Why are indoor particles counted as PM10 and PM2.5, and where do they come from?

Key facts

• PM10 particles deposit in the nose and throat, while PM2.5 particles pass both and reach the deep lung, which is why the two sizes are counted separately.

  Established

  Aerodynamic deposition in the respiratory tract; the basis for the size cuts used by the EPA and WHO.

• In the UAE Unified Aerosol Experiment, mineral dust oxides accounted for 72 percent of PM10 mass and 38 percent of PM2.5 mass at a coastal site near Abu Dhabi.

  72 ± 11 / 38 ± 26 % of mass Established

  Kaku et al. 2016, Atmospheric Research 169, chemical reconstruction of 51 PM10 and 52 PM2.5 filter samples, August to September 2004. Outdoor, coastal, two months.

• In one monitored Dubai home, fine particles sat near 4 micrograms per cubic metre all morning and reached 80 within a single half-hour around lunch, returning to baseline about four hours later.

  4 to 80 µg/m³ Estimated Time-bound claim

  One monitored home, 48 consecutive 30-minute readings on 1 July 2026, published with the household's consent. Absolute levels uncalibrated.

• Optical particle counters overread at high relative humidity because hygroscopic particles absorb water and scatter light like larger ones.

  Established

  Optical sizing infers mass from scattered light under an assumed density and refractive index.

Why are indoor particles counted in two sizes?

Indoor particles are counted as PM10 and PM2.5 because particle size decides where a particle stops in the body, not because the two are different substances. Both labels describe aerodynamic diameter — the diameter of a unit-density sphere with the same aerodynamic behaviour — rather than any chemical property.

PM10 covers particles below 10 micrometres, most of which impact in the nose, throat and upper airway. PM2.5 covers particles below 2.5 micrometres, which follow the airflow past those defences into the bronchioles and alveoli. The cut points were chosen to match the sizes at which deposition changes, which is why an indoor air index carries both.

What produces particles inside a Dubai home?

Particles inside a Dubai home come mostly from cooking and from burning, and both are ordinary domestic activity rather than anything unusual. Frying and other high-temperature cooking generate fine particles in quantity; so do candles, incense and any unvented flame. Vacuuming and cleaning re-suspend settled coarse material, which is why a sealed filter matters more than suction.

One monitored Dubai home shows the shape this takes. On 1 July 2026 the home sat near 4 micrograms per cubic metre through the whole morning, rose to 80 within a single half-hour around lunch, and took about four hours to return to that baseline. The cooking is brief and the exposure is not, which is why extraction after cooking does more than extraction during it.

Does a high particle reading in Dubai mean pollution?

A high particle reading in Dubai is a particle count, not a pollution count. A counter measures aerodynamic size and reports mass; it has no way to determine what a particle is made of or where it came from. In this region that distinction carries real weight, because a large share of airborne mass is mineral rather than combustion-derived.

The chemical reconstruction of the UAE Unified Aerosol Experiment, at a coastal site about 50 kilometres north-east of Abu Dhabi, attributed 72 percent plus or minus 11 of PM10 mass and 38 percent plus or minus 26 of PM2.5 mass to mineral dust oxides. The spread on the fine fraction is wide and the authors report it as wide. Two limits belong beside those numbers: they are outdoor readings taken over two months in 2004, not indoor readings, and the same work finds the dust arrives chemically processed by industrial sulfate and nitrate rather than clean off the ground.

None of this makes mineral dust harmless, and none of it reduces the case for extraction while cooking. It supports something narrower: a raw particle reading in the Gulf is not a clean proxy for combustion or traffic emissions, because much of what the instrument counted arrived from the ground. Indoor sources add to that regional baseline rather than replacing it.

What are the limits of a low-cost particle reading?

A low-cost particle reading has two limits worth stating whenever one is quoted. The first is that most affordable sensors are optical: they infer mass from light scattered by a sampled airstream, using an assumed particle density and refractive index. Mass is inferred rather than weighed, so the assumption carries into every number.

The second is humidity. Hygroscopic particles absorb water at high relative humidity and grow large enough to scatter light like a substantially bigger particle, so optical readings drift upward on humid days for reasons unconnected to emissions. In a coastal Gulf summer this is not a marginal effect.

Cut indoor particles at the source

1. Run the extractor before you startSwitch the range hood on before the pan heats, not once the smoke appears.
2. Keep it running afterwardsLeave the extractor on for a good fifteen minutes after the hob goes off — the decay takes far longer than the cooking did.
3. Treat candles and incense as combustionBoth produce fine particulate. Ventilate the room while they burn.
4. Vacuum with a sealed pathA vacuum without sealed filtration resuspends settled material instead of removing it.

Definitions

Aerodynamic diameter

The diameter of a unit-density sphere that behaves the same way in air as the particle in question. It governs where a particle deposits in the airway, and it is not the same as the diameter measured under a microscope.

Coarse fraction

The mass remaining when PM2.5 is subtracted from PM10, covering particles between roughly 2.5 and 10 micrometres.

What we are sure of, and what we are not