Home PM2.5 Monitors: Sensors and the EPA Correction
Last updated 2026-07-15
A home air-quality monitor puts a live PM2.5 number in the room you are actually breathing in — something no regional AirNow station can do. During a smoke event, indoor levels depend on your windows, your purifier, and how leaky your house is, so the official number for the whole county tells you little about your bedroom.
But consumer monitors have a well-known quirk: the cheap sensors that make them affordable tend to over-read in wildfire smoke. This guide covers what the reading means, what to look for, and how to line your monitor up against AirNow.
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How a home monitor measures PM2.5
Almost every affordable indoor monitor uses an optical laser particle counter (often a Plantower or Sensirion module). A small fan pulls air past a laser; particles scatter the light, and the sensor counts the flashes and estimates how much PM2.5 — fine particles 2.5 microns and smaller — is in the air. PM2.5 is the pollutant that dominates wildfire smoke, so it is the reading that matters most.
- The device does not weigh particles; it infers mass in micrograms per cubic meter (ug/m3) from how they scatter light.
- That inference assumes a typical mix of particle sizes and densities. Wildfire smoke breaks that assumption, which is the root of the over-reading problem below.
What to look for in a home monitor
Not every gadget labeled air monitor is worth buying. Prioritize:
- A real PM2.5 number in ug/m3, not just a colored happy or sad face. You want the concentration so you can compare it to EPA breakpoints and to AirNow.
- A laser particle counter, not a cheap resistive dust sensor. Laser-scatter sensors are the standard for a reason.
- Logging or an app so you can watch the trend over hours, which matters far more than any single reading during a smoke event.
- Adequate range. Some budget units flatten out (saturate) above a few hundred ug/m3 — exactly when heavy smoke makes the reading most useful.
A capable countertop unit is enough for most homes; see our air-quality monitor pick.
Why cheap sensors read high in smoke
This is the single most important thing to understand. Low-cost optical sensors are calibrated against everyday urban and dust particles. Wildfire smoke particles are smaller, differently shaped, and take up water differently, so the sensor overestimates PM2.5 — often by 30 to 50 percent or more during dense smoke.
The fix is a published EPA correction equation (the Barkjohn US-wide correction) that scales the raw sensor value down to better match reference-grade monitors. EPA already applies this correction to PurpleAir sensors shown on AirNow's Fire and Smoke Map; we explain how we treat sensor data on our methodology page.
- If your monitor shows a raw, uncorrected number, treat a heavy-smoke reading as an upper bound — the true level is likely somewhat lower.
- Most consumer indoor monitors do not apply any smoke correction, so their app number can run higher than the corrected AirNow value for the same air.
Consumer monitors vs PurpleAir
PurpleAir is a specific brand of sensor whose data feeds public maps. The difference that matters is not the hardware — both use similar laser modules — but the correction and context:
- PurpleAir on a corrected map (like AirNow's Fire and Smoke Map) shows an EPA-corrected value you can compare directly to regulatory monitors.
- PurpleAir's own map can show raw or differently processed numbers — check which layer you are viewing.
- A standalone consumer monitor usually shows the raw sensor number with no correction. That is great for indoor trends, less reliable as an absolute outdoor AQI.
Use a home monitor to track your indoor air and to prove a purifier is working; use AirNow or a corrected map for the official outdoor number.
How to use your monitor alongside AirNow
The two tools answer different questions. AirNow and our per-city pages give the authoritative outdoor AQI from reference-grade stations; your home monitor gives the indoor reality no station can see.
- Set the outdoor baseline from AirNow. Before deciding whether to go out, check the official number — our Can I go outside? tool reads the current EPA AQI for your spot.
- Watch the indoor trend on your monitor. If indoor PM2.5 climbs toward outdoor levels, your sealing is failing; if it stays low, your right-sized purifier or Corsi-Rosenthal box is doing its job.
- Compare corrected to corrected. When you sanity-check your monitor against AirNow, use AirNow's corrected value, and do not expect a raw indoor reading to match it exactly, especially in heavy smoke.
For orientation: on the 2024 EPA scale, PM2.5 up to 9.0 ug/m3 is Good and 9.1 to 35.4 is Moderate. A well-sealed home with a running purifier should hold indoor PM2.5 in that lower range even when it is orange or red outside. Get a heads-up when outdoor smoke arrives with our free smoke alerts.
FAQ
Why does my home monitor read higher than AirNow?
Low-cost laser sensors overestimate fine particles in wildfire smoke, and most consumer monitors do not apply any correction. AirNow uses reference-grade monitors, and the sensor data on its Fire and Smoke Map has the EPA correction applied, so its number is usually lower and more accurate during heavy smoke.
Are cheap PM2.5 monitors accurate enough to be useful?
Yes, for tracking indoor trends over time and confirming a purifier is lowering the level in your room. Treat absolute readings during dense smoke as an upper bound rather than a precise figure, and compare against AirNow for the official outdoor AQI.
Is a PurpleAir better than a consumer countertop monitor?
They use similar laser hardware, so the accuracy difference comes down to correction and context, not the sensor. PurpleAir data shown on a corrected map like AirNow's is comparable to regulatory monitors; a standalone consumer unit shows a raw, uncorrected number that is best used for indoor trends.
Where should I place a home air monitor?
Put it at breathing height in a room you use most, away from direct sources like the stove, candles, or a fireplace, and away from open windows and supply vents. Those spots spike the reading and hide the whole-room level you actually want to track.
Check the air where you are
Live AQI on the 2024 EPA standard, with the reporting monitor and observation time on every reading.
Where this guidance comes from
We paraphrase published guidance rather than inventing our own. The primary sources for everything on this page:
- AirNow — Air Quality and Health (U.S. EPA): the AQI scale and its activity guidance by level.
- EPA — Particulate Matter (PM) Basics: what PM2.5 is and why it reaches deep into the lungs.
- CDC — Wildfire Smoke Safety: who is at higher risk, and when to seek care.
- EPA — Wildfires and Indoor Air Quality: clean rooms, filtration and what actually works indoors.
- AHAM: the CADR standard and the two-thirds room-sizing rule.
Our own AQI arithmetic is documented on the methodology page.
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Some links are affiliate links — see disclosure. This guide is general health information and is not a substitute for medical advice — see disclaimer.