Fine particulate matter smaller than 2.5 micrometres — small enough to penetrate deep into the lungs and enter the bloodstream. The most harmful common air pollutant.
measurementWhat is PM2.5?
PM2.5 stands for “Particulate Matter 2.5 micrometres or smaller.” These tiny particles are about 1/30th the diameter of a human hair. They are the single most dangerous air pollutant for human health because their small size allows them to:
- Bypass nasal and throat defenses that would trap larger particles.
- Penetrate deep into the alveoli of the lungs where gas exchange happens.
- Cross into the bloodstream and reach the heart, brain and other organs.
In South Asia — and especially the Indo-Gangetic plain spanning Pakistan, India, Nepal and Bangladesh — PM2.5 is the dominant air-quality concern. The Global Burden of Disease study attributes around 1.7 million premature deaths annually in India alone to PM2.5 exposure, more than any other environmental risk factor.
Where PM2.5 comes from
PM2.5 has many sources, broadly split into primary (directly emitted) and secondary (formed in the atmosphere from other pollutants):
Primary sources:
- Vehicle exhaust — diesel especially produces large quantities of fine soot.
- Coal-fired power plants — major contributor across India and Pakistan.
- Industrial emissions — brick kilns, steel mills, refineries.
- Crop-residue burning — Punjab and Haryana farmers burning paddy stubble in October–November.
- Household cooking with biomass (wood, dung, charcoal) — still common in rural areas.
- Fireworks — sharp temporary spikes during Diwali, weddings.
- Construction and demolition dust — common across rapidly developing cities.
Secondary sources:
- Ammonia + nitrogen oxides + sulphates combining in the atmosphere to form ammonium sulphate and ammonium nitrate particles.
- Volatile organic compounds reacting with sunlight to form organic aerosols.
WHO and CPCB guidelines
The World Health Organization updated its global air quality guidelines in 2021. The new standards are stricter:
| Average | WHO 2021 limit |
|---|---|
| Annual mean | 5 µg/m³ |
| 24-hour mean | 15 µg/m³ |
For comparison, India’s National Ambient Air Quality Standards (NAAQS) are far more lenient:
| Average | India NAAQS |
|---|---|
| Annual mean | 40 µg/m³ (8× WHO) |
| 24-hour mean | 60 µg/m³ |
In practice, Delhi’s annual average PM2.5 is around 100–120 µg/m³ — roughly 20× the WHO limit and 3× even the lenient Indian standard.
PM2.5 in South Asia
The World Air Quality Report 2024 found that 11 of the world’s 12 most polluted cities are in South Asia. Annual average PM2.5 in selected cities:
- Lahore, Pakistan: ~100 µg/m³
- Delhi NCR, India: ~100–120 µg/m³
- Dhaka, Bangladesh: ~80–90 µg/m³
- Kolkata, India: ~75–85 µg/m³
- Mumbai, India: ~50–60 µg/m³
- Karachi, Pakistan: ~50–60 µg/m³
Seasonality matters enormously. Indian winters (November–January) routinely see daily PM2.5 above 300 µg/m³ in Delhi, with peaks beyond 500 µg/m³ in the worst smog events. By contrast, the same cities see PM2.5 below 50 µg/m³ during the monsoon, when rain scrubs the air.
Health effects
The link between PM2.5 and disease is now well established. Chronic exposure raises the risk of:
- Cardiovascular disease — heart attack, stroke, hypertension
- Lung cancer — PM2.5 is classified as a Group 1 carcinogen by the IARC
- COPD and asthma — exacerbation and increased mortality
- Premature birth and low birth weight — emerging evidence in South Asian cohorts
- Type 2 diabetes — recent meta-analyses confirm the association
- Cognitive decline and dementia — particles cross the blood-brain barrier
Short-term spikes (1–7 day exposure) cause measurable increases in hospital admissions for asthma, heart attacks and respiratory infections.
How to protect yourself
- Check your city’s live PM2.5 before going outdoors. Mausam Online displays the live reading alongside AQI on every city page.
- Wear an N95 or KN95 mask when PM2.5 exceeds 60 µg/m³. Cloth and surgical masks do not filter PM2.5.
- Run a HEPA air purifier indoors — sized to your room (CADR roughly 5× room area in m²).
- Keep windows closed during high-pollution episodes.
- Avoid outdoor exercise when PM2.5 is above 100 — exercise increases your lung intake by 5–10×.
- Special caution for children, elderly, pregnant women and anyone with asthma, COPD or heart disease.
Frequently asked questions
How small is PM2.5 really? A PM2.5 particle is about 2.5 micrometres across — for scale, a human hair is around 70 micrometres in diameter. PM2.5 is invisible to the naked eye and stays suspended in the air for hours to days.
What does “µg/m³” mean? Micrograms of particulate matter per cubic metre of air. A typical city street during winter smog in Delhi might contain 300 µg/m³ — so each cubic metre of air contains 300 millionths of a gram of PM2.5.
Does PM2.5 only matter outdoors? No. Indoor PM2.5 from cooking (especially with biomass fuels), candles, incense, smoking and infiltration of outdoor air can be just as harmful. A HEPA air purifier is the most effective indoor measure.
Are cloth masks good against PM2.5? No. Cloth masks filter only about 20–30% of PM2.5. N95 or KN95 masks filter at least 95% and are recommended whenever AQI is high. Make sure the mask seals around your nose and mouth.
Where can I see PM2.5 for my city? Mausam Online shows live PM2.5 and PM10 on every city page — see Delhi, Lahore, Kanpur, Patna, Dhaka.