PM2.5 and PM10: Why Fine Particles Are a Major Air-Quality Threat

What PM10 and PM2.5 Mean

PM10 refers to particles with an aerodynamic diameter up to 10 micrometres; PM2.5 is up to 2.5 micrometres. Aerodynamic diameter describes how a particle behaves in air — particles of different shape and density can belong to the same fraction. A human hair is typically tens of micrometres wide; PM2.5 is far smaller and not necessarily visible. Haze can indicate high particle levels, but apparently clear air may still contain substantial PM2.5.

What Particles Are Made Of

PM composition depends on the source: soot contains carbonaceous material, road dust contains minerals and metals, sea aerosol contains salts, and industrial particles may include specific metals and organic chemicals. Fires create complex mixtures from wood, fuel, plastics, and building materials. The same mass concentration does not always carry the same risk — chemical composition, solubility, and surface reactivity all vary.

Primary and Secondary Particles

Primary particles are emitted directly: ash, soot, cutting dust, construction dust, soil, tyre wear, and brake wear. Secondary particles form in the atmosphere when SO₂, NOₓ, ammonia, and organic gases become sulfates, nitrates, ammonium salts, and organic aerosol. Secondary formation means particles can appear far from precursor sources — local street cleaning is useful but cannot replace reductions from energy, industry, transport, and agriculture.

Major Sources

Burning coal, wood, heavy fuel oil, diesel, and waste produces fine particles. Industrial furnaces, cement production, quarries, construction, traffic, bulk-material handling, and re-suspended road dust also contribute. Natural sources include dust storms, sea salt, wildfires, and volcanic ash. Natural origin does not make particles automatically harmless — and controls for soot, road dust, and secondary nitrate are all different.

How Particles Behave in the Atmosphere

Larger particles settle more quickly; PM2.5 can remain airborne for days and travel hundreds of kilometres. Humidity changes the size of hygroscopic particles, and temperature affects the gas-particle balance. Buildings and terrain create accumulation zones; wind can either clean local air or import pollution from elsewhere. Episode analysis requires information about weather, mixing height, and transport direction.

Why Fine Particles Affect Health

PM10 can enter the respiratory tract; PM2.5 can reach the alveolar region deep in the lungs. Exposure is associated with inflammation, worsening of asthma and chronic respiratory illness, and cardiovascular effects. Dose depends on concentration, time, and breathing rate. Children, older people, pregnant people, those with heart or lung disease, and outdoor workers may be more vulnerable. The absence of immediate symptoms does not mean there is no long-term risk.

How PM Is Measured

Reference methods collect particles on filters for mass determination or use automatic instruments measuring mass change or radiation attenuation. These require controlled sampling, flow checks, temperature and humidity management, and regular maintenance. Optical sensors infer concentration from light scattering — but results depend on particle size, shape, colour, and humidity, so calibration for one aerosol type can be inaccurate for another. Such data should be labelled as indicative when it has not undergone appropriate verification.

How to Reduce Emissions and Personal Exposure

Combustion sources benefit from cleaner energy, electrification, efficient combustion, fabric filters, and electrostatic precipitators. Industrial and construction sites need enclosure, water suppression, covered materials, and wheel washing. Secondary PM is reduced by controlling SO₂, NOₓ, ammonia, and organic gases — requiring coordinated policy across energy, transport, industry, and agriculture. During high-pollution episodes: follow official information, avoid intense exercise near roads, and use a properly sized air cleaner with an appropriate filter.