A low-cost indoor air monitor can show a reassuring green light while still missing a problem it was not designed to measure. PM2.5 is fine particulate matter, not a score for every indoor hazard. The first decision is whether a sensor’s measurement answers the question you actually have.
This page is about interpreting and placing a monitor. For reducing particles during smoke events, use the existing wildfire cleaner-air room guide and air cleaners and HVAC filter guide.
Distinguish readings before comparing numbers
| Display or claim | What it may tell you | What it cannot prove |
|---|---|---|
| PM2.5, usually in µg/m³ | Estimate of fine suspended particles near the device | Whether carbon monoxide, radon or every gas is absent |
| “Indoor air-quality score” | Manufacturer’s combined or proprietary indicator | A nationally standardized indoor health threshold |
| Temperature and humidity | Environmental conditions affecting comfort and some sensors | Particle composition or cause |
| Trend after switching on filtration | Whether the displayed concentration changes at that location | Exact filter efficiency in every room |
| Smartphone notification | A configured device and network alert | What happens if the sensor, Wi-Fi or cloud stops working |
The EPA’s low-cost indoor monitor guidance emphasizes that these devices measure only their designed factors and that widely accepted indoor limits are unavailable for many pollutants. Do not apply outdoor AQI color scales to a consumer sensor as though they were identical.
Location can change the result
A sensor directly beside a frying pan, bathroom shower, air purifier outlet or open window may show an unusual short-lived local condition, not the average conditions where a person spends hours. The EPA siting guide recommends unobstructed airflow and locations relevant to the actual monitoring purpose.
Write down whether you are measuring the most occupied room, a bedroom or the area near a known particle source. Keep wires out of walking routes, follow the manufacturer’s location and power rules, and avoid obstructing vents.
Treat precision as a claim to verify
The EPA Air Sensor Toolbox FAQ notes that lower-cost sensors can over- or under-estimate real concentrations and may be affected by humidity and temperature. Two differently designed monitors disagreeing does not prove either one is correct.
Check the exact model’s evaluation evidence, calibration and maintenance instructions, and whether readings are raw, corrected, averaged or delayed. An alert threshold chosen by a vendor is not automatically a medical decision threshold.
Use the number alongside an action
For wildfire smoke, prioritize official local alerts and safe exposure-reduction guidance. A rising particle trend can prompt a check of closed windows, cooking smoke or filter function. A low displayed number must not delay evacuation for fire, gas, carbon monoxide, heat danger or official orders.
If the household has symptoms or vulnerable occupants, follow medical and local public-health guidance rather than experimenting to force a reading down.
Indoor PM2.5 evidence record
| Observation | Actual evidence |
|---|---|
| Exact monitor, sensor and firmware | |
| Pollutant and units shown | |
| Placement and nearby local sources | |
| Humidity, ventilation and filtration state | |
| Trend and time period, not just one sample | |
| Limitations and independent next action |
Next action: pick the room and pollutant you need to understand, then write a repeatable, safe measurement routine before buying a second sensor.
Does PM2.5 equal indoor AQI?
No. PM2.5 is one pollutant concentration. Public outdoor AQI combines standard pollutant-specific rules, while consumer indoor displays may use proprietary interpretation. Check the product’s exact definition.