A PM2.5 particulate matter sample collected from urban air. The brown-grey residue on the filter represents particles smaller than 2.5 micrometres. Source: Wikimedia Commons.
Particle Size Categories
Airborne particulate matter is classified by aerodynamic diameter. The two primary categories relevant to indoor monitoring are PM10 — particles smaller than 10 micrometres — and PM2.5, which covers particles smaller than 2.5 micrometres. A human hair is roughly 50–70 micrometres in diameter; PM2.5 particles are therefore 20–30 times smaller and remain suspended in indoor air for hours before settling.
PM1, the fraction below 1 micrometre, is increasingly monitored by newer consumer sensors but is not yet covered by most national ambient air quality standards. Its health relevance is the subject of ongoing research, and several studies suggest it may penetrate more deeply into lung tissue than larger PM2.5 particles.
| Fraction | Diameter | Common sources | Suspension time |
|---|---|---|---|
| PM10 | < 10 µm | Road dust, construction, pollen | Minutes to hours |
| PM2.5 | < 2.5 µm | Combustion, cooking, printing | Hours to days |
| PM1 | < 1 µm | Combustion byproducts, tobacco | Days (in still air) |
Indoor Sources of Particulate Matter
Indoor PM2.5 originates from both internal generation and infiltration from outside. The relative contribution of each varies considerably by building type, occupant behaviour and outdoor conditions.
Cooking
Frying and high-temperature cooking are among the highest indoor PM2.5 sources. Oil-based cooking at temperatures above 180 °C generates aerosol droplets and combustion particles that can drive PM2.5 readings above 200 µg/m³ in a typical kitchen within minutes. Gas burners add ultrafine combustion products even without visible smoke. An operating range hood exhausted to the outside reduces but does not eliminate this source.
Candles and Incense
A single burning paraffin candle in a medium-sized room can raise PM2.5 to 30–50 µg/m³ within 30 minutes. Beeswax candles generally produce less soot than paraffin equivalents. Incense sticks produce particulate concentrations that can reach several hundred µg/m³ in poorly ventilated spaces — a range that exceeds WHO annual mean guideline values by a substantial margin even during a short exposure.
Printers
Laser printers emit ultrafine particles during operation. Studies reviewed by several European environmental agencies found peak PM0.1 concentrations near operating laser printers exceeding 1,000 particles/cm³. The health significance of short-duration exposures of this kind remains under discussion, but the finding is consistent across multiple independent measurement campaigns.
Outdoor Particulate Infiltration in Poland
Poland consistently records some of the highest PM2.5 concentrations among EU member states during the heating season. The Chief Inspectorate for Environmental Protection (Główny Inspektorat Ochrony Środowiska, GIOŚ) maintains a public monitoring network covering over 150 stations. During severe smog episodes in Kraków, Katowice or Nowy Sącz, 24-hour mean PM2.5 values above 100 µg/m³ — four times the current WHO 24-hour guideline of 15 µg/m³ — have been recorded.
Real-time air quality data for Polish monitoring stations is publicly accessible at powietrze.gios.gov.pl. The site displays current PM10, PM2.5, ozone and nitrogen dioxide readings by station, with historical data available for download.
The infiltration factor — the fraction of outdoor PM2.5 that penetrates indoors — depends heavily on building envelope airtightness, window sealing quality and ventilation system type. In older multi-family buildings with gravity ventilation and unrefurbished window frames, infiltration factors of 0.5–0.8 are typical; that is, indoor concentrations during peak outdoor episodes reach 50–80% of outdoor values. In newer, well-sealed construction with mechanical ventilation and HEPA-stage filtration, infiltration factors below 0.1 are achievable.
How Indoor PM Sensors Work
Consumer-grade PM sensors use optical particle counting (OPC) or light scattering (nephelometry). A laser beam illuminates the sample air; particles passing through the beam scatter light in a pattern that correlates with particle size and concentration. Dedicated firmware translates raw scattering data into PM2.5 and PM10 mass concentration estimates (µg/m³) using calibration constants derived from laboratory testing with standardised aerosols.
Calibration Limitations
Consumer sensor PM readings are estimates, not direct gravimetric measurements. Sensor accuracy is typically stated as ±15–30 µg/m³ or ±15% for PM2.5 in the 0–200 µg/m³ range. Two sensors of the same model reading the same air can disagree by 15–30%. The readings are more useful for tracking trends and identifying events — a cooking episode, an open window during a smog alert — than for precise compliance measurement against regulatory limits.
Placement for Indoor Monitoring
Optimal placement for indoor PM monitoring follows similar logic to humidity sensors: central room position, 1–1.5 m height, away from air supply vents (which dilute readings) and away from obvious point sources like cooking areas if general room-level conditions are the target. For source characterisation — understanding how much a specific activity affects air quality — placing a sensor near the activity and a reference sensor in another room simultaneously is more informative.
Reference Concentration Levels
The 2021 WHO Global Air Quality Guidelines revised PM2.5 targets downward from earlier recommendations:
Indoor monitoring targets are not directly codified in Polish law for residential buildings. The WHO guidelines for indoor air quality (2010, under revision) reference outdoor guideline values as the relevant benchmark for infiltration-sourced indoor pollution.