How accurate are optical PM2.5 sensors?

Modern low-cost optical sensors agree well with reference instruments at typical indoor concentrations, with bias of roughly ±10–15%. They are excellent for trends and intercomparison; gravimetric sampling remains the reference for regulatory measurements.

What PM2.5 level should trigger action indoors?

WHO 2021 guideline values place the 24-hour PM2.5 target at 15 μg/m³. Sustained indoor readings well above outdoor levels usually indicate an indoor source or inadequate filtration.

Can PM2.5 monitoring replace ventilation assessments?

No. PM2.5 sensors measure outcomes — they do not characterise air change rate, filter condition or supply-air balance. They complement ventilation assessment rather than replace it.

Particulate monitoring

PM2.5 monitoring — turning fine particulates into actionable building data

Fine particulate matter is one of the most consequential indoor pollutants and one of the most measurable. Continuous PM2.5 monitoring shows the building exactly when, where and how often concentrations rise.

Fine particulates illuminated in sunlight near a window — PM2.5 indoor context

AirQualityTech.uk · Editorial

Sources

Where indoor PM2.5 actually comes from

Indoor concentrations are driven by a mix of outdoor infiltration and indoor activity. Monitoring data only tells the right story when both are considered.

Cooking

Frying, searing and gas combustion are dominant indoor PM2.5 sources.

Outdoor infiltration

Traffic and urban background drift inside through windows, doors and ventilation.

Combustion

Candles, wood burners and tobacco produce intense short-duration peaks.

Resuspension

Movement and cleaning lift settled dust back into the breathing zone.

Sensing

How optical PM2.5 sensing works

A small laser illuminates particles passing through a sampled airflow. The scattered light is detected and translated into particle count and size by the sensor's processor, then aggregated to mass concentration in micrograms per cubic metre.

Optical sensing is fast and inexpensive, which is why it sits inside virtually every continuous PM monitor. It is, however, sensitive to humidity and to the optical properties of the aerosol — two reasons high-humidity environments and unusual particle sources benefit from gravimetric reference validation.

For commercial deployments the practical answer is to use optical sensors continuously and run a gravimetric reference comparison once per year, or whenever an unusual source profile is suspected.

Interpretation

Indicative PM2.5 thresholds

Indoor 24-hour PM2.5	Interpretation	Typical action
<10 μg/m³	Clean indoor air, below WHO target	Monitor and verify
10–15 μg/m³	At or near WHO 24-hour guideline	Maintain ventilation and filtration
15–35 μg/m³	Elevated, investigate sources	Check filtration, identify source events
>35 μg/m³	Sustained exceedance	Engineer mitigation: filtration, source control

Limits

What PM2.5 monitoring will not tell you

Chemical composition

Mass alone does not identify what the particles are made of.

Long-term exposure

Continuous data shows patterns, not validated personal exposure dose.

Source attribution

Spatial sensor density and event correlation are needed to attribute peaks to a source.

Air change rate

Ventilation effectiveness still needs pressure, flow and tracer measurements.

FAQ

PM2.5 monitoring questions

PM2.5 refers to fine particulate matter 2.5 microns in diameter or smaller. Particles in this size range penetrate deep into the respiratory system and are a leading global air pollution health concern. Indoors, they originate from cooking, combustion, outdoor infiltration and resuspended dust.

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