigerating and Air-Conditioning Engineers recommends keeping indoor CO2 below 1,000 ppm. A study exposing participants to different CO2 levels found moderate declines in decision-making ability at 1,000 ppm, with more substantial impairment at 2,500 ppm. If your CO2 readings climb, it means the room needs more fresh air exchange.
VOCs (volatile organic compounds) are gases released by paint, cleaning products, new furniture, adhesives, and building materials. Many consumer monitors report a “Total VOC” number, but this reading has a significant limitation: it cannot distinguish between specific chemicals. A TVOC sensor reacts to a broad mix of gases and reports a single combined number, so a high reading could mean harmless compounds or something genuinely toxic. There are also no widely accepted health thresholds for TVOC. Think of the reading as a general alert that something is off-gassing, not a diagnosis.
Radon is a radioactive gas that seeps into buildings from the ground. It’s the second leading cause of lung cancer, and you can’t smell or see it. Radon requires its own dedicated testing approach, covered below.
Checking Outdoor Air Quality for Free
You don’t need any equipment to monitor outdoor air quality. The EPA’s AirNow platform (airnow.gov) reports the Air Quality Index for locations across the United States, updated throughout the day. The AQI translates raw pollution measurements into a simple 0 to 500 scale with color-coded categories. For PM2.5, a “Good” rating (AQI 0 to 50) corresponds to concentrations of 0 to 9.0 micrograms per cubic meter. “Moderate” (AQI 51 to 100) ranges from 9.1 to 35.4. Once you’re in the “Unhealthy” zone (AQI 151 to 200), concentrations hit 55.5 to 125.4 micrograms per cubic meter.
For more neighborhood-level data, PurpleAir maintains a network of low-cost sensors installed by individuals and organizations. Their real-time map (map.purpleair.com) shows readings from thousands of sensors worldwide, often giving you block-by-block resolution that EPA regulatory monitors can’t match. Research using PurpleAir data in Los Angeles County confirmed the network reliably detects pollution disparities between different communities, consistent with EPA monitor readings. If you live near a wildfire-prone area or an industrial zone, checking PurpleAir gives you a much more localized picture than AirNow alone.
Choosing an Indoor Air Quality Monitor
Consumer-grade indoor monitors typically measure PM2.5, CO2, temperature, humidity, and sometimes TVOC. Prices range from about $80 for basic models to $300 or more for units that track multiple pollutants with better accuracy. Popular options include devices from Airthings, Aranet, IQAir, and Qingping.
Accuracy varies significantly. The EPA notes there are currently no widely accepted indoor performance criteria for low-cost air pollutant monitors, and precision can be affected by humidity, temperature, placement, and even how long the sensor has been in use. Accuracy varies not just between manufacturers but between individual units from the same product line. Most independent evaluations have been conducted under outdoor conditions, which may not reflect how the sensors perform indoors.
That said, consumer monitors are still useful for identifying trends and relative changes. You may not get a lab-precise PM2.5 number, but you’ll clearly see when cooking sends particle levels soaring or when CO2 builds up overnight in a closed bedroom. For CO2 specifically, sensors based on NDIR (nondispersive infrared) technology tend to be more reliable than cheaper alternatives. The Aranet4, for example, has become a go-to for CO2 tracking because of its proven NDIR sensor.
Where to Place Your Monitor
Placement matters more than most people realize. The EPA recommends positioning sensors at breathing zone height, between 3 and 6 feet above the floor. Below 3 feet, readings can be skewed by ground-level effects. Prioritize the rooms where you spend the most time: bedrooms, living rooms, and home offices.
Keep the monitor away from direct pollution sources like toasters, stoves, or candles, and away from pollution sinks like air purifiers. Either will give you a skewed picture of overall room air quality. Avoid placing sensors near windows, exterior doors, or HVAC vents, since these spots experience rapid temperature and humidity swings that can throw off readings. They’re also disproportionately influenced by outdoor air, so they won’t represent what you’re actually breathing in the center of the room. The sensor needs open airflow around it, so don’t tuck it behind furniture or into a corner.
Testing for Radon
Radon requires a separate approach because standard air quality monitors don’t detect it. The EPA’s action level is 4 picocuries per liter (pCi/L). Above that threshold, mitigation is recommended.
Testing follows a two-step process. Start with a short-term test lasting 2 to 10 days, conducted with windows and exterior doors kept closed as much as possible. Place the test device on the lowest occupied level of your home. Research indicates a minimum of 4 days is necessary for results that reliably correlate with long-term averages. You can buy charcoal-based test kits for under $20 at most hardware stores, then mail them to a lab for analysis.
If the short-term result comes back above 4 pCi/L, a follow-up test is recommended. For borderline cases, a long-term test using a passive detector over three months or longer gives a more accurate picture of your actual annual exposure before you commit to installing a mitigation system. Continuous electronic radon monitors are also available for around $150 to $200 and provide ongoing readings, which is useful if you want to track how radon levels fluctuate seasonally, since they tend to be highest in winter when homes are sealed up tight.
Making Sense of Your Data
Raw numbers are only useful if you know what to compare them to. Here are practical benchmarks to reference:
- PM2.5: Keep daily indoor levels below 9 micrograms per cubic meter when possible. Cooking, vacuuming, and burning candles can spike levels into the hundreds temporarily. If your baseline stays elevated, it points to inadequate filtration or an ongoing source.
- CO2: Below 800 ppm indicates good ventilation. Between 1,000 and 1,500 ppm, the room is stuffy and ventilation needs improvement. Above 2,000 ppm, cognitive performance drops measurably, and you should open windows or increase mechanical ventilation immediately.
- Humidity: Aim for 30 to 50 percent relative humidity. Below 30 percent dries out mucous membranes and increases susceptibility to respiratory infections. Above 50 percent promotes mold growth and dust mites.
- Radon: Below 2 pCi/L is considered low risk. Between 2 and 4 pCi/L, consider mitigation. Above 4 pCi/L, take action.
The most valuable thing a monitor can do is reveal patterns. Check your data after specific activities: cooking with a gas stove, running a space heater, painting a room, sleeping with the door closed. Over a week or two, you’ll develop a clear picture of what drives pollution levels in your home and which changes (running an exhaust fan, opening a window, upgrading your HVAC filter) actually make a measurable difference.