PM2.5 — What It Is and Why It Matters
PM2.5 refers to fine particulate matter 2.5 micrometres across or smaller — about thirty times thinner than a human hair. Because the particles are so small, they bypass the body's natural defences, lodge deep in the lungs and can pass into the bloodstream, which is why PM2.5 is the pollutant most closely linked to serious health effects and is usually the "dominant" pollutant that drives a city's AQI.
Where PM2.5 comes from
Major sources include vehicle exhaust, coal and biomass burning, industrial emissions, construction dust, and — across North India in winter — crop-residue burning and household heating. Secondary particles also form in the air from gases such as sulphur dioxide and nitrogen oxides.
What is a safe PM2.5 level?
The World Health Organization's 2021 guideline recommends a 24-hour average PM2.5 below 15 µg/m³ and an annual average below 5 µg/m³. On the Indian AQI scale, a 24-hour PM2.5 concentration up to 30 µg/m³ maps to "Good" and up to 60 µg/m³ to "Satisfactory"; above that, the index rises into levels that affect health. Because much of India routinely exceeds these figures in winter, reducing exposure — masks, purifiers, timing outdoor activity for cleaner hours — matters.
Frequently asked questions
Is PM2.5 more harmful than PM10?
Yes — PM2.5 particles are smaller, penetrate deeper into the lungs and bloodstream, and are more strongly linked to health harm than the coarser PM10.
What PM2.5 level is safe?
The WHO recommends a 24-hour average under 15 µg/m³. On the Indian AQI, PM2.5 up to 30 µg/m³ is "Good".