What is PM2.5, and why does it matter most?
The tiny particles that do the most harm, where Delhi's come from, and which other pollutants matter.
The particles you can’t see
PM2.5 means particulate matter 2.5 micrometres across or smaller, roughly 30 times thinner than a human hair. These particles are small enough to get past the nose and throat, travel deep into the lungs, and the finest can enter the bloodstream.
PM10 includes larger particles up to 10 micrometres, such as road and construction dust. They irritate the nose, throat and airways but mostly don’t reach as deep.
Most of the health harm from air pollution worldwide is linked to PM2.5, which is why this guide focuses on it, and why HEPA filters and respirators, which capture fine particles, are the core tools.
Where Delhi’s PM2.5 comes from
Delhi’s pollution is a mix that changes with the season:
- Vehicles, especially diesel engines and traffic congestion
- Industry and power generation around the region
- Burning: crop stubble in neighbouring states (October–November), rubbish, biomass for cooking and heating, and firecrackers
- Dust from roads and construction (more PM10, but some PM2.5)
- Secondary particles that form in the air from gases such as sulphur dioxide, nitrogen oxides and ammonia
Winter weather makes all of it worse: cold, still air near the ground traps pollution, and lower wind speeds stop it dispersing. See Delhi’s pollution seasons.
Other pollutants
| Pollutant | What it is | Does a HEPA filter or N95 help? |
|---|---|---|
| PM10 | Coarser dust | Yes |
| Nitrogen dioxide (NO₂) | Gas from vehicles, gas stoves | No. Reduce sources and ventilate kitchens |
| Ozone (O₃) | Gas formed in sunlight, worse in summer afternoons | No. Avoid ozone-making “purifiers” |
| Carbon monoxide (CO) | Gas from burning fuel | No. Never burn charcoal indoors, use a CO alarm |
| Sulphur dioxide (SO₂) | Gas from coal and industry | No |
Particle filters handle particles. For gases, the main tools are reducing sources and choosing when to ventilate.
Short-term and long-term
- Short-term spikes (hours to days) can trigger asthma attacks, COPD flare-ups, heart attacks and strokes in people at risk, and cause coughing and irritation in everyone.
- Long-term exposure (months to years) raises the risk of heart disease, stroke, lung cancer, chronic lung disease and other conditions, and affects children’s lung growth.
That’s why both matter: protecting people during bad spells, and lowering what they breathe on average across the season. See health effects.