The short answer: buy for the sensor type, not the screen. A CO2 monitor with a true infrared sensor and a PM2.5 monitor with published test results beat a cheaper box that lists ten gases and explains none of them.
What a home monitor can and cannot tell you
The U.S. EPA says indoor air quality problems mostly come from sources inside the building that release gases or particles. Too little fresh air makes it worse, because pollutants build up instead of being diluted. Cooking, burning candles, smoking, cleaning products and new furnishings are common sources.
A monitor shows you what is in the air at one spot, right now. It does not tell you the cause, and it does not diagnose anything. Use it the way you would use a thermometer: to see a change, then act on it, such as opening a window or running the range hood.
Source: U.S. EPA, "Introduction to Indoor Air Quality," epa.gov.
CO2: a clue about fresh air, not a pollutant meter
People breathe out CO2, so a crowded or sealed room shows it rising. That makes CO2 a useful sign that fresh air is running low. It does not measure dust, smoke or chemicals.
Look for a sensor described as NDIR, short for non-dispersive infrared. It measures CO2 directly with light. Some cheap units instead guess a CO2 number from other gas signals, which is a different and less direct method. If the listing says "equivalent CO2" or "eCO2," treat the number as an estimate.
Even real CO2 sensors can be off. In 2010, researchers at Lawrence Berkeley National Laboratory reported on CO2 sensors used in California buildings. They found that 47 percent had errors above 75 ppm at a reading of 760 ppm, and 40 percent had errors above 75 ppm at 1010 ppm. Some had errors above 300 ppm. Those were commercial building sensors, not home monitors, but the lesson carries over: a single reading can miss by a lot.
The same report noted that a sensor which tracks fresh outdoor air alongside the indoor reading cancels out some error. At home, you can copy that idea. Take the monitor outside for a few minutes and see whether it settles near typical outdoor levels. If it sits far off, trust trends more than exact numbers.
Source: W. Fisk, D. Sullivan, D. Faulkner and E. Eliseeva, "CO2 Monitoring for Demand Controlled Ventilation in Commercial Buildings," Lawrence Berkeley National Laboratory, report to the California Energy Commission, March 2010.
PM2.5: fine particles and how they are measured
PM2.5 means particles 2.5 micrometers across or smaller. They come from cooking, candles, fireplaces and smoking, and they also drift in from outdoors. The EPA notes that in homes without smoking or other strong particle sources, indoor levels are expected to be the same as or lower than outdoor levels.
Most home monitors count particles with a small laser and a fan. The device turns the count into a mass figure in micrograms per cubic meter. That step is an estimate, and it can shift with humidity and with the kind of particle, so smoke and dust do not read the same way.
For context, in February 2024 the EPA set its annual PM2.5 standard at 9.0 micrograms per cubic meter. That standard is for outdoor air and is judged over a year, so a short spike on your monitor is not the same thing as a standards breach. It is still a handy yardstick for what "low" looks like.
Sources: U.S. EPA, "Indoor Particulate Matter," and "Final Reconsideration of the National Ambient Air Quality Standards for Particulate Matter (PM)," epa.gov.
How to judge accuracy before you buy
Do not rely on the maker's own claim. Look for independent testing. The South Coast Air Quality Management District in California started its AQ-SPEC program in 2014 to tell the public how commercially available low-cost air sensors really perform. AQ-SPEC tests sensors in the field and in the laboratory, and it publishes evaluation reports.
The EPA has also published target values for PM2.5 sensors. Its February 2021 report suggests these field-test goals: a slope near 1.0 (within 0.35), an R-squared of 0.70 or higher, and a root mean square error of 7 micrograms per cubic meter or less. The report is voluntary, and the EPA does not certify any product. It also covers outdoor, fixed sensors, so it is a guide to what good performance looks like, not a rating for a specific home unit.
In plain terms, ask three questions. Has an independent lab or agency tested this model? Does the test compare it to a reference instrument? Does the maker explain its calibration? If the answer to all three is no, the readings are a rough guide at best.
Sources: South Coast AQMD, AQ-SPEC program page, aqmd.gov; U.S. EPA, "Performance Testing Protocols, Metrics, and Target Values for Fine Particulate Matter Air Sensors," February 2021, and the EPA Air Sensor Toolbox page.
A simple buying checklist
- Sensor type: NDIR for CO2, and a laser particle counter for PM2.5.
- Independent test results, such as an AQ-SPEC report, for the exact model.
- A stated calibration method, and whether the CO2 reading adjusts itself over time.
- A display or app that shows live numbers and a history, so you can spot trends.
- Placement: put it at breathing height, away from windows, vents and the stove.
One honest limit: no home monitor replaces a professional survey. If you suspect a real problem, such as a gas appliance that is not working right, call a qualified technician. Do not rely on a consumer gadget to tell you a space is safe.
Sources: as listed above.