How to read an air quality monitor

What PM2.5 actually tells you

PM2.5 refers to particulate matter 2.5 microns or smaller in diameter, small enough to stay suspended in air for extended periods rather than settling quickly. A PM2.5 sensor reports a mass concentration, typically in micrograms per cubic meter, and rising or falling numbers over time in the same room are a directly useful, measurable signal of whether particulate levels are increasing or decreasing.

A PM2.5 reading does not identify the source; smoke, dust, cooking particles, and outdoor pollution infiltrating through a window all register on the same scale. Watching a PM2.5 number spike right after cooking or right after opening a window during a smoke event is a useful diagnostic even without knowing the exact particle type.

Most consumer PM2.5 sensors use a low-cost laser or optical scattering method, which is genuinely useful for tracking relative changes but is generally less precise than the gravimetric reference method used in regulatory air quality monitoring. This gap in absolute precision rarely matters for the household question this page is concerned with, whether a specific room’s air is getting better or worse over time, since that question depends on the trend rather than the exact number.

Humidity also affects some low-cost optical PM2.5 sensors, since water droplets in very humid air can scatter light in a way the sensor misreads as particulate matter, inflating the reported number during a humid spell without an actual increase in dust, smoke, or pollen. A PM2.5 spike that coincides with a sudden jump in humidity, rather than an obvious source like cooking or an open window, is worth treating with a bit of skepticism for this reason.

What CO2 actually tells you

Carbon dioxide concentration in an occupied room rises with the number of people present and the amount of fresh outdoor air being exchanged in, which makes CO2 a widely used proxy for ventilation rate rather than a pollutant tracked for its own sake at typical indoor levels. A rising CO2 reading in a closed bedroom overnight, for example, indicates the room is not getting fresh air exchange, independent of anything to do with particulate or VOC levels.

Dedicated CO2 sensors commonly use NDIR (non-dispersive infrared) technology, which is generally more accurate and more expensive than the metal-oxide sensors bundled into many budget all-in-one monitors; a listing that specifically states NDIR CO2 sensing is disclosing a different, more reliable measurement approach than one that only lists a generic multi-gas sensor.

A cheaper alternative some budget monitors use is an "eCO2" or estimated CO2 reading, calculated indirectly from a VOC sensor rather than measured directly, which is a meaningfully different and less reliable approach than a true, direct CO2 measurement. A listing that distinguishes between "CO2" and "eCO2" is disclosing a real difference in measurement method, not just marketing language, and the direct CO2 reading is the more trustworthy of the two for the ventilation-tracking purpose this section describes. Checking a listing’s specification sheet for which of the two terms it actually uses is worth the extra minute before assuming a "CO2 monitor" is measuring CO2 directly.

What a VOC index actually is, and what it is not

A VOC index reported by most consumer monitors is a relative score representing a shift in the combined signal from a broad group of volatile organic compounds, not a measurement of a specific named chemical like formaldehyde or benzene at a stated concentration. The sensor reacts to a mixture of gases together and reports a single index number that moves up or down as that mixture changes.

This makes a VOC index genuinely useful for noticing when something changed, a new paint smell, cleaning product use, or off-gassing from new furniture, but it cannot answer "how much formaldehyde is in this room" on its own. A dedicated formaldehyde or specific-gas meter, a separate and typically more expensive instrument, is required to answer that narrower question.

Different manufacturers also scale their VOC index numbers differently, so a reading of, for example, 150 on one brand’s monitor is not necessarily comparable to a 150 reading on a different brand’s device, even though both are called a VOC index. This makes VOC index most useful as a same-device, same-room trend indicator rather than a number to compare across two different monitors from different manufacturers.

What a cheap sensor can and cannot do

A budget all-in-one monitor combining PM2.5, CO2, and VOC sensing in one device is generally reliable for tracking relative trends in the same room over time: is the number going up or down compared to an hour ago, or compared to before and after turning on a purifier. Budget sensors are less reliable for precise absolute accuracy compared to laboratory-grade or dedicated single-purpose instruments, particularly for CO2 measured with a metal-oxide sensor rather than NDIR, and for VOC index, which by design is a relative rather than absolute figure.

For most household purposes, particularly checking whether a purifier is actually reducing PM2.5 in a specific room, or whether a bedroom needs better ventilation based on rising overnight CO2, a budget monitor’s trend data is genuinely useful even without laboratory precision.

Sensor drift is a real, if usually gradual, limitation worth knowing about: most consumer sensors lose a small amount of calibration accuracy over months of continuous use, and a monitor that reported accurately when new may read somewhat differently a year or two later. Manufacturers vary in whether they support recalibration, and checking this before buying matters more for a household planning to rely on the monitor’s absolute numbers over a long period than for one mainly watching short-term before-and-after trends.

Using a monitor alongside a purifier, not instead of one

A monitor measures air quality; it does not change it. Pairing a PM2.5 reading with the sizing and placement guidance in `/guides/how-to-size-an-air-purifier/` and `/guides/where-to-place-an-air-purifier/` turns a monitor from a passive number on a screen into a way to actually verify whether a specific purifier setup is working in a specific room, before and after running it.

A practical routine worth adopting: note the PM2.5 reading before turning the purifier on, then check again after roughly the time predicted by the 90 percent clearance formula, ln(10) divided by the room’s ACH, in hours, covered in `/guides/air-changes-per-hour/`. A reading that drops close to the predicted amount confirms the setup is performing close to its sizing math; a reading that barely moves is a signal to check placement, filter condition, or whether the room is actually as sealed as assumed.