Indoor/Outdoor PM2.5 Monitor

How it works

A laser particle counter draws air past a beam and counts the scattering events, converting them to an estimated PM2.5 concentration in µg/m³. It tells you what the air in a specific room actually is, rather than what a regional monitoring station 40 km away reports. That distinction matters, because indoor concentrations during a smoke event can be anywhere from a tenth to well above the outdoor reading depending on your building.

What the market looks like

Consumer air monitors split cleanly. Cheap handheld units at CAD $40–$120 use uncalibrated sensors and drift badly — useful for relative changes, not for absolute numbers. Mid-range fixed monitors at CAD $150–$400 (PurpleAir, AirGradient, Awair and similar) use Plantower or Sensirion sensor modules with reasonable calibration and log data over time. Reference-grade instruments start around CAD $3,000 and are not a consumer product. The mid-range is where the value is, and PurpleAir in particular feeds a public map that has become genuinely useful infrastructure during western North American smoke seasons.

Features that actually matter

Laser scattering sensor

Plantower and Sensirion modules dominate and are decent. The brand on the box often matters less than which module is inside — worth asking.

Indoor and outdoor variants

An outdoor unit tells you when to close up; an indoor unit tells you whether it worked. If you buy one, buy the indoor one.

Data logging and history

The real value. A number now is mildly interesting; a graph across a week tells you whether your purifier and your window discipline are working.

Public network contribution

PurpleAir outdoor units publish to a public map, which improves local awareness in exactly the rural areas government monitors do not cover.

Humidity correction

Laser sensors over-read in high humidity. The better units and networks apply a correction; the cheap handhelds do not.

What it really costs

Up front

CAD $150–$400 for a credible unit.

Consumables

None, though the sensor module degrades over roughly 3–5 years of continuous use.

What else you'll need

Power and Wi-Fi. Outdoor units need weather-protected mounting.

Over five years

Purchase price plus eventual replacement. Some units allow a CAD $40 sensor module swap, which is worth checking.

Figures are typical street prices in Canadian and US dollars as of September 2026, gathered from public retail listings. They move with the season and with the exchange rate — treat them as a planning band, not a quote.

How it differs from the alternatives

Versus government AQHI or AirNow

Free, reference-grade, and possibly measuring a valley you do not live in. During smoke events the difference between the station reading and your street can be large. A local monitor resolves that.

Versus the PurpleAir public map

If a neighbour already runs an outdoor unit, you get their outdoor data free. What you still lack is your indoor number, which is the actionable one.

What it changes

This is a behaviour device. It tells you when to close windows, when it is safe to open them again, whether the purifier in the bedroom is winning, and whether the filter needs changing. Without it people guess, and they mostly guess wrong in the direction of opening windows too early.

Accuracy expectations

Expect ±10–20% against reference in the ranges that matter. That is plenty for deciding whether to open a window and not enough for any clinical purpose.

If not this, then what

• The free AQHI or AirNow appsFree, well-calibrated, potentially measuring somewhere else. Use them as well.
• A neighbour's PurpleAir on the public mapFree outdoor data. Does not tell you about your own indoor air.