Reading AQI properly: what PM2.5 is really telling you

One number, several pollutants

AQI is computed for each pollutant separately — fine particles (PM2.5), coarse particles (PM10), nitrogen dioxide, ozone, sulphur dioxide and carbon monoxide — and the highest of those sub-indices becomes the headline figure.

In most South Asian cities the headline is PM2.5 almost every day of the year. That is why a single reading can jump while the sky looks unchanged: the particles doing the damage are around 30 times thinner than a human hair.

Why PM2.5 matters more than the rest

Particles under 2.5 micrometres bypass the nose and throat entirely, settle deep in the lungs and pass into the bloodstream. The World Health Organization's guideline is an annual average of 5 µg/m³ — a level most large cities exceed many times over.

Long-term exposure is linked to heart disease, stroke, reduced lung development in children and lower birth weight. Short spikes mostly show up as asthma attacks and hospital admissions within a day or two.

What the bands mean in practice

US AQI bands are colour-coded for a reason — the advice changes sharply between them.

• 0–50 Good — no precautions needed
• 51–100 Moderate — unusually sensitive people may notice symptoms
• 101–150 Unhealthy for sensitive groups — children, older adults and asthmatics should ease off outdoor exertion
• 151–200 Unhealthy — everyone should cut back hard outdoor activity; run a filter indoors
• 201+ Very unhealthy to hazardous — stay indoors, seal gaps, use N95 masks outside

Where trees fit

Trees are not an air filter you can switch on. A single street tree removes a modest amount of particulate matter, but a continuous canopy along a corridor measurably lowers dust and roadside PM, and shade cuts the heat that drives ozone formation.

The honest framing: trees are a long-term, compounding intervention alongside cleaner fuel and traffic policy — not a substitute for them.