Evidence guide · Indoor smoke monitoring
Published 9 October 2026 · Sources checked 9 October 2026
A PM2.5 monitor can help you follow particle levels in a room during wildfire smoke. The useful question is not simply whether its display turns green: it is whether comparable measurements show a sustained change, and what else changed at the same time.
Short answer: keep the monitor in a representative position, compare the same units and averaging periods, and record filtration settings alongside indoor and outdoor conditions. A falling indoor reading is an observation. It is not, by itself, a measurement of filter efficiency or proof that the room is safe.
Where should you put an indoor PM2.5 monitor?
EPA recommends measuring near the occupied breathing zone, typically 3–6 feet above the floor, with unobstructed airflow. Avoid the immediate vicinity of a stove, an air-cleaner outlet, windows, doors or HVAC vents when your aim is a representative room reading. A source, a clean-air stream or a draft can dominate that location. Follow the device instructions as well. [1]
Choose and document one position before starting a comparison. A photo and a short note such as “bedroom shelf, away from purifier outlet” make subsequent readings easier to interpret. If you move the monitor, mark the change in your log rather than treating both positions as one continuous experiment.
Moving a sensor between indoors and outdoors also mixes changes in location with changes over time. If the outdoor plume is changing while you move it, the two readings are not a simultaneous pair.
Check these four things before comparing numbers
| Check | Record | Common mistake |
|---|---|---|
| Quantity | PM2.5 concentration in µg/m³, or the named AQI system | Subtracting an AQI number from a concentration |
| Time | Timestamp, time zone and averaging interval | Comparing a live display with an earlier hourly mean |
| Location | Indoor room and outdoor station or sensor | Treating a distant outdoor station as a measurement at your window |
| Processing | Raw or corrected data, device settings and missing intervals | Treating differently processed displays as interchangeable |
The AirNow Fire and Smoke Map processes sensor observations, including quality screening, correction and NowCast calculations. Its displayed information can therefore differ from another interface for the same device. Nearby locations can also experience different particle levels. Use the map for outdoor context, checking the station and observation time. [3] For the units, see AQI versus PM2.5.
Worked example: why “75% lower” is not filter efficiency
Illustrative data only: the following numbers are invented to demonstrate arithmetic. They are not field measurements, a product test or recommended exposure levels. Assume synchronized hourly means from indoor and outdoor instruments with comparable processing.
| Period | Indoor PM2.5 | Outdoor PM2.5 | Indoor/outdoor ratio |
|---|---|---|---|
| A | 40 µg/m³ | 80 µg/m³ | 0.50 |
| B | 10 µg/m³ | 20 µg/m³ | 0.50 |
The indoor decrease is (40 − 10) ÷ 40 × 100 = 75%. But outdoor concentration also decreased by 75%. The indoor/outdoor ratio remained 0.50 in both periods.
An accurate report is: “The indoor hourly reading fell from 40 to 10 µg/m³ while the outdoor reading fell from 80 to 20 µg/m³.” A claim that “the purifier removed 75% of smoke” is not established by those observations. The example contains no controlled comparison that isolates the purifier.
Now change only the illustrative outdoor value for period B to 80 µg/m³. The ratio becomes 10 ÷ 80 = 0.125. That is a more informative change relative to the outdoor reference, but it still does not separate filtration from changes in air exchange, indoor sources, particle deposition or measurement error. Nor does the ratio measure a filter’s single-pass efficiency.
A ratio also becomes unstable when the denominator approaches zero. Record the actual concentrations alongside it; do not report a large percentage based on two tiny, uncertain readings. Where no suitably matched outdoor measurement exists, describe the indoor trend and identify the missing comparison.
A monitoring log you can copy
This is a Tecnobosque editorial recording template, not an EPA test protocol. Keep your normal protective filtration operating: do not deliberately introduce smoke or switch off protection to manufacture a baseline.
| Field | What to enter |
|---|---|
| Time | Date, time zone, interval start and end |
| Indoor observation | PM2.5 mean, units, device, room and any missing samples |
| Outdoor context | PM2.5 value and interval, station name or URL, distance/context if known |
| Filtration | Purifier speed, HVAC operating state and changes |
| Room events | Door/window opening, cooking, cleaning or sensor movement |
| Environment | Temperature and relative humidity, if available |
| Interpretation | Observed change, plausible alternatives and information missing |
Choose a reporting interval your instruments can support and keep it consistent. Preserve the underlying readings when available. An interval average without a completeness note can hide a device that stopped reporting halfway through.
Write the observation first and the explanation second. “Readings rose after the bedroom door opened” is a useful note. “The hallway was the only source” requires evidence that the log may not contain.
Common questions
Does a green display mean the room is safe?
No single consumer display establishes that. EPA explains that low-cost monitors measure only their specified pollutants, and their alerts are not equivalent to certified smoke or carbon monoxide alarms. A PM-only device does not assess carbon monoxide. Its reading also does not establish an individual’s health risk. [2]
Why does my monitor disagree with another one?
EPA identifies placement, temperature, humidity and data processing among factors affecting performance. Check those factors before attributing disagreement to a real room difference. Similar readings do not, on their own, establish accuracy. [2]
Can an outdoor correction automatically be used indoors?
No. EPA cautions that a correction developed from outdoor collocation may not transfer to indoor conditions. Document the correction actually used rather than silently applying one from a different setting. [1]
What should I do with the observations?
Use them alongside your cleaner-air room plan and the official AirNow Fire and Smoke Map. Follow local public-health and emergency instructions; this logging method does not decide evacuation or override an alarm. For equipment sizing, use the smoke CADR calculator; a room trend is not a substitute for a standardized CADR test.
Sources and method
This guide synthesizes public agency guidance and adds an explicitly hypothetical arithmetic example and an editorial log template. Tecnobosque did not conduct a sensor or purifier test for this article. Source review: 9 October 2026.
- US EPA — A Guide to Siting and Installing Air Sensors. Placement and limits of transferring outdoor corrections indoors.
- US EPA — Low-Cost Air Pollution Monitors and Indoor Air Quality. Performance, pollutant coverage and interpretation limits.
- AirNow — Fire and Smoke Map: Questions and Answers. Sensor processing, spatial differences and outdoor context.
Related: Why a satellite smoke plume is not a ground-level PM2.5 measurement · Smoke and indoor air quality.