What Is PM2.5, and Why Does It Matter More Than Anything Else?

The human respiratory system is a reasonably good filter for large particles. Nasal hairs, the mucous lining and the cilia of the upper airway trap most of what is above about 10 microns, and cough and mucus clearance remove much of the rest.

Below 2.5 microns that defence largely fails. The particles are small enough to follow the airflow all the way to the alveoli, the gas-exchange sacs at the end of the airway, where there are no cilia and no mucus escalator. From there the finest fraction crosses into the bloodstream, which is why the strongest evidence on PM2.5 concerns cardiovascular disease rather than only lung disease.

• Vehicle exhaust — the dominant year-round urban source in most Indian cities, and the reason exposure is worst at street level on a main road.
• Industry and thermal power — a large regional contributor, particularly across the Indo-Gangetic plain.
• Construction and road dust — technically more of a PM10 source, but a significant PM2.5 contributor through resuspension and secondary formation.
• Waste and biomass burning — municipal waste, leaf burning and residential solid fuel, all of which spike in winter evenings.
• Crop residue burning — a seasonal north Indian contributor in October and November, superimposed on an already high baseline.
• Indoor sources — cooking on a gas hob, incense, mosquito coils and candles all produce PM2.5 at concentrations that can exceed outdoor levels in a closed kitchen.

The gap between the WHO guideline and India's national standard is worth understanding rather than glossing over. National standards balance health evidence against what is achievable; the WHO guideline is the health evidence alone. Meeting CPCB's limit is a genuine policy achievement and is still several times the level at which harm is measurable.

For your own home, the sensible target is not a regulatory threshold at all. It is whatever your purifier can actually hold — single digits to low teens in a closed bedroom with a correctly sized unit, even on a bad outdoor day.

• Purify the bedroom first. It is eight uninterrupted hours in a closable room, which is the highest-value intervention available to most households.
• Keep windows closed during peak hours — typically early morning and late evening, when inversions are strongest — and ventilate in the early afternoon instead.
• Extract at the source while cooking. A chimney or exhaust fan removes cooking PM2.5 far faster than any purifier can filter it afterwards.
• Stop indoor combustion where you can. Incense, mosquito coils and candles are significant, controllable indoor PM2.5 sources.
• Wear a fitted N95 outdoors on severe days. A cloth or surgical mask does very little for PM2.5; fit matters more than filter grade.
• Use recirculation mode in the car during traffic, and fit a HEPA-grade cabin filter — cabin concentrations in traffic are often higher than at the roadside.